28.7.23 C API Multiple Statement Execution Support

By default, mysql_query() and mysql_real_query() interpret their statement string argument as a single statement to be executed, and you process the result according to whether the statement produces a result set (a set of rows, as for SELECT) or an affected-rows count (as for INSERT, UPDATE, and so forth).

MySQL also supports the execution of a string containing multiple statements separated by semicolon (;) characters. This capability is enabled by special options that are specified either when you connect to the server with mysql_real_connect() or after connecting by calling mysql_set_server_option().

Executing a multiple-statement string can produce multiple result sets or row-count indicators. Processing these results involves a different approach than for the single-statement case: After handling the result from the first statement, it is necessary to check whether more results exist and process them in turn if so. To support multiple-result processing, the C API includes the mysql_more_results() and mysql_next_result() functions. These functions are used at the end of a loop that iterates as long as more results are available. Failure to process the result this way may result in a dropped connection to the server.

Multiple-result processing also is required if you execute CALL statements for stored procedures. Results from a stored procedure have these characteristics:

The multiple statement and result capabilities can be used only with mysql_query() or mysql_real_query(). They cannot be used with the prepared statement interface. Prepared statement handlers are defined to work only with strings that contain a single statement. See Section 28.7.8, “C API Prepared Statements”.

To enable multiple-statement execution and result processing, the following options may be used:

The following procedure outlines a suggested strategy for handling multiple statements:

One possible implementation of the preceding strategy is shown following. The final part of the loop can be reduced to a simple test of whether mysql_next_result() returns nonzero. The code as written distinguishes between no more results and an error, which enables a message to be printed for the latter occurrence.

/* connect to server with the CLIENT_MULTI_STATEMENTS option */
if (mysql_real_connect (mysql, host_name, user_name, password,
    db_name, port_num, socket_name, CLIENT_MULTI_STATEMENTS) == NULL)
{
  printf("mysql_real_connect() failed\n");
  mysql_close(mysql);
  exit(1);
}

/* execute multiple statements */
status = mysql_query(mysql,
                     "DROP TABLE IF EXISTS test_table;\
                      CREATE TABLE test_table(id INT);\
                      INSERT INTO test_table VALUES(10);\
                      UPDATE test_table SET id=20 WHERE id=10;\
                      SELECT * FROM test_table;\
                      DROP TABLE test_table");
if (status)
{
  printf("Could not execute statement(s)");
  mysql_close(mysql);
  exit(0);
}

/* process each statement result */
do {
  /* did current statement return data? */
  result = mysql_store_result(mysql);
  if (result)
  {
    /* yes; process rows and free the result set */
    process_result_set(mysql, result);
    mysql_free_result(result);
  }
  else          /* no result set or error */
  {
    if (mysql_field_count(mysql) == 0)
    {
      printf("%lld rows affected\n",
            mysql_affected_rows(mysql));
    }
    else  /* some error occurred */
    {
      printf("Could not retrieve result set\n");
      break;
    }
  }
  /* more results? -1 = no, >0 = error, 0 = yes (keep looping) */
  if ((status = mysql_next_result(mysql)) > 0)
    printf("Could not execute statement\n");
} while (status == 0);

mysql_close(mysql);