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| Pro*C/C++ Precompiler Programmer's Guide Release 8.1.5 A68022-01 |
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This chapter helps you to understand and apply the basic techniques of embedded SQL programming. Topics are:
Oracle uses host variables to pass data and status information to your program; your program uses host variables to pass data to Oracle.
Depending on how they are used, host variables are called output or input host variables.
Host variables in the INTO clause of a SELECT or FETCH statement are called output host variables because they hold column values output by Oracle. Oracle assigns the column values to corresponding output host variables in the INTO clause.
All other host variables in a SQL statement are called input host variables because your program inputs their values to Oracle. For example, you use input host variables in the VALUES clause of an INSERT statement and in the SET clause of an UPDATE statement. They are also used in the WHERE, HAVING, and FOR clauses. Input host variables can appear in a SQL statement wherever a value or expression is allowed.
Attention: In an ORDER BY clause, you can use a host variable, but it is treated as a constant or literal, and hence the contents of the host variable have no effect. For example, the SQL statement
EXEC SQL SELECT ename, empno INTO :name,:number FROM emp ORDER BY :ord;
appears to contain an input host variable :ord. However, the host variable in this case is treated as a constant, and regardless of the value of :ord, no ordering is done.
You cannot use input host variables to supply SQL keywords or the names of database objects. Thus, you cannot use input host variables in data definition statements such as ALTER, CREATE, and DROP. In the following example, the DROP TABLE statement is invalid:
char table_name[30]; printf("Table name? "); gets(table_name); EXEC SQL DROP TABLE :table_name; -- host variable not allowed
If you need to change database object names at runtime, use dynamic SQL. See "Oracle Dynamic SQL" for more information about dynamic SQL.
Before Oracle executes a SQL statement containing input host variables, your program must assign values to them. An example follows:
int emp_number; char temp[20]; VARCHAR emp_name[20]; /* get values for input host variables */ printf("Employee number? "); gets(temp); emp_number = atoi(temp); printf("Employee name? "); gets(emp_name.arr); emp_name.len = strlen(emp_name.arr); EXEC SQL INSERT INTO EMP (EMPNO, ENAME) VALUES (:emp_number, :emp_name);
Notice that the input host variables in the VALUES clause of the INSERT statement are prefixed with colons.
You can associate any host variable with an optional indicator variable. Each time the host variable is used in a SQL statement, a result code is stored in its associated indicator variable. Thus, indicator variables let you monitor host variables.
You use indicator variables in the VALUES or SET clause to assign NULLs to input host variables and in the INTO clause to detect NULLs or truncated values in output host variables.
The values your program can assign to an indicator variable have the following meanings:
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-1 |
Oracle will assign a NULL to the column, ignoring the value of the host variable. |
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>=0 |
Oracle will assign the value of the host variable to the column. |
The values Oracle can assign to an indicator variable have the following meanings:
Remember, an indicator variable must be defined as a 2-byte integer and, in SQL statements, must be prefixed with a colon and must immediately follow its host variable.
You can use indicator variables to INSERT NULLs. Before the INSERT, for each column you want to be NULL, set the appropriate indicator variable to -1, as shown in the following example:
set ind_comm = -1; EXEC SQL INSERT INTO emp (empno, comm) VALUES (:emp_number, :commission:ind_comm);
The indicator variable ind_comm specifies that a NULL is to be stored in the COMM column.
You can hard code the NULL instead, as follows:
EXEC SQL INSERT INTO emp (empno, comm) VALUES (:emp_number, NULL);
While this is less flexible, it might be more readable. Typically, you insert NULLs conditionally, as the next example shows:
printf("Enter employee number or 0 if not available: "); scanf("%d", &emp_number); if (emp_number == 0) ind_empnum = -1; else ind_empnum = 0; EXEC SQL INSERT INTO emp (empno, sal) VALUES (:emp_number:ind_empnum, :salary);
You can also use indicator variables to manipulate returned NULLs, as the following example shows:
EXEC SQL SELECT ename, sal, comm INTO :emp_name, :salary, :commission:ind_comm FROM emp WHERE empno = :emp_number; if (ind_comm == -1) pay = salary; /* commission is null; ignore it */ else pay = salary + commission;
When DBMS=V7 or DBMS=V8, if you SELECT or FETCH NULLs into a host variable not associated with an indicator variable, Oracle issues the following error message:
ORA-01405: fetched column value is NULL
For more information about the DBMS option, see "DBMS".
You can use indicator variables in the WHERE clause to test for NULLs, as the following example shows:
EXEC SQL SELECT ename, sal INTO :emp_name, :salary FROM emp WHERE :commission INDICATOR :ind_comm IS NULL ...
However, you cannot use a relational operator to compare NULLs with each other or with other values. For example, the following SELECT statement fails if the COMM column contains one or more NULLs:
EXEC SQL SELECT ename, sal INTO :emp_name, :salary FROM emp WHERE comm = :commission;
The next example shows how to compare values for equality when some of them might be NULLs:
EXEC SQL SELECT ename, sal INTO :emp_name, :salary FROM emp WHERE (comm = :commission) OR ((comm IS NULL) AND (:commission INDICATOR :ind_comm IS NULL));
When DBMS=V7 or V8, if you SELECT or FETCH a truncated column value into a host variable not associated with an indicator variable, a warning is generated instead of an error.
Executable SQL statements let you query, manipulate, and control Oracle data and create, define, and maintain Oracle objects such as tables, views, and indexes. This chapter focuses on the statements that query and manipulate data.
When executing a data manipulation statement such as INSERT, UPDATE, or DELETE, your only concern, besides setting the values of any input host variables, is whether the statement succeeds or fails. To find out, you simply check the SQLCA. (Executing any SQL statement sets the SQLCA variables.) You can check in the following two ways:
For more information about the SQLCA and the WHENEVER statement, see Chapter 9, "Handling Runtime Errors".
When executing a SELECT statement (query), however, you must also deal with the rows of data it returns. Queries can be classified as follows:
Queries that return more than one row require explicitly declared cursors or the use of host arrays (host variables declared as arrays).
Note: Host arrays let you process "batches" of rows. For more information, see Chapter 8, "Host Arrays". This chapter assumes the use of scalar host variables.
The following embedded SQL statements let you query and manipulate Oracle data:
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SELECT |
Returns rows from one or more tables. |
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INSERT |
Adds new rows to a table. |
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UPDATE |
Modifies rows in a table. |
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DELETE |
Removes unwanted rows from a table. |
The following embedded SQL statements let you define and manipulate an explicit cursor:
In the coming sections, first you learn how to code INSERT, UPDATE, DELETE, and single-row SELECT statements. Then, you progress to multirow SELECT statements. For a detailed discussion of each statement and its clauses, see the appendix "Embedded SQL Statements and Directives" and the Oracle8i SQL Reference .
Querying the database is a common SQL operation. To issue a query you use the SELECT statement. In the following example, you query the EMP table:
EXEC SQL SELECT ename, job, sal + 2000 INTO :emp_name, :job_title, :salary FROM emp WHERE empno = :emp_number;
The column names and expressions following the keyword SELECT make up the select list. The select list in our example contains three items. Under the conditions specified in the WHERE clause (and following clauses, if present), Oracle returns column values to the host variables in the INTO clause.
The number of items in the select list should equal the number of host variables in the INTO clause, so there is a place to store every returned value.
In the simplest case, when a query returns one row, its form is that shown in the last example. However, if a query can return more than one row, you must FETCH the rows using a cursor or SELECT them into a host-variable array. Cursors and the FETCH statement are discussed later in this chapter; array processing is discussed in Chapter 8, "Host Arrays".
If a query is written to return only one row but might actually return several rows, the result of the SELECT is indeterminate. Whether this causes an error depends on how you specify the SELECT_ERROR option. The default value, YES, generates an error if more than one row is returned.
You can use all of the following standard SQL clauses in your
SELECT statements:
Except for the INTO clause, the text of embedded SELECT statements can be executed and tested interactively using SQL*Plus. In SQL*Plus, you use substitution variables or constants instead of input host variables.
Use the INSERT statement to add rows to a table or view. In the following example, you add a row to the EMP table:
EXEC SQL INSERT INTO emp (empno, ename, sal, deptno) VALUES (:emp_number, :emp_name, :salary, :dept_number);
Each column you specify in the column list must belong to the table named in the INTO clause. The VALUES clause specifies the row of values to be inserted. The values can be those of constants, host variables, SQL expressions, SQL functions such as USER and SYSDATE, or user-defined PL/SQL functions.
The number of values in the VALUES clause must equal the number of names in the column list. However, you can omit the column list if the VALUES clause contains a value for each column in the table, in the order that they are defined in the table.
For more details, see "INSERT (Executable Embedded SQL)".
A subquery is a nested SELECT statement. Subqueries let you conduct multi-part searches. They can be used to
The following example uses a subquery in an INSERT statement to copy rows from one table to another:
EXEC SQL INSERT INTO emp2 (empno, ename, sal, deptno) SELECT empno, ename, sal, deptno FROM emp WHERE job= :job_title ;
Note that this INSERT statement uses the subquery to obtain intermediate results.
Use the UPDATE statement to change the values of specified columns in a table or view. In the following example, we update the SAL and COMM columns in the EMP table:
EXEC SQL UPDATE emp SET sal = :salary, comm = :commission WHERE empno = :emp_number;
Use the optional WHERE clause to specify the conditions under which rows are updated. See "Using the WHERE Clause".
The SET clause lists the names of one or more columns for which you must provide values. You can use a subquery to provide the values, as the following example shows:
EXEC SQL UPDATE emp SET sal = (SELECT AVG(sal)*1.1 FROM emp WHERE deptno = 20) WHERE empno = :emp_number;
The UPDATE statement has an optional returning clause, like the INSERT and DELETE statements. It is only allowed after the optional WHERE condition.
For more details, see "UPDATE (Executable Embedded SQL)".
Use the DELETE statement to remove rows from a table or view. In the following example, you delete all employees in a given department from the EMP table:
EXEC SQL DELETE FROM emp WHERE deptno = :dept_number ;
We have used the optional WHERE clause to specify the condition under which rows are deleted.
The returning clause option can be used in DELETE statements also. It is allowed after the optional WHERE condition. In the above example, it is good practice to record the field values of each employee that is deleted.
For more details, see "DELETE (Executable Embedded SQL)".
Use the WHERE clause to SELECT, UPDATE, or DELETE only those rows in a table or view that meet your search condition. The WHERE-clause search condition is a Boolean expression, which can include scalar host variables, host arrays (not in SELECT statements), subqueries, and user-defined stored functions.
If you omit the WHERE clause, all rows in the table or view are processed. If you omit the WHERE clause in an UPDATE or DELETE statement, Oracle sets sqlwarn[4] in the SQLCA to 'W' to warn that all rows were processed.
The INSERT, UPDATE, and DELETE statements can have an optional DML returning clause which returns column value expressions expr, into host variables hv, with host indicator variables iv. The DML returning clause looks like this:
{RETURNING | RETURN} {expr [,expr]} INTO {:hv [[INDICATOR]:iv] [, :hv [[INDICATOR]:iv]]}
The number of expressions must equal the number of host variables. This clause eliminates the need for selecting the rows after an INSERT or UPDATE, and before a DELETE when you need to record that information for your application. The returning clause eliminates inefficient network round-trips, extra processing, and server memory.
Oracle Dynamic SQL Method 4 does not support the DML returning clause; but ANSI Dynamic SQL Method 4 does. See Chapter 14, "ANSI Dynamic SQL".
When a query returns multiple rows, you can explicitly define a cursor to
Or, you can use host arrays; see Chapter 8, "Host Arrays".
A cursor identifies the current row in the set of rows returned by the query. This allows your program to process the rows one at a time. The following statements let you define and manipulate a cursor:
First you use the DECLARE CURSOR statement to name the cursor and associate it with a query.
The OPEN statement executes the query and identifies all the rows that meet the query search condition. These rows form a set called the active set of the cursor. After OPENing the cursor, you can use it to retrieve the rows returned by its associated query.
Rows of the active set are retrieved one by one (unless you use host arrays). You use a FETCH statement to retrieve the current row in the active set. You can execute FETCH repeatedly until all rows have been retrieved.
When done FETCHing rows from the active set, you disable the cursor with a CLOSE statement, and the active set becomes undefined.
The following sections show you how to use these cursor control statements in your application program.
You use the DECLARE CURSOR statement to define a cursor by giving it a name and associating it with a query, as the following example shows:
EXEC SQL DECLARE emp_cursor CURSOR FOR SELECT ename, empno, sal FROM emp WHERE deptno = :dept_number;
The cursor name is an identifier used by the precompiler, not a host or program variable, and should not be defined in the Declare Section. Cursor names cannot be hyphenated. They can be any length, but only the first 31 characters are significant. For ANSI compatibility, use cursor names no longer than 18 characters.
The SELECT statement associated with the cursor cannot include an INTO clause. Rather, the INTO clause and list of output host variables are part of the FETCH statement.
Because it is declarative, the DECLARE CURSOR statement must physically (not just logically) precede all other SQL statements referencing the cursor. That is, forward references to the cursor are not allowed. In the following example, the OPEN statement is misplaced:
... EXEC SQL OPEN emp_cursor; EXEC SQL DECLARE emp_cursor CURSOR FOR SELECT ename, empno, sal FROM emp WHERE ename = :emp_name;
The cursor control statements (DECLARE, OPEN, FETCH, CLOSE) must all occur within the same precompiled unit. For example, you cannot DECLARE a cursor in file A, then OPEN it in file B.
Your host program can DECLARE as many cursors as it needs. However, in a given file, every DECLARE statement must be unique. That is, you cannot DECLARE two cursors with the same name in one precompilation unit, even across blocks or procedures, because the scope of a cursor is global within a file.
If you will be using many cursors, you might want to specify the MAXOPENCURSORS option. For more information, see Chapter 10, "Precompiler Options", and Appendix C, "Performance Tuning".
You use the OPEN statement to execute the query and identify the active set. In the following example, you OPEN a cursor named emp_cursor:
EXEC SQL OPEN emp_cursor;
OPEN positions the cursor just before the first row of the active set. It also zeroes the rows-processed count kept by the third element of SQLERRD in the SQLCA. However, none of the rows is actually retrieved at this point. That will be done by the FETCH statement.
Once you OPEN a cursor, the query's input host variables are not re-examined until you reOPEN the cursor. Thus, the active set does not change. To change the active set, you must reOPEN the cursor.
Generally, you should CLOSE a cursor before reOPENing it. However, if you specify MODE=ORACLE (the default), you need not CLOSE a cursor before reOPENing it. This can increase performance; for details, see Appendix C, "Performance Tuning".
The amount of work done by OPEN depends on the values of three precompiler options: HOLD_CURSOR, RELEASE_CURSOR, and MAXOPENCURSORS. For more information, see the section "Using the Precompiler Options".
You use the FETCH statement to retrieve rows from the active set and specify the output host variables that will contain the results. Recall that the SELECT statement associated with the cursor cannot include an INTO clause. Rather, the INTO clause and list of output host variables are part of the FETCH statement. In the following example, you FETCH INTO three host variables:
EXEC SQL FETCH emp_cursor INTO :emp_name, :emp_number, :salary;
The cursor must have been previously DECLAREd and OPENed. The first time you execute FETCH, the cursor moves from before the first row in the active set to the first row. This row becomes the current row. Each subsequent execution of FETCH advances the cursor to the next row in the active set, changing the current row. The cursor can only move forward in the active set. To return to a row that has already been FETCHed, you must reOPEN the cursor, then begin again at the first row of the active set.
If you want to change the active set, you must assign new values to the input host variables in the query associated with the cursor, then reOPEN the cursor. When MODE=ANSI, you must CLOSE the cursor before reOPENing it.
As the next example shows, you can FETCH from the same cursor using different sets of output host variables. However, corresponding host variables in the INTO clause of each FETCH statement must have the same datatype.
EXEC SQL DECLARE emp_cursor CURSOR FOR SELECT ename, sal FROM emp WHERE deptno = 20; ... EXEC SQL OPEN emp_cursor; EXEC SQL WHENEVER NOT FOUND GOTO ... for (;;) { EXEC SQL FETCH emp_cursor INTO :emp_name1, :salary1; EXEC SQL FETCH emp_cursor INTO :emp_name2, :salary2; EXEC SQL FETCH emp_cursor INTO :emp_name3, :salary3; ... }
If the active set is empty or contains no more rows, FETCH returns the "no data found" error code to sqlcode in the SQLCA, or to the SQLCODE or SQLSTATE status variables. The status of the output host variables is indeterminate. (In a typical program, the WHENEVER NOT FOUND statement detects this error.) To re-use the cursor, you must reOPEN it.
It is an error to FETCH on a cursor under the following conditions:
When done FETCHing rows from the active set, you CLOSE the cursor to free the resources, such as storage, acquired by OPENing the cursor. When a cursor is closed, parse locks are released. What resources are freed depends on how you specify the HOLD_CURSOR and RELEASE_CURSOR options. In the following example, you CLOSE the cursor named emp_cursor:
EXEC SQL CLOSE emp_cursor;
You cannot FETCH from a closed cursor because its active set becomes undefined. If necessary, you can reOPEN a cursor (with new values for the input host variables, for example).
When MODE=ORACLE, issuing a COMMIT or ROLLBACK closes cursors referenced in a CURRENT OF clause. Other cursors are unaffected by COMMIT or ROLLBACK and if open, remain open. However, when MODE=ANSI, issuing a COMMIT or ROLLBACK closes all explicit cursors. For more information about COMMIT and ROLLBACK, see Chapter 3, "Database Concepts". For more information about the CURRENT OF clause, see the next section.
The CLOSE_ON_COMMIT micro precompiler option allows you to choose whether or not to close all cursors when a COMMIT is executed and the macro option MODE=ANSI. When MODE=ANSI, CLOSE_ON_COMMIT has the default value YES. Explicitly setting CLOSE_ON_COMMIT=NO results in better performance because cursors will not be closed when a COMMIT is executed, removing the need to re-open the cursors and incur extra parsing.
See "Macro and Micro Options" for a discussion of how micro options affect macro options. See "CLOSE_ON_COMMIT" for a complete discussion of this option.
The precompiler option PREFETCH allows for more efficient queries by pre-fetching a given number of rows. This decreases the number of server round-trips needed. The number of rows set by the PREFETCH option value in a configuration file or on the command line is used for all queries involving explicit cursors, subject to the standard precedence rules. When used inline, the PREFETCH option must precede any of these cursor statements:
When an OPEN is executed, the value of PREFETCH gives the number of rows to be pre-fetched when the query is executed. You can set the value from 0 (no pre-fetching) to 65535.
The Pro*C/C++ Precompiler supports optimizer hints in SQL statements. An optimizer hint is a suggestion to the Oracle SQL optimizer that can override the optimization approach that would normally be taken. You can use hints to specify the
Hints allow you to choose between rule-based and cost-based optimization. With cost-based optimization, you can use further hints to maximize throughput or response time.
You can issue an optimizer hint inside a C or C++ style comment, immediately after a SELECT, DELETE, or UPDATE command. You indicate that the comment contains one or more hints by following the comment opener with a plus sign, leaving no space between the opener and the '+'. For example, the following statement uses the ALL_ROWS hint to let the cost-based approach optimize the statement for the goal of best throughput:
EXEC SQL SELECT /*+ ALL_ROWS (cost-based) */ empno, ename, sal, job INTO :emp_rec FROM emp WHERE deptno = :dept_number;
As shown in this statement, the comment can contain optimizer hints as well as other comments.
For more information about the cost-based optimizer, and optimizer hints, see Oracle8i Application Developer's Guide - Fundamentals.
You use the CURRENT OF cursor_name clause in a DELETE or UPDATE statement to refer to the latest row FETCHed from the named cursor. The cursor must be open and positioned on a row. If no FETCH has been done or if the cursor is not open, the CURRENT OF clause results in an error and processes no rows.
The FOR UPDATE OF clause is optional when you DECLARE a cursor that is referenced in the CURRENT OF clause of an UPDATE or DELETE statement. The CURRENT OF clause signals the precompiler to add a FOR UPDATE clause if necessary. For more information, see "Using FOR UPDATE OF".
In the following example, you use the CURRENT OF clause to refer to the latest row FETCHed from a cursor named emp_cursor:
EXEC SQL DECLARE emp_cursor CURSOR FOR SELECT ename, sal FROM emp WHERE job = 'CLERK' FOR UPDATE OF sal; ... EXEC SQL OPEN emp_cursor; EXEC SQL WHENEVER NOT FOUND GOTO ... for (;;) { EXEC SQL FETCH emp_cursor INTO :emp_name, :salary; ... EXEC SQL UPDATE emp SET sal = :new_salary WHERE CURRENT OF emp_cursor; }
You cannot use CURRENT OF clause on an index-organized table.
An explicit FOR UPDATE OF or an implicit FOR UPDATE acquires exclusive row locks. All rows are locked at the OPEN, not as they are FETCHed, and are released when you COMMIT or ROLLBACK. Therefore, you cannot FETCH from a FOR UPDATE cursor after a COMMIT. If you try to do this, Oracle returns a 1002 error code.
Also, you cannot use host arrays with the CURRENT OF clause. For an alternative, see "Mimicking CURRENT OF".
Furthermore, you cannot reference multiple tables in an associated FOR UPDATE OF clause, which means that you cannot do joins with the CURRENT OF clause.
Finally, you cannot use dynamic SQL with the CURRENT OF clause.
The following example shows the typical sequence of cursor control statements in an application program:
... /* define a cursor */ EXEC SQL DECLARE emp_cursor CURSOR FOR SELECT ename, job FROM emp WHERE empno = :emp_number FOR UPDATE OF job; /* open the cursor and identify the active set */ EXEC SQL OPEN emp_cursor; /* break if the last row was already fetched */ EXEC SQL WHENEVER NOT FOUND DO break; /* fetch and process data in a loop */ for (;;) { EXEC SQL FETCH emp_cursor INTO :emp_name, :job_title; /* optional host-language statements that operate on the FETCHed data */ EXEC SQL UPDATE emp SET job = :new_job_title WHERE CURRENT OF emp_cursor; } ... /* disable the cursor */ EXEC SQL CLOSE emp_cursor; EXEC SQL COMMIT WORK RELEASE; ...
The following complete program illustrates the use of a cursor and the FETCH statement. The program prompts for a department number, then displays the names of all employees in that department.
All FETCHes except the final one return a row and, if no errors were detected during the FETCH, a success status code. The final FETCH fails and returns the "no data found" Oracle error code to sqlca.sqlcode. The cumulative number of rows actually FETCHed is found in sqlerrd[2] in the SQLCA.
#include <stdio.h> /* declare host variables */ char userid[12] = "SCOTT/TIGER"; char emp_name[10]; int emp_number; int dept_number; char temp[32]; void sql_error(); /* include the SQL Communications Area */ #include <sqlca.h> main() { emp_number = 7499; /* handle errors */ EXEC SQL WHENEVER SQLERROR do sql_error("Oracle error"); /* connect to Oracle */ EXEC SQL CONNECT :userid; printf("Connected.\n"); /* declare a cursor */ EXEC SQL DECLARE emp_cursor CURSOR FOR SELECT ename FROM emp WHERE deptno = :dept_number; printf("Department number? "); gets(temp); dept_number = atoi(temp); /* open the cursor and identify the active set */ EXEC SQL OPEN emp_cursor; printf("Employee Name\n"); printf("-------------\n"); /* fetch and process data in a loop exit when no more data */ EXEC SQL WHENEVER NOT FOUND DO break; while (1) { EXEC SQL FETCH emp_cursor INTO :emp_name; printf("%s\n", emp_name); } EXEC SQL CLOSE emp_cursor; EXEC SQL COMMIT WORK RELEASE; exit(0); } void sql_error(msg) char *msg; { char buf[500]; int buflen, msglen; EXEC SQL WHENEVER SQLERROR CONTINUE; EXEC SQL ROLLBACK WORK RELEASE; buflen = sizeof (buf); sqlglm(buf, &buflen, &msglen); printf("%s\n", msg); printf("%*.s\n", msglen, buf); exit(1); }
