MySQL Cookbook

Obtaining the Number of Rows Affected by a Query

Problem

You want to know how many rows were changed by a query.

Solution

Some APIs provide the count as the return value of the function that issues the query. Others have a separate function that you call after issuing the query.

Discussion

For queries that affect rows (UPDATE, DELETE, INSERT, REPLACE), each API provides a way to determine the number of rows involved. For MySQL, “affected by” normally means “changed by,” so rows that are not changed by a query are not counted, even if they match the conditions specified in the query. For example, the following UPDATE statement would result in an “affected by” value of zero because it does not change any columns from their current values, no matter how many rows the WHERE clause matches:

UPDATE limbs SET arms = 0 WHERE arms = 0;

Perl

In DBI scripts, the affected-rows count is returned by do( ) or by execute( ), depending on how you execute the query:

# execute $query using do( )
my $count = $dbh->do ($query);
# report 0 rows if an error occurred
printf "%d rows were affected\n", (defined ($count) ? $count : 0);

# execute query using prepare( ) plus execute( )
my $sth = $dbh->prepare ($query);
my $count = $sth->execute ( );
printf "%d rows were affected\n", (defined ($count) ? $count : 0);

When you use DBI, you have the option of asking MySQL to return the “matched by” value rather than the “affected by” value. To do this, specify mysql_client_found_rows=1 in the options part of the data ...