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mysql> select count(*) from wp_comments where comment_post_id = (select ID from wp_posts where post_name = '') and comment_approved = 1;

ERROR 1242 (21000): Subquery returns more than 1 row

mysql> select ID from wp_posts where post_name = ''

    -> ;

+-------+

| ID    |

+-------+

|  1087 |

| 20541 |

| 23011 |

+-------+

3 rows in set (0.00 sec)