5
Maintaining Data Integrity

This chapter explains how to enforce the business rules associated with your database and prevent the entry of invalid information into tables by using integrity constraints. Topics include the following:

Using Integrity Constraints

You can define integrity constraints to enforce business rules on data in your tables. Once an integrity constraint is enabled, all data in the table must conform to the rule that it specifies. If you subsequently issue a SQL statement that modifies data in the table, then Oracle ensures that the resulting data satisfies the integrity constraint. Without integrity constraints, such business rules must be enforced programmatically by your application.

When to Use Integrity Constraints

Enforcing rules with integrity constraints is less costly than enforcing the equivalent rules by issuing SQL statements in your application. The semantics of integrity constraints are very clearly defined, so the internal operations that Oracle performs to enforce them are optimized beneath the level of SQL statements in Oracle. Because your applications use SQL, they cannot achieve this level of optimization.

Enforcing business rules with SQL statements can be even more costly in a networked environment because the SQL statements must be transmitted over a network. In such cases, using integrity constraints eliminates the performance overhead incurred by this transmission.

Example

To ensure that each employee in the EMP_TAB table works for a department that is listed in the DEPT_TAB table, first create a PRIMARY KEY constraint on the DEPTNO column of the DEPT_TAB table with the following statement:

ALTER TABLE Dept_tab
     
ADD PRIMARY KEY (Deptno);

Then create a referential integrity constraint on the DEPTNO column of the EMP_TAB table that references the primary key of the DEPT_TAB table. For example:

ALTER TABLE Emp_tab
     
ADD FOREIGN KEY (Deptno) REFERENCES Dept_tab(Deptno);

If you subsequently add a new employee record to the table, then Oracle automatically ensures that its department number appears in the department table.

To enforce this rule without integrity constraints, your application must test each new employee record to ensure that its department number belongs to an existing department. This testing involves issuing a SELECT statement to query the DEPT_TAB table.

Taking Advantage of Integrity Constraints

For best performance, define and enable integrity constraints and develop your applications to rely on them, rather than on SQL statements in your applications, to enforce business rules.

However, in some cases, you might want to enforce business rules through your application as well as through integrity constraints. Enforcing a business rule in your application might provide faster feedback to the user than an integrity constraint. For example, if your application accepts 20 values from the user and then issues an INSERT statement containing these values, then you might want your user to be notified immediately after entering a value that violates a business rule.

Because integrity constraints are enforced only when a SQL statement is issued, an integrity constraint can only notify the user of a bad value after the user has entered all 20 values and the application has issued the INSERT statement. However, you can design your application to verify the integrity of each value as it is entered, and notify the user immediately in the event of a bad value.

Using NOT NULL Integrity Constraints

By default, all columns can contain nulls. Only define NOT NULL constraints for columns of a table that absolutely require values at all times.

For example, in the EMP_TAB table, it might not be detrimental if an employee's manager or hire date were temporarily omitted. Also, some employees might not have a commission. Therefore, these three columns would not be good candidates for NOT NULL integrity constraints. However, it might not be permitted to have a row that does not have an employee name. Therefore, this column is a good candidate for the use of a NOT NULL integrity constraint.

NOT NULL constraints are often combined with other types of integrity constraints to further restrict the values that can exist in specific columns of a table. Use the combination of NOT NULL and UNIQUE key integrity constraints to force the input of values in the UNIQUE key; this combination of data integrity rules eliminates the possibility that any new row's data will ever attempt to conflict with an existing row's data.

Figure 5-1 NOT NULL Integrity Constraints

Setting Default Column Values

Legal default values include any literal, or any expression that does not refer to a column, LEVEL, ROWNUM, or PRIOR. Default values can include the expressions SYSDATE, USER, USERENV, and UID. The datatype of the default literal or expression must match or be convertible to the column datatype.

If you do not explicitly define a default value for a column, the default for the column is implicitly set to NULL.

When to Use Default Values

Only assign default values to columns that contain a typical value. For example, in the DEPT_TAB table, if most departments are located at one site, then the default value for the LOC column can be set to this value (such as NEW YORK).

Defaults are also useful when you use a view to make a subset of a table's columns visible. For example, you might allow users to insert rows into a table through a view. The view is defined to show all columns pertinent to end-user operations; however, the base table might also have a column named INSERTER, not included in the definition of the view, which logs the user that originally inserts each row of the table. The column named INSERTER can record the name of the user that inserts a row by defining the column with the USER function. For example:

. . ., inserter VARCHAR2(30) DEFAULT USER, . . .

See Also:
For another example of assigning a default column value, refer to the section "Creating Tables".

Figure 5-2 A UNIQUE Key Constraint

Choosing a Table's Primary Key

Each table can have one primary key. A primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist. Use the following guidelines when selecting a primary key:

Choose a column whose data values are unique.

The purpose of a table's primary key is to uniquely identify each row of the table. Therefore, the column or set of columns in the primary key must contain unique values for each row.
Choose a column whose data values are never changed.

A primary key value is only used to identify a row in the table; primary key values should never contain any data that is used for any other purpose. Therefore, primary key values should rarely need to be changed.
Choose a column that does not contain any nulls.

A PRIMARY KEY constraint, by definition, does not allow the input of any row with a null in any column that is part of the primary key.
Choose a column that is short and numeric.

Short primary keys are easy to type. You can use sequence numbers to easily generate numeric primary keys.
Avoid choosing composite primary keys.

Although composite primary keys are allowed, they do not satisfy the previous recommendations. For example, composite primary key values are long and cannot be assigned by sequence numbers.

Using UNIQUE Key Integrity Constraints

Choose unique keys carefully. In many situations, unique keys are incorrectly comprised of columns that should be part of the table's primary key (see the previous section for more information about primary keys). When deciding whether to use a UNIQUE key constraint, use the rule that a UNIQUE key constraint is only required to prevent the duplication of the key values within the rows of the table. The data in a unique key is such that it cannot be duplicated in the table.

Note:
Although UNIQUE key constraints allow the input of nulls, because of the search mechanism for UNIQUE constraints on more than one column, you cannot have identical values in the non-null columns of a partially null composite UNIQUE key constraint.

Do not confuse the concept of a unique key with that of a primary key. Primary keys are used to identify each row of the table uniquely. Therefore, unique keys should not have the purpose of identifying rows in the table.

Some examples of good unique keys include

An employee's social security number (the primary key is the employee number)
A truck's license plate number (the primary key is the truck number)
A customer's phone number, consisting of the two columns AREA and PHONE (the primary key is the customer number)
A department's name and location (the primary key is the department number)

Using Referential Integrity Constraints

Whenever two tables are related by a common column (or set of columns), define a PRIMARY or UNIQUE key constraint on the column in the parent table, and define a FOREIGN KEY constraint on the column in the child table, to maintain the relationship between the two tables.

See Also:
Depending on this relationship, you may want to define additional integrity constraints including the foreign key, as listed in the section "Relationships Between Parent and Child Tables".

Figure 5-3 shows a foreign key defined on the DEPTNO column of the EMP_TAB table. It guarantees that every value in this column must match a value in the primary key of the DEPT_TAB table (the DEPTNO column); therefore, no erroneous department numbers can exist in the DEPTNO column of the EMP_TAB table.

Foreign keys can be comprised of multiple columns. However, a composite foreign key must reference a composite primary or unique key of the exact same structure (the same number of columns and datatypes). Because composite primary and unique keys are limited to 16 columns, a composite foreign key is also limited to 16 columns.

Nulls and Foreign Keys

By default (without any NOT NULL or CHECK clauses), and in accordance with the ANSI/ISO standard, the FOREIGN KEY constraint enforces the "match none" rule for composite foreign keys. The "full" and "partial" rules can also be enforced by using CHECK and NOT NULL constraints, as follows:

To enforce the "match full" rule for nulls in composite foreign keys, which requires that all components of the key be null or all be non-null, define a CHECK constraint that allows only all nulls or all non-nulls in the composite foreign key as follows, assuming a composite key comprised of columns A, B, and C:
```
CHECK ((A IS NULL AND B IS NULL AND C IS NULL) OR  
       (A IS NOT NULL AND B IS NOT NULL AND C IS NOT NULL))
```
In general, it is not possible to use declarative referential integrity to enforce the "match partial" rule for nulls in composite foreign keys, which requires the non-null portions of the key to appear in the corresponding portions in the primary or unique key of a single row in the referenced table. You can often use triggers to handle this case, as described in Chapter 13, "Using Triggers".

Figure 5-3 Referential Integrity Constraints

Relationships Between Parent and Child Tables

Several relationships between parent and child tables can be determined by the other types of integrity constraints defined on the foreign key in the child table.

No Constraints on the Foreign Key

When no other constraints are defined on the foreign key, any number of rows in the child table can reference the same parent key value. This model allows nulls in the foreign key.

This model establishes a "one-to-many" relationship between the parent and foreign keys that allows undetermined values (nulls) in the foreign key. An example of such a relationship is shown in Figure 4-3 on page 8 between EMP_TAB and DEPT_TAB; each department (parent key) has many employees (foreign key), and some employees might not be in a department (nulls in the foreign key).

NOT NULL Constraint on the Foreign Key

When nulls are not allowed in a foreign key, each row in the child table must explicitly reference a value in the parent key because nulls are not allowed in the foreign key. However, any number of rows in the child table can reference the same parent key value.

This model establishes a "one-to-many" relationship between the parent and foreign keys. However, each row in the child table must have a reference to a parent key value; the absence of a value (a null) in the foreign key is not allowed. The same example in the previous section can be used to illustrate such a relationship. However, in this case, employees must have a reference to a specific department.

UNIQUE Constraint on the Foreign Key

When a UNIQUE constraint is defined on the foreign key, one row in the child table can reference a parent key value. This model allows nulls in the foreign key.

This model establishes a "one-to-one" relationship between the parent and foreign keys that allows undetermined values (nulls) in the foreign key. For example, assume that the EMP_TAB table had a column named MEMBERNO, referring to an employee's membership number in the company's insurance plan. Also, a table named INSURANCE has a primary key named MEMBERNO, and other columns of the table keep respective information relating to an employee's insurance policy. The MEMBERNO in the EMP_TAB table should be both a foreign key and a unique key:

To enforce referential integrity rules between the EMP_TAB and INSURANCE tables (the FOREIGN KEY constraint)
To guarantee that each employee has a unique membership number (the UNIQUE key constraint)

UNIQUE and NOT NULL Constraints on the Foreign Key

When both UNIQUE and NOT NULL constraints are defined on the foreign key, only one row in the child table can reference a parent key value. Because nulls are not allowed in the foreign key, each row in the child table must explicitly reference a value in the parent key.

This model establishes a "one-to-one" relationship between the parent and foreign keys that does not allow undetermined values (nulls) in the foreign key. If you expand the previous example by adding a NOT NULL constraint on the MEMBERNO column of the EMP_TAB table, in addition to guaranteeing that each employee has a unique membership number, then you also ensure that no undetermined values (nulls) are allowed in the MEMBERNO column of the EMP_TAB table.

Multiple FOREIGN KEY Constraints

Oracle allows a column to be referenced by multiple FOREIGN KEY constraints; effectively, there is no limit on the number of dependent keys. This situation might be present if a single column is part of two different composite foreign keys.

Concurrency Control, Indexes, and Foreign Keys

Oracle maximizes the concurrency control of parent keys in relation to dependent foreign key values. You can control what concurrency mechanisms are used to maintain these relationships, and, depending on the situation, this can be highly beneficial. The following sections explain the possible situations and give recommendations for each.

No Index on the Foreign Key

Figure 5-4 illustrates the locking mechanisms used by Oracle when no index is defined on the foreign key and when rows are being updated or deleted in the parent table. Inserts into the parent table do not require any locks on the child table.

Notice that a share lock of the entire child table is required until the transaction containing the DELETE statement for the parent table is committed. If the foreign key specifies ON DELETE CASCADE, then the DELETE statement results in a table-level share-subexclusive lock on the child table. A share lock of the entire child table is also required for an UPDATE statement on the parent table that affects any columns referenced by the child table. Share locks allow reading only; therefore, no INSERT, UPDATE, or DELETE statements can be issued on the child table until the transaction containing the UPDATE or DELETE is committed. Queries are allowed on the child table.

This situation is tolerable if updates and deletes can be avoided on the parent.

INSERT, UPDATE, and DELETE statements on the child table do not acquire any locks on the parent table, although INSERT and UPDATE statements will wait for a row-lock on the index of the parent table to clear.

Figure 5-4 Locking Mechanisms When No Index Is Defined on the Foreign Key

Index on the Foreign Key

Figure 5-5 illustrates the locking mechanisms used by Oracle when an index is defined on the foreign key, and new rows are inserted, updated or deleted in the child table.

Notice that no table locks of any kind are acquired on the parent table or any of its indexes as a result of the insert, update or delete. Therefore, any type of DML statement can be issued on the parent table, including inserts, updates, deletes, and queries.

This situation is preferable if there is any update or delete activity on the parent table while update activity is taking place on the child table. Inserts, updates, and deletes on the parent table do not require any locks on the child table, although updates and deletes will wait for row-level locks on the indexes of the child table to clear.

Figure 5-5 Locking Mechanisms When Index Is Defined on the Foreign Key

If the child table specifies ON DELETE CASCADE, then deletes from the parent table may result in deletes from the child table. In this case, waiting and locking rules are the same as if you deleted yourself from the child table after performing the delete from the parent table.

Referential Integrity in a Distributed Database

Oracle does not permit declarative referential integrity constraints to be defined across nodes of a distributed database (in other words, a declarative referential integrity constraint on one table cannot specify a foreign key that references a primary or unique key of a remote table).

However, parent/child table relationships across nodes can be maintained using triggers.

See Also:
For more information about triggers that enforce referential integrity, refer to Chapter 13, "Using Triggers".
Using triggers to maintain referential integrity requires the distributed option; for more information refer to Oracle8 Distributed Database Systems

Note:
If you decide to define referential integrity across the nodes of a distributed database using triggers, then be aware that network failures can limit the accessibility of not only the parent table, but also the child table. For example, assume that the child table is in the SALES database, and the parent table is in the HQ database.
If the network connection between the two databases fails, then some DML statements against the child table (those that insert rows into the child table or update a foreign key value in the child table) cannot proceed, because the referential integrity triggers must have access to the parent table in the HQ database.

Using CHECK Integrity Constraints

Use CHECK constraints when you need to enforce integrity rules that can be evaluated based on logical expressions. Never use CHECK constraints when any of the other types of integrity constraints can provide the necessary checking.

Examples of appropriate CHECK constraints include the following:

A CHECK constraint on the SAL column of the EMP_TAB table so that no salary value is greater than 10000
A CHECK constraint on the LOC column of the DEPT_TAB table so that only the locations "BOSTON", "NEW YORK", and "DALLAS" are allowed
A CHECK constraint on the SAL and COMM columns to compare the SAL and COMM values of a row and prevent the COMM value from being greater than the SAL value

Restrictions on CHECK Constraints

A CHECK integrity constraint requires that a condition be true or unknown for every row of the table. If a statement causes the condition to evaluate to false, then the statement is rolled back. The condition of a CHECK constraint has the following limitations:

The condition must be a Boolean expression that can be evaluated using the values in the row being inserted or updated.
The condition cannot contain subqueries or sequences.
The condition cannot include the SYSDATE, UID, USER, or USERENV SQL functions.
The condition cannot contain the pseudocolumns LEVEL, PRIOR, or ROWNUM;

See Also:
Oracle8i SQL Reference for an explanation of these pseudocolumns.
The condition cannot contain a user-defined SQL function.

Designing CHECK Constraints

When using CHECK constraints, consider the ANSI/ISO standard which states that a CHECK constraint is violated only if the condition evaluates to false; true and unknown values do not violate a check condition. Therefore, make sure that any CHECK constraint that you define actually enforces the rule you need enforced.

For example, consider the following CHECK constraint:

CHECK (Sal > 0 OR Comm >= 0)

At first glance, this rule may be interpreted as "do not allow a row in the EMP_TAB table unless the employee's salary is greater than zero or the employee's commission is greater than or equal to zero." However, note that if a row is inserted with a null salary and a negative commission, then the row does not violate the CHECK constraint, because the entire check condition is evaluated as unknown. In this particular case, you can account for such violations by placing NOT NULL integrity constraints on both the SAL and COMM columns.

Note:
If you are not sure when unknown values result in NULL conditions, review the truth tables for the logical operators AND and OR in Oracle8i SQL Reference

Multiple CHECK Constraints

A single column can have multiple CHECK constraints that reference the column in its definition. There is no limit to the number of CHECK constraints that can be defined that reference a column.

CHECK and NOT NULL Integrity Constraints

According to the ANSI/ISO standard, a NOT NULL integrity constraint is an example of a CHECK integrity constraint, where the condition is the following:

CHECK (Column_name IS NOT NULL)

Therefore, NOT NULL integrity constraints for a single column can, in practice, be written in two forms: using the NOT NULL constraint or a CHECK constraint. For ease of use, you should always choose to define NOT NULL integrity constraints, instead of CHECK constraints with the IS NOT NULL condition.

In the case where a composite key can allow only all nulls or all values, you must use a CHECK integrity constraint. For example, the following expression of a CHECK integrity constraint allows a key value in the composite key made up of columns C1 and C2 to contain either all nulls or all values:

CHECK ((C1 IS NULL AND C2 IS NULL) OR 
     
(C1 IS NOT NULL AND C2 IS NOT NULL))

Defining Integrity Constraints

Define an integrity constraint using the constraint clause of the SQL commands CREATE TABLE or ALTER TABLE. The next two sections describe how to use these commands to define integrity constraints.

Note:
There are additional considerations if you are using Trusted Oracle; see the Trusted Oracle for more information.

The CREATE TABLE Command

The following examples of CREATE TABLE statements show the definition of several integrity constraints:

CREATE TABLE Dept_tab (
     
Deptno  NUMBER(3) PRIMARY KEY, 
Dname   VARCHAR2(15), 
Loc     VARCHAR2(15), 
        CONSTRAINT Dname_ukey UNIQUE (Dname, Loc), 
        CONSTRAINT Loc_check1 
                CHECK (loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')));

     
CREATE TABLE Emp_tab ( 
     
Empno    NUMBER(5) PRIMARY KEY, 
Ename    VARCHAR2(15) NOT NULL, 
Job      VARCHAR2(10), 
Mgr      NUMBER(5) CONSTRAINT Mgr_fkey 
                 REFERENCES Emp_tab,
Hiredate DATE, 
Sal      NUMBER(7,2), 
Comm     NUMBER(5,2), 
Deptno   NUMBER(3) NOT NULL 
         CONSTRAINT dept_fkey 
     
       REFERENCES Dept_tab ON DELETE CASCADE);

The ALTER TABLE Command

You can also define integrity constraints using the constraint clause of the ALTER TABLE command. For example, the following examples of ALTER TABLE statements show the definition of several integrity constraints:

ALTER TABLE Dept_tab 
     
ADD PRIMARY KEY (deptno); 

     
ALTER TABLE Emp_tab 
     
ADD CONSTRAINT Dept_fkey FOREIGN KEY (Deptno) REFERENCES Dept_tab 
MODIFY (Ename VARCHAR2(15) NOT NULL);

Restrictions with the ALTER TABLE Command

Because data is likely to be in the table at the time an ALTER TABLE statement is issued, there are several restrictions to be aware of. Table 5-1 lists each type of constraint and the associated restrictions with the ALTER TABLE command.

Table 5-1 Restrictions for Defining Integrity Constraints with ALTER TABLE

Type of Constraint	Added to Existing Columns of the Table	Added with New Columns to the Table
`NOT NULL`	Cannot be defined if any row contains a null value for this column*	Cannot be defined if the table contains any rows
`UNIQUE`	Cannot be defined if duplicate values exist in the key*	Always OK
`PRIMARY KEY`	Cannot be defined if duplicate or null values exist in the key*	Cannot be defined if the table contains any rows
`FOREIGN KEY`	Cannot be defined if the foreign key has values that do not reference a parent key value*	Always OK
`CHECK`	Cannot be defined if the volume has values that do not comply with the check condition*	Always OK
* Assumes DISABLE clause not included in statement.

If you attempt to define a constraint with an ALTER TABLE statement and you violate one of these restrictions, then the statement is rolled back, and an informative error is returned explaining the violation.

Required Privileges

The creator of a constraint must have the ability to create tables (the CREATE TABLE or CREATE ANY TABLE system privilege), or the ability to alter the table (the ALTER object privilege for the table or the ALTER ANY TABLE system privilege) with the constraint. Additionally, UNIQUE and PRIMARY KEY integrity constraints require that the owner of the table have either a quota for the tablespace that contains the associated index or the UNLIMITED TABLESPACE system privilege. FOREIGN KEY integrity constraints also require some additional privileges.

Naming Integrity Constraints

Assign names to NOT NULL, UNIQUE KEY, PRIMARY KEY, FOREIGN KEY, and CHECK constraints using the CONSTRAINT option of the constraint clause. This name must be unique with respect to other constraints that you own. If you do not specify a constraint name, then one is assigned by Oracle.

See the previous examples of the CREATE TABLE and ALTER TABLE statements for examples of the CONSTRAINT option of the constraint clause. Note that the name of each constraint is included with other information about the constraint in the data dictionary.

See Also:
"Listing Integrity Constraint Definitions" for examples of data dictionary views.

Enabling and Disabling Constraints Upon Definition

By default, whenever an integrity constraint is defined in a CREATE or ALTER TABLE statement, the constraint is automatically enabled (enforced) by Oracle unless it is specifically created in a disabled state using the DISABLE clause.

See Also:
"Enabling and Disabling Integrity Constraints" has more information about important issues for enabling and disabling constraints.

UNIQUE Key, PRIMARY KEY, and FOREIGN KEY

When defining UNIQUE, PRIMARY KEY, and FOREIGN KEY integrity constraints, you should be aware of several important issues and prerequisites.

See Also:
For information about defining and managing FOREIGN KEY constraints, see "Managing FOREIGN KEY Integrity Constraints".
UNIQUE key and PRIMARY KEY constraints are usually enabled by the database administrator; see the Oracle8i Administrator's Guide.

Enabling and Disabling Integrity Constraints

This section explains the mechanisms and procedures for manually enabling and disabling integrity constraints.

enabled constraint

When a constraint is enabled, the rule defined by the constraint is enforced on the data values in the columns that define the constraint. The definition of the constraint is stored in the data dictionary.

disabled constraint

When a constraint is disabled, the rule defined by the constraint is not enforced on the data values in the columns included in the constraint; however, the definition of the constraint is retained in the data dictionary.

In summary, an integrity constraint can be thought of as a statement about the data in a database. This statement is always true when the constraint is enabled; however, the statement may or may not be true when the constraint is disabled because data in violation of the integrity constraint can be in the database.

Why Enable or Disable Constraints?

To enforce the rules defined by integrity constraints, the constraints should always be enabled; however, in certain situations, it is desirable to disable the integrity constraints of a table temporarily for performance reasons. For example:

When loading large amounts of data into a table using SQL*Loader
When performing batch operations that make massive changes to a table (such as changing everyone's employee number by adding 1000 to the existing number)
When importing or exporting one table at a time

In cases such as these, integrity constraints may be temporarily turned off to improve the performance of the operation.

Integrity Constraint Violations

If a row of a table does not adhere to an integrity constraint, then this row is said to be in violation of the constraint and is known as an exception to the constraint. If any exceptions exist, then the constraint cannot be enabled. The rows that violate the constraint must be either updated or deleted in order for the constraint to be enabled.

Exceptions for a specific integrity constraint can be identified while attempting to enable the constraint.

See Also:
This procedure is discussed in the section "Exception Reporting".

On Definition

When you define an integrity constraint in a CREATE TABLE or ALTER TABLE statement, you can enable the constraint by including the ENABLE clause in its definition or disable it by including the DISABLE clause in its definition. If neither the ENABLE nor the DISABLE clause is included in a constraint's definition, Oracle automatically enables the constraint.

Enabling Constraints

The following CREATE TABLE and ALTER TABLE statements both define and enable integrity constraints:

CREATE TABLE Emp_tab (
    Empno NUMBER(5) PRIMARY KEY);
 ALTER TABLE Emp_tab 
    ADD PRIMARY KEY (Empno);

An ALTER TABLE statement that defines and attempts to enable an integrity constraint may fail because rows of the table may violate the integrity constraint. In this case, the statement is rolled back and the constraint definition is not stored and not enabled.

See Also:
Refer to the section "Exception Reporting" for more information about rows that violate integrity constraints.

Disabling Constraints

The following CREATE TABLE and ALTER TABLE statements both define and disable integrity constraints:

CREATE TABLE Emp_tab (  
     
Empno NUMBER(5) PRIMARY KEY DISABLE); 


     ALTER TABLE Emp_tab  
     
ADD PRIMARY KEY (Empno) DISABLE;

An ALTER TABLE statement that defines and disables an integrity constraints never fails. The definition of the constraint is always allowed because its rule is not enforced.

Enabling and Disabling Defined Integrity Constraints

Use the ALTER TABLE command to

Enable a disabled constraint, using the ENABLE clause
Disable an enabled constraint, using the DISABLE clause

Enabling Disabled Constraints

The following statements are examples of statements that enable disabled integrity constraints:

ALTER TABLE Dept_tab  
     
ENABLE CONSTRAINT Dname_ukey; 
 

     ALTER TABLE Dept_tab  
     
ENABLE PRIMARY KEY  
ENABLE UNIQUE (Dname)
ENABLE UNIQUE (Loc);

An ALTER TABLE statement that attempts to enable an integrity constraint fails when the rows of the table violate the integrity constraint. In this case, the statement is rolled back and the constraint is not enabled.

See Also:
Refer to the section "Exception Reporting" for more information about rows that violate integrity constraints.

Disabling Enabled Constraints

The following statements are examples of statements that disable enabled integrity constraints:

ALTER TABLE Dept_tab  
     
DISABLE CONSTRAINT Dname_ukey;
 

     ALTER TABLE Dept_tab
     
DISABLE PRIMARY KEY
DISABLE UNIQUE (Dname)
DISABLE UNIQUE (Loc);

       






Tip -- Using the Data Dictionary for Reference:



The example statements in the previous sections require that you have some information about a constraint to enable or disable it. 


For example, the first statement of each section requires that you know the constraint's name, while the second statement of each section requires that you know the unique key's column list. If you do not have such information, then you can query one of the data dictionary views defined for constraints; for more information about these views, see "Listing Integrity Constraint Definitions" and Oracle8i Reference.

Enabling and Disabling Key Integrity Constraints

When enabling or disabling UNIQUE, PRIMARY KEY, and FOREIGN KEY integrity constraints, you should be aware of several important issues and prerequisites. UNIQUE key and PRIMARY KEY constraints are usually managed by the database administrator.

Enabling Constraints after a Parallel Direct Path Load

SQL*Loader permits multiple concurrent sessions to perform a direct path load into the same table. Because each SQL*Loader session can attempt to re-enable constraints on a table after a direct path load, there is a danger that one session may attempt to re-enable a constraint before another session is finished loading data. In this case, the first session to complete the load will be unable to enable the constraint because the remaining sessions possess share locks on the table.

Because there is a danger that some constraints might not be re-enabled after a direct path load, you should check the status of the constraint after completing the load to ensure that it was enabled properly.

PRIMARY and UNIQUE KEY constraints

PRIMARY KEY and UNIQUE key constraints create indexes on a table when they are enabled, and subsequently can take a significantly long time to enable after a direct path loading session if the table is very large.

You should consider enabling these constraints manually after a load (and not specify the automatic enable feature). This allows you to manually create the required indexes in parallel to save time before enabling the constraint.

See Also:
Oracle8i Tuning for more information about creating indexes in parallel.

Exception Reporting

If no exceptions are present when you issue a CREATE TABLE... ENABLE... or ALTER TABLE... ENABLE... statement, then the integrity constraint is enabled and all subsequent DML statements are subject to the enabled integrity constraints.

If exceptions exist when you enable a constraint, then an error is returned, and the integrity constraint remains disabled. When a statement is not successfully executed because integrity constraint exceptions exist, the statement is rolled back. If exceptions exist, then you cannot enable the constraint until all exceptions to the constraint are either updated or deleted.

To determine which rows violate the integrity constraint, include the EXCEPTIONS option in the ENABLE clause of a CREATE TABLE or ALTER TABLE statement. The EXCEPTIONS option places the ROWID, table owner, table name, and constraint name of all exception rows into a specified table. For example, the following statement attempts to enable the primary key of the DEPT_TAB table; if exceptions exist, information is inserted into a table named EXCEPTIONS:

Caution:
You may need to set up data structures for certain examples to work:

ALTER TABLE Dept_tab ENABLE PRIMARY KEY EXCEPTIONS INTO exceptions;

Create an appropriate exceptions report table to accept information from the EXCEPTIONS option of the ENABLE clause. Create an exception table by submitting the script UTLEXCPT.SQL. The script creates a tabled named EXCEPTIONS. You can create additional exceptions tables with different names by modifying and resubmitting the script.

If duplicate primary key values exist in the DEPT_TAB table and the name of the PRIMARY KEY constraint on DEPT_TAB is SYS_C00301, the following rows might be placed in the table EXCEPTIONS by the previous statement:

SELECT * FROM Exceptions;  
 
ROWID                       OWNER   TABLE_NAME       CONSTRAINT  
------------------          ------  ------------     -----------  
AAAA5bAADAAAAEQAAA          SCOTT   DEPT_TAB         SYS_C00301  
AAAA5bAADAAAAEQAAB          SCOTT   DEPT_TAB         SYS_C00301

A more informative query would be to join the rows in an exception report table and the master table to list the actual rows that violate a specific constraint. For example:

SELECT Deptno, Dname, Loc FROM Dept_tab, Exceptions  
     
WHERE Exceptions.Constraint = 'SYS_C00301'  
AND Dept_tab.Rowid = Exceptions.Row_id;  

      
DEPTNO     DNAME          LOC  
---------- -------------- -------------  
10         ACCOUNTING     NEW YORK  
10         RESEARCH       DALLAS

Rows that violate a constraint must be either updated or deleted from the table that contains the constraint. If updating exceptions, then you must change the value that violates the constraint to a value consistent with the constraint or a null (if allowed). After updating or deleting a row in the master table, delete the corresponding rows for the exception in the exception report table to avoid confusion with later exception reports. The statements that update the master table and the exception report table should be in the same transaction to ensure transaction consistency.

For example, to correct the exceptions in the previous examples, the following transaction might be issued:

UPDATE Dept_tab SET Deptno = 20 WHERE Dname = 'RESEARCH';

DELETE FROM Exceptions WHERE Constraint = 'SYS_C00301';

COMMIT;

When you manage exceptions, your goal should be to eliminate all exceptions in your exception report table. After eliminating all exceptions, you must re-enable the constraint; the constraint is not automatically enabled after the exceptions are handled.

While you are correcting current exceptions for a table with the constraint disabled, other users can issue statements creating new exceptions.

Altering Integrity Constraints

In Oracle 8.0, only certain constraint states could be changed using the ENABLE or DISABLE clauses. With Oracle 8.1, there are expanded capabilities to alter the state of an existing constraint with the MODIFY CONSTRAINT clause.

See Also:
For information on the parameters you can modify, see the ALTER TABLE section in Oracle8i SQL Reference.

Examples of MODIFY CONSTRAINT

Modify Constraint Example #1

CREATE TABLE X1_tab (a1 NUMBER CONSTRAINT y CHECK (a1>3) DEFERRABLE 
DISABLE);
    

    ALTER TABLE X1_tab MODIFY CONSTRAINT Y_cnstrt ENABLE;
ALTER TABLE X1_tab MODIFY CONSTRAINT Y_cnstrt RELY;
ALTER TABLE X1_tab MODIFY CONSTRAINT Y_cnstrt INITIALLY DEFERRED;
ALTER TABLE X1_tab MODIFY CONSTRAINT Y_cnstrt ENABLE NOVALIDATE;

Modify Constraint Example #2

CREATE TABLE X1_tab (A1 NUMBER CONSTRAINT Y_cnstrt 
NOT NULL DEFERRABLE INITIALLY DEFERRED NORELY DISABLE);

ALTER TABLE X1_tab ADD CONSTRAINT One_cnstrt UNIQUE(A1) 
DEFERRABLE INITIALLY IMMEDIATE RELY USING INDEX PCTFREE = 30
ENABLE VALIDATE;

ALTER TABLE X1_tab MODIFY UNIQUE(A1)
INITIALLY DEFERRED NORELY USING INDEX PCTFREE = 40
ENABLE NOVALIDATE;

ALTER TABLE X1_tab MODIFY CONSTRAINT One_cnstrt
INITIALLY IMMEDIATE RELY;

Modify Constraint Example #3

CREATE TABLE T1_tab (A1 INT, B1 INT);
ALTER TABLE T1_tab add CONSTRAINT P1_cnstrt PRIMARY KEY(a1) DISABLE;
ALTER TABLE T1_tab MODIFY PRIMARY KEY INITIALLY IMMEDIATE
USING INDEX PCTFREE = 30 ENABLE NOVALIDATE;
ALTER TABLE T1_tab MODIFY PRIMARY KEY
USING INDEX PCTFREE = 35 ENABLE;
ALTER TABLE T1_tab MODIFY PRIMARY KEY ENABLE NOVALIDATE;

Note:
RELY and NORELY are new states that can be set or reset when a constraint is created or modified.

Dropping Integrity Constraints

Drop an integrity constraint if the rule that it enforces is no longer true or if the constraint is no longer needed. Drop an integrity constraint using the ALTER TABLE command and the DROP clause. For example, the following statements drop integrity constraints:

ALTER TABLE Dept_tab  
     
DROP UNIQUE (Dname);  

     ALTER TABLE Dept_tab  
     
DROP UNIQUE (Loc);  

       
ALTER TABLE Emp_tab  
     
DROP PRIMARY KEY,  
DROP CONSTRAINT Dept_fkey; 

     
DROP TABLE Emp_tab CASCADE CONSTRAINTS;

When dropping UNIQUE, PRIMARY KEY, and FOREIGN KEY integrity constraints, you should be aware of several important issues and prerequisites. UNIQUE and PRIMARY KEY constraints are usually managed by the database administrator.

Managing FOREIGN KEY Integrity Constraints

General information about defining, enabling, disabling, and dropping all types of integrity constraints is given in the previous sections. The following section supplements this information, focusing specifically on issues regarding FOREIGN KEY integrity constraints.

Defining FOREIGN KEY Integrity Constraints

The following topics are of interest when defining FOREIGN KEY integrity constraints.

Matching of Datatypes

When defining referential integrity constraints, the corresponding column names of the dependent and referenced tables do not need to match. However, they must be of the same datatype.

Composite Foreign Keys

Because foreign keys reference primary and unique keys of the parent table, and PRIMARY KEY and UNIQUE key constraints are enforced using indexes, composite foreign keys are limited to 16 columns.

Implied Referencing of a Primary Key

If the column list is not included in the REFERENCES option when defining a FOREIGN KEY constraint (single column or composite), then Oracle assumes that you intend to reference the primary key of the specified table. Alternatively, you can explicitly specify the column(s) to reference in the parent table within parentheses. Oracle automatically checks to verify that this column list references a primary or unique key of the parent table. If it does not, then an informative error is returned.

Privileges Required for FOREIGN KEY Integrity Constraints

To create a FOREIGN KEY constraint, the creator of the constraint must have privileged access to both the parent and the child table.

The Parent Table The creator of the referential integrity constraint must own the parent table or have REFERENCES object privileges on the columns that constitute the parent key of the parent table.
The Child Table The creator of the referential integrity constraint must have the ability to create tables (that is, the CREATE TABLE or CREATE ANY TABLE system privilege) or the ability to alter the child table (that is, the ALTER object privilege for the child table or the ALTER ANY TABLE system privilege).

In both cases, necessary privileges cannot be obtained via a role; they must be explicitly granted to the creator of the constraint.

These restrictions allow:

The owner of the child table to explicitly decide what constraints are enforced on her or his tables and the other users that can create constraints on her or his tables
The owner of the parent table to explicitly decide if foreign keys can depend on the primary and unique keys in her tables

Specifying Referential Actions for Foreign Keys

Oracle allows two different types of referential integrity actions to be enforced, as specified with the definition of a FOREIGN KEY constraint:

The UPDATE/DELETE No Action Restriction This action prevents the update or deletion of a parent key if there is a row in the child table that references the key. By default, all FOREIGN KEY constraints enforce the no action restriction; no option needs to be specified when defining the constraint to enforce the no action restriction. For example:
```
CREATE TABLE Emp_tab (  
FOREIGN KEY (Deptno) REFERENCES Dept_tab);
 
```
The ON DELETE CASCADE Action This action allows referenced data in the parent key to be deleted (but not updated). If referenced data in the parent key is deleted, all rows in the child table that depend on the deleted parent key values are also deleted. To specify this referential action, include the ON DELETE CASCADE option in the definition of the FOREIGN KEY constraint. For example:
```
CREATE TABLE Emp_tab ( 
    FOREIGN KEY (Deptno) REFERENCES Dept_tab  
        ON DELETE CASCADE); 
```

Enabling FOREIGN KEY Integrity Constraints

FOREIGN KEY integrity constraints cannot be enabled if the referenced primary or unique key's constraint is not present or not enabled.

Listing Integrity Constraint Definitions

The data dictionary contains the following views that relate to integrity constraints:

ALL_CONSTRAINTS
ALL_CONS_COLUMNS
CONSTRAINT_COLUMNS
CONSTRAINT_DEFS
USER_CONSTRAINTS
USER_CONS_COLUMNS
USER_CROSS_REFS
DBA_CONSTRAINTS
DBA_CONS_COLUMNS
DBA_CROSS_REFS

See Also:
Refer to Oracle8i Reference for detailed information about each view.

Examples

Consider the following CREATE TABLE statements that define a number of integrity constraints:

CREATE TABLE Dept_tab (  
     
Deptno   NUMBER(3) PRIMARY KEY,  
Dname    VARCHAR2(15), 
Loc      VARCHAR2(15), 
CONSTRAINT Dname_ukey UNIQUE (Dname, Loc),  
CONSTRAINT LOC_CHECK1  
     
CHECK (Loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')));  

             
CREATE TABLE Emp_tab (  
     
Empno    NUMBER(5) PRIMARY KEY,  
Ename    VARCHAR2(15) NOT NULL,  
Job      VARCHAR2(10),  
Mgr      NUMBER(5) CONSTRAINT Mgr_fkey  
     
 REFERENCES Emp_tab ON DELETE CASCADE,  

     Hiredate DATE,
Sal      NUMBER(7,2),  
Comm     NUMBER(5,2),  
Deptno   NUMBER(3) NOT NULL  
CONSTRAINT Dept_fkey REFERENCES Dept_tab);

Example 1: Listing All of Your Accessible Constraints

The following query lists all constraints defined on all tables accessible to the user:

SELECT Constraint_name, Constraint_type, Table_name, 
     
R_constraint_name 
     
FROM User_constraints;

Considering the example statements at the beginning of this section, a list similar to the one below is returned:

CONSTRAINT_NAME  C TABLE_NAME  R_CONSTRAINT_NAME  
---------------  - ----------- ------------------  
SYS_C00275       P DEPT_TAB  
DNAME_UKEY       U DEPT_TAB  
LOC_CHECK1       C DEPT_TAB  
SYS_C00278       C EMP_TAB  
SYS_C00279       C EMP_TAB  
SYS_C00280       P EMP_TAB  
MGR_FKEY         R EMP_TAB     SYS_C00280  
DEPT_FKEY        R EMP_TAB     SYS_C00275

Notice the following:

Some constraint names are user specified (such as DNAME_UKEY), while others are system specified (such as SYS_C00275).
Each constraint type is denoted with a different character in the CONSTRAINT_TYPE column. The table below summarizes the characters used for each constraint type.

Constraint Type Character

PRIMARY KEY

P

UNIQUE KEY

U

FOREIGN KEY

R

CHECK, NOT NULL

C

Note:
An additional constraint type is indicated by the character "V" in the CONSTRAINT_TYPE column. This constraint type corresponds to constraints created by the WITH CHECK OPTION for views. See Chapter 3, "Managing Schema Objects" for more information about views and the WITH CHECK OPTION.

Example 2: Distinguishing NOT NULL Constraints from CHECK Constraints

In the previous example, several constraints are listed with a constraint type of "C". To distinguish which constraints are NOT NULL constraints and which are CHECK constraints in the EMP_TAB and DEPT_TAB tables, issue the following query:

SELECT Constraint_name, Search_condition  
     
FROM User_constraints  
WHERE (Table_name = 'DEPT_TAB' OR Table_name = 'EMP_TAB') AND  
     
Constraint_type = 'C';

Considering the example CREATE TABLE statements at the beginning of this section, a list similar to the one below is returned:

CONSTRAINT_NAME  SEARCH_CONDITION  
---------------  ----------------------------------------  
LOC_CHECK1       loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')  
SYS_C00278       ENAME IS NOT NULL  
SYS_C00279       DEPTNO IS NOT NULL