Release 8.0
A54661_01
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| Programmer's Guide to the Oracle Pro*C/C++ Precompiler Release 8.0 A54661_01 |
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This chapter looks at using arrays to simplify coding and improve program performance. You learn how to manipulate Oracle data using arrays, how to operate on all the elements of an array with a single SQL statement, and how to limit the number of array elements processed. The following issues are addressed:
Arrays reduce programming time and result in improved performance.
With arrays, you manipulate an entire array with a single SQL statement. Thus, Oracle communication overhead is reduced markedly, especially in a networked environment. A major portion of runtime is spent on network roundtrips between the client program and the server database. Arrays reduce the roundtrips.
For example, suppose you want to insert information about 300 employees into the EMP table. Without arrays your program must do 300 individual INSERTs-one for each employee. With arrays, only one INSERT needs to be done.
You declare host arrays just like scalar host variables. Dimension the arrays when they are declared. The following example declares three host arrays, each with a dimension of 50 elements:
char emp_name[50][10];
int emp_number[50];
float salary[50];
Arrays of VARCHARs are also allowed. The following declaration is a valid host language declaration:
VARCHAR v_array[10][30];
You cannot declare host arrays of pointers, except for object types.
Except for character arrays (strings), host arrays that might be referenced in a SQL statement are limited to one dimension. So, the two-dimensional array declared in the following example is invalid:
int hi_lo_scores[25][25]; /* not allowed */
The maximum size of a host array is 32,767 divided by the size of the datatype. If you use a host array that exceeds the maximum, you get a "parameter out of range" runtime error.
You can use host arrays as input variables in the INSERT, UPDATE, and DELETE statements and as output variables in the INTO clause of SELECT and FETCH statements.
The embedded SQL syntax used for host arrays and simple host variables is nearly the same. One difference is the optional FOR clause, which lets you control array processing. Also, there are restrictions on mixing host arrays and simple host variables in a SQL statement.
The following sections illustrate the use of host arrays in data manipulation statements.
If you use multiple host arrays in a single SQL statement, their dimensions should be the same. Otherwise, an "array size mismatch" warning message is issued at precompile time. If you ignore this warning, the precompiler uses the smallest dimension for the SQL operation.
In this example, only 25 rows are INSERTed:
int emp_number[50];
char emp_name[50][10];
int dept_number[25];
/* Populate host arrays here. */
EXEC SQL INSERT INTO emp (empno, ename, deptno)
VALUES (:emp_number, :emp_name, :dept_number);
It is possible to subscript host arrays in SQL statements, and use them in a loop to INSERT or fetch data. For example, you could INSERT every fifth element in an array using a loop such as:
for (i = 0; i < 50; i += 5)
EXEC SQL INSERT INTO emp (empno, deptno)
VALUES (:emp_number[i], :dept_number[i]);
However, if the array elements that you need to process are contiguous, you should not process host arrays in a loop. Simply use the unsubscripted array names in your SQL statement. Oracle treats a SQL statement containing host arrays of dimension n like the same statement executed n times with n different scalar variables.
You can use indicator arrays to assign NULLs to input host arrays, and to detect NULL or truncated values in output host arrays. The following example shows how to INSERT with indicator arrays:
int emp_number[50];
int dept_number[50];
float commission[50];
short comm_ind[50]; /* indicator array */
/* Populate the host and indicator arrays. To insert a null
into the comm column, assign -1 to the appropriate
element in the indicator array. */
EXEC SQL INSERT INTO emp (empno, deptno, comm)
VALUES (:emp_number, :dept_number,
:commission INDICATOR :comm_ind);
Mixing scalar host variables with host arrays in the VALUES, SET, INTO, or WHERE clause is not allowed. If any of the host variables is an array, all must be arrays.
You cannot use host arrays with the CURRENT OF clause in an UPDATE or DELETE statement.
The array interface is an Oracle extension to the ANSI/ISO embedded SQL standard. However, when you precompile with MODE=ANSI, array SELECTs and FETCHes are still allowed. The use of arrays can be flagged using the FIPS flagger precompiler option, if desired.
When DBMS=V6, no error is generated if you SELECT or FETCH NULL columns into a host array that is not associated with an indicator array. Still, when doing array SELECTs and FETCHes, always use indicator arrays. That way, you can test for NULLs in the associated output host array.
When you precompile with the precompiler option DBMS=V7 or V8, if a NULL is selected or fetched into a host variable that has no associated indicator variable, Oracle stops processing, sets sqlca.sqlerrd[2] to the number of rows processed, and returns the following error:
ORA-01405: fetched column value is NULL
When DBMS=V6, if you SELECT or FETCH a truncated column value into a host array that is not associated with an indicator array, Oracle stops processing, sets sqlca.sqlerrd[2] to the number of rows processed, and issues the following error message:
ORA-01406: fetched column value was truncated
When DBMS=V7 or V8, Oracle does not consider truncation to be an error.
You can use host arrays as output variables in the SELECT statement. If you know the maximum number of rows the SELECT will return, simply dimension the host arrays with that number of elements. In the following example, you select directly into three host arrays. Knowing the SELECT will return no more than 50 rows, you dimension the arrays with 50 elements:
char emp_name[50][20];
int emp_number[50];
float salary[50];
EXEC SQL SELECT ENAME, EMPNO, SAL
INTO :emp_name, :emp_number, :salary
FROM EMP
WHERE SAL > 1000;
In this example, the SELECT statement returns up to 50 rows. If there are fewer than 50 eligible rows or you want to retrieve only 50 rows, this method will suffice. However, if there are more than 50 eligible rows, you cannot retrieve all of them this way. If you re-execute the SELECT statement, it just returns the first 50 rows again, even if more are eligible. You must either dimension a larger array or declare a cursor for use with the FETCH statement.
If a SELECT INTO statement returns more rows than the number of elements you dimensioned, Oracle issues the error message
ORA-02112: PCC: SELECT ...INTO returns too many rows
unless you specify SELECT_ERROR=NO. For more information about the SELECT_ERROR option, see the section "Using the Precompiler Options" on page 9-9 .
If you do not know the maximum number of rows a SELECT will return, you can declare and open a cursor, then fetch from it in "batches."
Batch fetches within a loop let you retrieve a large number of rows with ease. Each FETCH returns the next batch of rows from the current active set. In the following example, you fetch in 20-row batches:
int emp_number[20];
float salary[20];
EXEC SQL DECLARE emp_cursor CURSOR FOR
SELECT empno, sal FROM emp;
EXEC SQL OPEN emp_cursor;
EXEC SQL WHENEVER NOT FOUND do break;
for (;;)
{
EXEC SQL FETCH emp_cursor
INTO :emp_number, :salary;
/* process batch of rows */
...
}
Each FETCH returns, at most, the number of rows in the array dimension. Fewer rows are returned in the following cases:
The cumulative number of rows returned can be found in the third element of sqlerrd in the SQLCA, called sqlerrd[2] in this guide. This applies to each open cursor. In the following example, notice how the status of each cursor is maintained separately:
EXEC SQL OPEN cursor1;
EXEC SQL OPEN cursor2;
EXEC SQL FETCH cursor1 INTO :array_of_20;
/* now running total in sqlerrd[2] is 20 */
EXEC SQL FETCH cursor2 INTO :array_of_30;
/* now running total in sqlerrd[2] is 30, not 50 */
EXEC SQL FETCH cursor1 INTO :array_of_20;
/* now running total in sqlerrd[2] is 40 (20 + 20) */
EXEC SQL FETCH cursor2 INTO :array_of_30;
/* now running total in sqlerrd[2] is 60 (30 + 30) */
The demonstration program in this section shows how you can use host arrays when writing a query in Pro*C/C++. Pay particular attention to the use of the "rows processed count" in the SQLCA (sqlca.sqlerrd[2]). See Chapter 11, "Handling Runtime Errors" for more information about the SQLCA. This program is available on-line in the file sample3.pc in your demo directory.
/*
* sample3.pc
* Host Arrays
*
* This program connects to ORACLE, declares and opens a cursor,
* fetches in batches using arrays, and prints the results using
* the function print_rows().
*/
#include <stdio.h>
#include <string.h>
#include <sqlca.h>
#define NAME_LENGTH 20
#define ARRAY_LENGTH 5
/* Another way to connect. */
char *username = "SCOTT";
char *password = "TIGER";
/* Declare a host structure tag. */
struct
{
int emp_number[ARRAY_LENGTH];
char emp_name[ARRAY_LENGTH][NAME_LENGTH];
float salary[ARRAY_LENGTH];
} emp_rec;
/* Declare this program's functions. */
void print_rows(); /* produces program output */
void sql_error(); /* handles unrecoverable errors */
main()
{
int num_ret; /* number of rows returned */
/* Connect to ORACLE. */
EXEC SQL WHENEVER SQLERROR DO sql_error("Connect error:");
EXEC SQL CONNECT :username IDENTIFIED BY :password;
printf("\nConnected to ORACLE as user: %s\n", username);
EXEC SQL WHENEVER SQLERROR DO sql_error("Oracle error:");
/* Declare a cursor for the FETCH. */
EXEC SQL DECLARE c1 CURSOR FOR
SELECT empno, ename, sal FROM emp;
EXEC SQL OPEN c1;
/* Initialize the number of rows. */
num_ret = 0;
/* Array fetch loop - ends when NOT FOUND becomes true. */
EXEC SQL WHENEVER NOT FOUND DO break;
for (;;)
{
EXEC SQL FETCH c1 INTO :emp_rec;
/* Print however many rows were returned. */
print_rows(sqlca.sqlerrd[2] - num_ret);
num_ret = sqlca.sqlerrd[2]; /* Reset the number. */
}
/* Print remaining rows from last fetch, if any. */
if ((sqlca.sqlerrd[2] - num_ret) > 0)
print_rows(sqlca.sqlerrd[2] - num_ret);
EXEC SQL CLOSE c1;
printf("\nAu revoir.\n\n\n");
/* Disconnect from the database. */
EXEC SQL COMMIT WORK RELEASE;
exit(0);
}
void
print_rows(n)
int n;
{
int i;
printf("\nNumber Employee Salary");
printf("\n------ -------- ------\n");
for (i = 0; i < n; i++)
printf("%-9d%-15.15s%9.2f\n", emp_rec.emp_number[i],
emp_rec.emp_name[i], emp_rec.salary[i]);
}
void
sql_error(msg)
char *msg;
{
EXEC SQL WHENEVER SQLERROR CONTINUE;
printf("\n%s", msg);
printf("\n% .70s \n", sqlca.sqlerrm.sqlerrmc);
EXEC SQL ROLLBACK WORK RELEASE;
exit(1);
}
Using host arrays in the WHERE clause of a SELECT statement is not allowed except in a subquery. For an example, see the section "Using the WHERE Clause" on page 12-15.
Also, you cannot mix simple host variables with host arrays in the INTO clause of a SELECT or FETCH statement. If any of the host variables is an array, all must be arrays.
Table 12-1 shows which uses of host arrays are valid in a SELECT INTO statement:
| INTO Clause | WHERE Clause | Valid? |
|
array |
array |
no |
|
scalar |
scalar |
yes |
|
array |
scalar |
yes |
|
scalar |
array |
no |
When DBMS=V6, if you SELECT or FETCH null column values into a host array not associated with an indicator array, no error is generated. So, when doing array SELECTs and FETCHes, always use indicator arrays. That way, you can test for nulls in the associated output host array.
When DBMS = V7 or V6_CHAR, if you SELECT or FETCH a null column value into a host array not associated with an indicator array, Oracle stops processing, sets sqlerrd[2] to the number of rows processed, and issues the following error message:
ORA-01405: fetched column value is NULL
When DBMS=V6, if you SELECT or FETCH a truncated column value into a host array not associated with an indicator array, Oracle stops processing, sets sqlerrd[2] to the number of rows processed, and issues the following error message:
ORA-01406: fetched column value was truncated
In either case, you can check sqlerrd[2] for the number of rows processed before the truncation occurred. The rows-processed count includes the row that caused the truncation error.
When DBMS=V7 or V6_CHAR, truncationresults in a warning message, but Oracle continues processing.
Again, when doing array SELECTs and FETCHes, always use indicator arrays. That way, if Oracle assigns one or more truncated column values to an output host array, you can find the original lengths of the column values in the associated indicator array.
You can use host arrays as input variables in an INSERT statement. Just make sure your program populates the arrays with data before executing the INSERT statement.
If some elements in the arrays are irrelevant, you can use the FOR clause to control the number of rows inserted. See the section "Using the FOR Clause" on page 12-13.
An example of inserting with host arrays follows:
char emp_name[50][20];
int emp_number[50];
float salary[50];
/* populate the host arrays */
...
EXEC SQL INSERT INTO EMP (ENAME, EMPNO, SAL)
VALUES (:emp_name, :emp_number, :salary);
The cumulative number of rows inserted can be found in the rows-processed count, sqlca.sqlerrd[2].
In the following example, the INSERT is done one row at a time. This is much less efficient than the previous example, since a call to the server must be made for each row inserted.
for (i = 0; i < array_dimension; i++)
EXEC SQL INSERT INTO emp (ename, empno, sal)
VALUES (:emp_name[i], :emp_number[i], :salary[i]);
You cannot use an array of pointers in the VALUES clause of an INSERT statement; all array elements must be data items.
Mixing scaler host variables with host arrays in the VALUES clause of an INSERT statement is not allowed. If any of the host variables is an array, all must be arrays.
You can also use host arrays as input variables in an UPDATE statement, as the following example shows:
int emp_number[50];
float salary[50];
/* populate the host arrays */
EXEC SQL UPDATE emp SET sal = :salary
WHERE EMPNO = :emp_number;
The cumulative number of rows updated can be found in sqlerrd[2]. The number does not include rows processed by an update cascade.
If some elements in the arrays are irrelevant, you can use the embedded SQL FOR clause to limit the number of rows updated.
The last example showed a typical update using a unique key (EMP_NUMBER). Each array element qualified just one row for updating. In the following example, each array element qualifies multiple rows:
char job_title [10][20];
float commission[10];
...
EXEC SQL UPDATE emp SET comm = :commission
WHERE job = :job_title;
Mixing simple host variables with host arrays in the SET or WHERE clause of an UPDATE statement is not recommended. If any of the host variables is an array, all should be arrays. Furthermore, if you use a host array in the SET clause, use one of equal dimension in the WHERE clause.
You cannot use host arrays with the CURRENT OF clause in an UPDATE statement. For an alternative, see the section "Mimicking CURRENT OF" on page 12-27.
Table 12-2 shows which uses of host arrays are valid in an
UPDATE statement:
| SET Clause | WHERE Clause | Valid? |
|
array |
array |
yes |
|
scalar |
scalar |
yes |
|
array |
scalar |
no |
|
scalar |
array |
no |
You can also use host arrays as input variables in a DELETE statement. It is like executing the DELETE statement repeatedly using successive elements of the host array in the WHERE clause. Thus, each execution might delete zero, one, or more rows from the table.
An example of deleting with host arrays follows:
...
int emp_number[50];
/* populate the host array */
...
EXEC SQL DELETE FROM emp
WHERE empno = :emp_number;
The cumulative number of rows deleted can be found in sqlerrd[2]. The number does not include rows processed by a delete cascade.
The last example showed a typical delete using a unique key (EMP_NUMBER). Each array element qualified just one row for deletion. In the following example, each array element qualifies multiple rows:
...
char job_title[10][20];
/* populate the host array */
...
EXEC SQL DELETE FROM emp
WHERE job = :job_title;
Mixing simple host variables with host arrays in the WHERE clause of a DELETE statement is not allowed. If any of the host variables is an array, all must be arrays.
You cannot use host arrays with the CURRENT OF clause in a DELETE statement. For an alternative, see the section "Mimicking CURRENT OF" on page 12-27.
You can use the optional embedded SQL FOR clause to set the number of array elements processed by any of the following SQL statements:
The FOR clause is especially useful in UPDATE, INSERT, and DELETE statements. With these statements you might not want to use the entire array. The FOR clause lets you limit the elements used to just the number you need, as the following example shows:
char emp_name[100][20];
float salary[100];
int rows_to_insert;
/* populate the host arrays */
rows_to_insert = 25; /* set FOR-clause variable */
EXEC SQL FOR :rows_to_insert /* will process only 25 rows */
INSERT INTO emp (ename, sal)
VALUES (:emp_name, :salary);
The FOR clause can use an integer host variable to count array elements, or an integer literal. A complex C expression that resolves to an integer cannot be used. For example, the following statement that uses an integer expression is illegal:
EXEC SQL FOR :rows_to_insert + 5 /* illegal */
INSERT INTO emp (ename, empno, sal)
VALUES (:emp_name, :emp_number, :salary);
The FOR clause variable specifies the number of array elements to be processed. Make sure the number is not larger than the smallest array dimension. Also, the number must be positive. If it is negative or zero, no rows are processed and Oracle issues an error message.
Two restrictions keep FOR clause semantics clear: you cannot use the FOR clause in a SELECT statement or with the CURRENT OF clause.
If you use the FOR clause in a SELECT statement, you get the following error message:
PCC-E-0056: FOR clause not allowed on SELECT statement at ...
The FOR clause is not allowed in SELECT statements because its meaning is unclear. Does it mean "execute this SELECT statement n times"? Or, does it mean "execute this SELECT statement once, but return n rows"? The problem in the former case is that each execution might return multiple rows. In the latter case, it is better to declare a cursor and use the FOR clause in a FETCH statement, as follows:
EXEC SQL FOR :limit FETCH emp_cursor INTO ...
You can use the CURRENT OF clause in an UPDATE or DELETE statement to refer to the latest row returned by a FETCH statement, as the following example shows:
EXEC SQL DECLARE emp_cursor CURSOR FOR
SELECT ename, sal FROM emp WHERE empno = :emp_number;
...
EXEC SQL OPEN emp_cursor;
...
EXEC SQL FETCH emp_cursor INTO :emp_name, :salary;
...
EXEC SQL UPDATE emp SET sal = :new_salary
WHERE CURRENT OF emp_cursor;
However, you cannot use the FOR clause with the CURRENT OF clause. The following statements are invalid because the only logical value of limit is 1 (you can only update or delete the current row once):
EXEC SQL FOR :limit UPDATE emp SET sal = :new_salary
WHERE CURRENT OF emp_cursor;
...
EXEC SQL FOR :limit DELETE FROM emp
WHERE CURRENT OF emp_cursor;
Oracle treats a SQL statement containing host arrays of dimension n like the same SQL statement executed n times with n different scalar variables (the individual array elements). The precompiler issues the following error message only when such treatment would be ambiguous:
PCC-S-0055: Array <name> not allowed as bind variable at ...
For example, assuming the declarations
int mgr_number[50];
char job_title[50][20];
it would be ambiguous if the statement
EXEC SQL SELECT mgr INTO :mgr_number FROM emp
WHERE job = :job_title;
were treated like the imaginary statement
for (i = 0; i < 50; i++)
SELECT mgr INTO :mgr_number[i] FROM emp
WHERE job = :job_title[i];
because multiple rows might meet the WHERE-clause search condition, but only one output variable is available to receive data. Therefore, an error message is issued.
On the other hand, it would not be ambiguous if the statement
EXEC SQL UPDATE emp SET mgr = :mgr_number
WHERE empno IN (SELECT empno FROM emp
WHERE job = :job_title);
were treated like the imaginary statement
for (i = 0; i < 50; i++)
UPDATE emp SET mgr = :mgr_number[i]
WHERE empno IN (SELECT empno FROM emp
WHERE job = :job_title[i]);
because there is a mgr_number in the SET clause for each row matching job_title in the WHERE clause, even if each job_title matches multiple rows. All rows matching each job_title can be SET to the same mgr_number. Therefore, no error message is issued.
Prior releases of Pro*C/C++ supported the use of simple aggregate datatypes for inserting data into and fetching data from the database.
Using arrays of scalars, users can perform multi-row operations involving a single column only. Using structs of scalars allows users to perform single row operations involving multiple columns.
In order to perform multi-row operations involving multiple columns, however, users previously needed to allocate several parallel arrays of scalars either separately or encapsulated within a single struct. In many cases, it is easier to reorganize this data structure more conveniently as a single array of structs instead.
Beginning with release 8.0, Pro*C/C++ supports the use of arrays of structs which enable an application programmer to perform multi-row, multi-column operations using an array of C structs. With this enhancement, Pro*C/C++ can handle simple arrays of structs of scalars as bind variables in embedded SQL statements for easier processing of user data. This makes programming more intuitive, and allows users greater flexibility in organizing their data.
In addition to supporting arrays of structs as bind variables, Pro*C/C++ also supports arrays of indicator structs when used in conjunction with an array of structs declaration.
Note: Binding structs to PL/SQL records and binding arrays of structs to PL/SQL tables of records are not part of this new functionality. Arrays of structs may also not be used within an embedded PL/SQL block. See the section "Restrictions on Arrays of Structs" on page 12-18 for further restrictions.
Since arrays of structs are intended to be used when performing multi-row operations involving multiple columns, it is generally anticipated that they will be used in the following ways.
The notion of an array of structs is not new to C programmers. It does, however, present a conceptual difference for data storage when it is compared to a struct of parallel arrays.
In a struct of parallel arrays, the data for the individual columns is stored contiguously. In an array of structs, on the other hand, the column data is interleaved, whereby each occurrence of a column in the array is separated by the space required by the other columns in the struct. This space is known as a `stride'.
The following restrictions apply to the use of arrays of structs in Pro*C/C++:
The syntax for declaring an array of structs doesn't really change. There are, however a few things to keep in mind when using an array of structs.
When declaring an array of structs which will be used in a Pro*C/C++ application, the programmer must keep in mind the following important points:
struct person {
char name[15];
int VARCHARage;
} people[10];
the person variable is the structure tag. This is so the precompiler can use the name of the struct to compute the size of the stride.
Given these restrictions regarding the use of arrays of structs, the following declaration is legal in Pro*C/C++
struct department {
int deptno;
char dname[15];
char loc[14];
} dept[4];
while the following declaration is illegal.
struct { /* the struct is missing a structure tag */
int empno[15]; /* struct members may not be arrays */
char ename[15][10]; /* character types may not be 2-dimensional */
struct nested {
int salary; /* nested struct not permitted in array of structs */
} sal_struct;
} bad[15];
It is also important to note that you may not apply datatype equivalencing to either the array of structs itself or to any of the individual fields within the struct. For example, assuming empno is not declared as an array in the above illegal struct, the following is illegal:
exec sql var bad[3].empno is integer(4);
The precompiler has no way to keep track of individual structure elements within the array of structs. One could do the following, on the other hand, to achieve the desired effect.
typedef int myint;
exec sql type myint is integer(4);
struct equiv {
myint empno; /* now legally considered an integer(4) datatype */
...
} ok[15];
This should come as no surprise since equivalencing individual array items has not been supported by previous releases of Pro*C/C++. For example, the following scalar array declarations illustrate what is legal and what is not.
int empno[15];
exec sql var empno[3] is integer(4); /* illegal */
myint empno[15]; /* legal */
In summary, you may not equivalence any individual array item.
Indicator variables for an array of structs declaration work in much the same way as a normal struct declaration. An indicator array of structs declaration must abide by the rules for an array of structs described in the section "Declaring an Array of Structs" on page 12-18, plus the following rules.
Note that these rules generally reflect the rules for using structs as implemented in prior releases of Pro*C/C++. The array restriction is also the same as that previously used for arrays of scalars.
Given these rules, assume the following struct declaration:
struct department {
int deptno;
char dname[15];
char loc[14];
} dept[4];
The following is a legal indicator variable struct declaration:
struct department_ind {
short deptno_ind;
short dname_ind;
short loc_ind;
} dept_ind[4];
while the following is illegal as an indicator variable
struct{ /* missing indicator structure tag */
int deptno_ind; /* indicator variable not of type short */
short dname_ind[15];/* array element forbidden in indicator struct */
short loc_ind[14]; /* array element forbidden in indicator struct */
} bad_ind[2]; /* indicator array size is smaller than host array */
In some cases, it may be desirable to declare a pointer to an array of structs. This allows pointers to arrays of structs to be passed to other functions or used directly in an embedded SQL statement.
Note: The length of the array referenced by a pointer to an array of structs cannot be known during precompilation. For this reason, an explicit FOR clause must be used when a bind variable whose type is a pointer to an array of structs is used in any embedded SQL statement.
Remember that FOR clauses may not be used in an embedded SQL SELECT statement. Therefore, to retrieve data into a pointer to an array of structs, an explicit cursor and FETCH statement must be used with the FOR clause.
The following examples demonstrate different uses of the array of structs functionality in Pro*C/C++.
Given the following structure declaration,
struct department {
int deptno;
char dname[15];
char loc[14];
} my_dept[4];
a user could then select the dept data into my_dept as follows:
exec sql select * into :my_dept from dept;
or the user could populate my_dept first and then bulk insert it into the dept table:
exec sql insert into dept values (:my_dept);
To use an indicator variable, a parallel indicator array of structs could be declared.
struct deptartment_ind {
short deptno_ind;
short dname_ind;
short loc_ind;
} my_dept_ind[4];
Data is then be selected using the same query except for the addition of the indicator variable:
exec sql select * into :my_dept indicator :my_dept_ind from dept;
Similarly, the indicator could be used when inserting the data as well:
exec sql insert into dept values (:my_dept indicator :my_dept_ind);
As in prior releases of Pro*C/C++, when using multiple arrays for bulk handling of user data, the size of the arrays must be the same. If they are not, the smallest array size is chosen leaving the remaining portions of the arrays unaffected.
Given the following declarations,
struct employee {
int empno;
char ename[11];
} emp[14];
float sal[14];
float comm[14];
it is possible to select multiple rows for all columns in one simple query:
exec sql select empno, ename, sal, comm into :emp, :sal, :comm from emp;
We also want to know whether the column values for the commissions are NULL or not. A single indicator array could be used given the following declaration:
short comm_ind[14];
...
exec sql select empno, ename, sal, comm
into :emp, :sal, :comm indicator :comm_ind from emp;
Note that you could not declare a single indicator array of structs which encapsulated all indicator information from the query. Therefore:
struct employee_ind { /* example of illegal usage */
short empno_ind;
short ename_ind;
short sal_ind;
short comm_ind;
} illegal_ind[15];
exec sql select empno, ename, sal, comm
into :emp, :sal, :comm indicator :illegal_ind from emp;
is illegal (as well as undesirable). The above statement associates the indicator array with the comm column only, not the entire SELECT...INTO list.
Assuming the array of structs and the sal, comm and comm_ind arrays were populated with the desired data, insertion is straightforward:
exec sql insert into emp (empno, ename, sal, comm)
values (:emp, :sal, :comm indicator :comm_ind);
For this example, we make the following declarations:
struct employee {
int empno;
char ename[11];
char job[10];
} emp[14];
struct compensation {
int sal;
int comm;
} wage[14];
struct compensation_ind {
short sal_ind;
short comm_ind;
} wage_ind[14];
Our program could then make use of these arrays of structs as follows:
exec sql declare c cursor for
select empno, ename, job, sal, comm from emp;
exec sql open c;
exec sql whenever not found do break;
while(1)
{
exec sql fetch c into :emp, :wage indicator :wage_ind;
... process batch rows returned by the fetch ...
}
printf("%d rows selected.\n", sqlca.sqlerrd[2]);
exec sql close c;
Alternatively, we could have used the FOR clause to instruct the fetch on how many rows to retrieve. Recall that the FOR clause is prohibited when using the SELECT statement, but not the INSERT or FETCH statements.
We add the following to our original declarations
int limit = 10;
and code our example accordingly.
exec sql for :limit
fetch c into :emp, :wage indicator :wage_ind;
Prior releases of Pro*C/C++ allowed array references to single structures in an array of structs. The following is therefore legal since the bind expression resolves to a simple struct of scalars.
exec sql select * into :dept[3] from emp;
Users can reference an individual scalar member of a specific struct in an array of structs as the following example shows.
exec sql select dname into :dept[3].dname from dept where ...;
Naturally, this requires that the query be a single row query so only one row is selected into the variable represented by this bind expression.
Prior releases of Pro*C/C++ required that an indicator struct have the same number of fields as its associated bind struct. This restriction has been relaxed when using structs in general. By following the above guidelines for indicator arrays of structs it is possible to construct the following example.
struct employee {
float comm;
float sal;
int empno;
char ename[10];
} emp[14];
struct employee_ind {
short comm;
} emp_ind[14];
exec sql select comm, sal, empno, ename
into :emp indicator :emp_ind from emp;
The mapping of indicator variables to bind values is one-to-one. They map in associative sequential order starting with the first field.
Be aware, however, that if any of the other fields has a fetched value of NULL and no indicator is provided, the
ORA-1405: fetched column value is NULL
error will be raised. As an example, such is the case if sal was nullable because there is no indicator for sal.
Suppose we change the array of structs as follows,
struct employee {
int empno;
char ename[10];
float sal;
float comm;
} emp[15];
but still used the same indicator array of structs from above.
Because the indicators map in associative sequential order, the comm indicator maps to the empno field leaving the comm bind variable without an indicator once again leading to the ORA-1405 error.
Generally, to avoid the ORA-1405 when using indicator structs that have fewer fields than their associative bind variable structs, the nullable attributes should appear first and in sequential order.
Note that we could easily change this into a single-row fetch involving multiple columns by using non-array structs and expect it to work as though the indicator struct was declared as follows.
struct employee_ind {
short comm;
short sal;
short empno;
short ename;
} emp_ind;
Because Pro*C/C++ no longer requires that the indicator struct have the same number of fields as its associated value struct, the above example is now legal in Pro*C/C++ whereas previously it was not.
Our indicator struct could now look like the following simple struct.
struct employee_ind {
short comm;
} emp_ind;
Using the non-array emp and emp_ind structs we are able to perform a single row fetch as follows.
exec sql fetch comm, sal, empno, ename
into :emp indicator :emp_ind from emp;
Note once again how the comm indicator maps to the comm bind variable in this case as well.
This example demonstrates how to use a pointer to an array of structs.
Given the following type declaration:
typedef struct dept {
int deptno;
char dname[15];
char loc[14];
} dept;
we can perform a variety of things, manipulating a pointer to an array of structs of that type. For example, we can pass pointers to arrays of structs to other functions.
void insert_data(d, n)
dept *d;
int n;
{
exec sql for :n insert into dept values (:d);
}
void fetch_data(d, n)
dept *d;
int n;
{
exec sql declare c cursor for select deptno, dname, loc from dept;
exec sql open c;
exec sql for :n fetch c into :d;
exec sql close c;
}
Such functions are invoked by passing the address of the array of structs as these examples indicate.
dept d[4];
dept *dptr = &d[0];
const int n = 4;
fetch_data(dptr, n);
insert_data(d, n); /* We are treating `&d[0]' as being equal to `d' */
Or we can simply use such pointers to arrays of structs directly in some embedded SQL statement.
exec sql for :n insert into dept values (:dptr);
The most important thing to remember is the use of the FOR clause.
You use the CURRENT OF cursor clause in a DELETE or UPDATE statement to refer to the latest row FETCHed from the cursor. (For more information, see "Using the CURRENT OF Clause" on page 5-17.) However, you cannot use CURRENT OF with host arrays. Instead, select the ROWID of each row, then use that value to identify the current row during the update or delete. An example follows:
char emp_name[20][10];
char job_title[20][10];
char old_title[20][10];
char row_id[20][18];
...
EXEC SQL DECLARE emp_cursor CURSOR FOR
SELECT ename, job, rowid FROM emp;
...
EXEC SQL OPEN emp_cursor;
EXEC SQL WHENEVER NOT FOUND do break;
for (;;)
{
EXEC SQL FETCH emp_cursor
INTO :emp_name, :job_title, :row_id;
...
EXEC SQL DELETE FROM emp
WHERE job = :old_title AND rowid = :row_id;
EXEC SQL COMMIT WORK;
}
However, the fetched rows are not locked because no FOR UPDATE OF clause is used. (You cannot use FOR UPDATE OF without CURRENT OF.) So, you might get inconsistent results if another user changes a row after you read it but before you delete it.
For INSERT, UPDATE, DELETE, and SELECT INTO statements, sqlca.sqlerrd[2] records the number of rows processed. For FETCH statements, it records the cumulative sum of rows processed.
When using host arrays with FETCH, to find the number of rows returned by the most recent iteration, subtract the current value of sqlca.sqlerrd[2] from its previous value (stored in another variable). In the following example, you determine the number of rows returned by the most recent fetch:
int emp_number[100];
char emp_name[100][20];
int rows_to_fetch, rows_before, rows_this_time;
EXEC SQL DECLARE emp_cursor CURSOR FOR
SELECT empno, ename
FROM emp
WHERE deptno = 30;
EXEC SQL OPEN emp_cursor;
EXEC SQL WHENEVER NOT FOUND CONTINUE;
/* initialize loop variables */
rows_to_fetch = 20; /* number of rows in each "batch" */
rows_before = 0; /* previous value of sqlerrd[2] */
rows_this_time = 20;
while (rows_this_time == rows_to_fetch)
{
EXEC SQL FOR :rows_to_fetch
FETCH emp_cursor
INTO :emp_number, :emp_name;
rows_this_time = sqlca.sqlerrd[2] - rows_before;
rows_before = sqlca.sqlerrd[2];
}
...
sqlca.sqlerrd[2] is also useful when an error occurs during an array operation. Processing stops at the row that caused the error, so sqlerrd[2] gives the number of rows processed successfully.
