MySQL 8.0 C API Developer Guide
unsigned int mysql_field_count(MYSQL *mysql)
Returns the number of columns for the most recent query on the connection.
The normal use of this function is when
mysql_store_result()
returned
NULL
(and thus you have no result set
pointer). In this case, you can call
mysql_field_count()
to
determine whether
mysql_store_result()
should
have produced a nonempty result. This enables the client
program to take proper action without knowing whether the
query was a SELECT
(or
SELECT
-like) statement. The
example shown here illustrates how this may be done.
See Section 3.6.9, “NULL mysql_store_result() Return After mysql_query() Success”.
MYSQL_RES *result; unsigned int num_fields; unsigned int num_rows; if (mysql_query(&mysql,query_string)) { // error } else // query succeeded, process any data returned by it { result = mysql_store_result(&mysql); if (result) // there are rows { num_fields = mysql_num_fields(result); // retrieve rows, then call mysql_free_result(result) } else // mysql_store_result() returned nothing; should it have? { if(mysql_field_count(&mysql) == 0) { // query does not return data // (it was not a SELECT) num_rows = mysql_affected_rows(&mysql); } else // mysql_store_result() should have returned data { fprintf(stderr, "Error: %s\n", mysql_error(&mysql)); } } }
An alternative is to replace the
mysql_field_count(&mysql)
call with
mysql_errno(&mysql)
. In
this case, you are checking directly for an error from
mysql_store_result()
rather
than inferring from the value of
mysql_field_count()
whether
the statement was a SELECT
.