mysql> select bar, baz from foo;
+------+-------+
| bar  | baz   |
+------+-------+
| a    | qux   |
| b    | qux   |
| c    | qux   |
| d    | other |
+------+-------+
4 rows in set (0.00 sec)

mysql> update foo set bar = 'b' where baz = 'qux';
Query OK, 2 rows affected (0.02 sec)
Rows matched: 3  Changed: 2  Warnings: 0

mysql> select row_count();
+-------------+
| row_count() |
+-------------+
|           2 |
+-------------+
1 row in set (0.00 sec)