unsigned int mysql_field_count(MYSQL *mysql)
Description
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 allows 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.
Return Values
An unsigned integer representing the number of columns in a result set.
Errors
None.
Example
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
.
User Comments
in the page mysql_field_count() page, the example didn't work.
mysql_store_result alaways return a value not zero, then it cannot be used to know if there are line returned in the query. The example of the page is below.
regards,
Flavia
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));
}
}
}
Do not follow the comment about using mysql_error as another approach - it doesn't work!
If you follow the example and use mysql_field_count, you will be fine. If you substitute mysql_error for it, you will always be told it is an error!
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