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34.3.1. Executing SQL Statements

34.3.2. Using Cursors

34.3.3. Managing Transactions

34.3.4. Prepared Statements

Creating a table:

EXEC SQL CREATE TABLE foo (number integer, ascii char(16));EXEC SQL CREATE UNIQUE INDEX num1 ON foo(number);EXEC SQL COMMIT;

Inserting rows:

EXEC SQL INSERT INTO foo (number, ascii) VALUES (9999, 'doodad');EXEC SQL COMMIT;

Deleting rows:

EXEC SQL DELETE FROM foo WHERE number = 9999;EXEC SQL COMMIT;

Updates:

EXEC SQL UPDATE foo    SET ascii = 'foobar'    WHERE number = 9999;EXEC SQL COMMIT;

SELECT statements that return a single result row can also be executed usingEXEC SQL directly. To handle result sets with multiple rows, an application has to use a cursor; seeSection 34.3.2 below. (As a special case, an application can fetch multiple rows at once into an array host variable; seeSection 34.4.4.3.1.)

Single-row select:

EXEC SQL SELECT foo INTO :FooBar FROM table1 WHERE ascii = 'doodad';

Also, a configuration parameter can be retrieved with theSHOW command:

EXEC SQL SHOW search_path INTO :var;

The tokens of the form:something arehost variables, that is, they refer to variables in the C program. They are explained inSection 34.4.

To retrieve a result set holding multiple rows, an application has to declare a cursor and fetch each row from the cursor. The steps to use a cursor are the following: declare a cursor, open it, fetch a row from the cursor, repeat, and finally close it.

Select using cursors:

EXEC SQL DECLARE foo_bar CURSOR FOR    SELECT number, ascii FROM foo    ORDER BY ascii;EXEC SQL OPEN foo_bar;EXEC SQL FETCH foo_bar INTO :FooBar, DooDad;...EXEC SQL CLOSE foo_bar;EXEC SQL COMMIT;

For more details about declaring a cursor, seeDECLARE; for more details about fetching rows from a cursor, seeFETCH.

In the default mode, statements are committed only whenEXEC SQL COMMIT is issued. The embedded SQL interface also supports autocommit of transactions (similar topsql's default behavior) via the-t command-line option toecpg (seeecpg) or via theEXEC SQL SET AUTOCOMMIT TO ON statement. In autocommit mode, each command is automatically committed unless it is inside an explicit transaction block. This mode can be explicitly turned off usingEXEC SQL SET AUTOCOMMIT TO OFF.

The following transaction management commands are available:

When the values to be passed to an SQL statement are not known at compile time, or the same statement is going to be used many times, then prepared statements can be useful.

The statement is prepared using the commandPREPARE. For the values that are not known yet, use the placeholder“?”:

EXEC SQL PREPARE stmt1 FROM "SELECT oid, datname FROM pg_database WHERE oid = ?";

If a statement returns a single row, the application can callEXECUTE afterPREPARE to execute the statement, supplying the actual values for the placeholders with aUSING clause:

EXEC SQL EXECUTE stmt1 INTO :dboid, :dbname USING 1;

If a statement returns multiple rows, the application can use a cursor declared based on the prepared statement. To bind input parameters, the cursor must be opened with aUSING clause:

EXEC SQL PREPARE stmt1 FROM "SELECT oid,datname FROM pg_database WHERE oid > ?";EXEC SQL DECLARE foo_bar CURSOR FOR stmt1;/* when end of result set reached, break out of while loop */EXEC SQL WHENEVER NOT FOUND DO BREAK;EXEC SQL OPEN foo_bar USING 100;...while (1){    EXEC SQL FETCH NEXT FROM foo_bar INTO :dboid, :dbname;    ...}EXEC SQL CLOSE foo_bar;

When you don't need the prepared statement anymore, you should deallocate it:

EXEC SQL DEALLOCATE PREPAREname;

For more details aboutPREPARE, seePREPARE. Also seeSection 34.5 for more details about using placeholders and input parameters.