Running SQL Statements

You can use the function PQexec to execute SQL query statements through the connection object. This includes querying data (SELECT), inserting data, updating data, and deleting data. If multiple SQL statements are executed concurrently as a transaction, use the transaction control function. For example, run SQL statements such as BEGIN, COMMIT, and ROLLBACK to control the start, committing, and rollback of a transaction. Handle the errors after executing the SQL query statements.

The following is an example (for details about the complete example, see Establishing a Database Connection, Executing SQL Statements, and Returning Results):

/*
 * After the connection is successful
 * Since a cursor is used in the test case, a transaction block is required.
 * Put all data in one "select * from pg_database".
 * PQexec() is too simple and is not recommended.
 */

/* Start a transaction block. */
res = PQexec(conn, "BEGIN");
if (PQresultStatus(res) != PGRES_COMMAND_OK)
{
    fprintf(stderr, "BEGIN command failed: %s", PQerrorMessage(conn));
    PQclear(res);
    exit_nicely(conn);
}

/*
 * PQclear PGresult should be executed when it is no longer needed, to avoid memory leakage.
 */
PQclear(res);

/*
 * Fetch data from the pg_database system catalog.
 */
res = PQexec(conn, "DECLARE myportal CURSOR FOR select * from pg_database");
if (PQresultStatus(res) != PGRES_COMMAND_OK)
{
    fprintf(stderr, "DECLARE CURSOR failed: %s", PQerrorMessage(conn));
    PQclear(res);
    exit_nicely(conn);
}
PQclear(res);

res = PQexec(conn, "FETCH ALL in myportal");
if (PQresultStatus(res) != PGRES_TUPLES_OK)
{
    fprintf(stderr, "FETCH ALL failed: %s", PQerrorMessage(conn));
    PQclear(res);
    exit_nicely(conn);
}