Syntax:
ROW_COUNT()

Before MySQL 5.5.5, ROW_COUNT() returns the number of rows changed,
deleted, or inserted by the last statement if it was an UPDATE, DELETE,
or INSERT. For other statements, the value may not be meaningful.

As of MySQL 5.5.5, ROW_COUNT() returns a value as follows:

o DDL statements: 0. This applies to statements such as CREATE TABLE or
  DROP TABLE.

o DML statements other than SELECT: The number of affected rows. This
  applies to statements such as UPDATE, INSERT, or DELETE (as before),
  but now also to statements such as ALTER TABLE and LOAD DATA INFILE.

o SELECT: -1 if the statement returns a result set, or the number of
  rows "affected" if it does not. For example, for SELECT * FROM t1,
  ROW_COUNT() returns -1. For SELECT * FROM t1 INTO OUTFILE
  'file_name', ROW_COUNT() returns the number of rows written to the
  file.

o SIGNAL statements: 0.

For UPDATE statements, the affected-rows value by default is the number
of rows actually changed. If you specify the CLIENT_FOUND_ROWS flag to
mysql_real_connect() when connecting to mysqld, the affected-rows value
is the number of rows "found"; that is, matched by the WHERE clause.

For REPLACE statements, the affected-rows value is 2 if the new row
replaced an old row, because in this case, one row was inserted after
the duplicate was deleted.

For INSERT ... ON DUPLICATE KEY UPDATE statements, the affected-rows
value is 1 if the row is inserted as a new row and 2 if an existing row
is updated.

The ROW_COUNT() value is the same as the value from the
mysql_affected_rows() C API function and the row count that the mysql
client displays following statement execution.

URL: http://dev.mysql.com/doc/refman/5.5/en/information-functions.html

