# > 问题： A表存储文章，B表存储文章类别，通过B.id进行连接，查询每文章类别种类的文章数量 > 易错点: > ```sql > SELECT COUNT(1) FROM A JOIN B ON (B.id = A.B_id) > ``` >A JOIN B 时，若B中某一类，在A中没有，则无法查询到改类 正确: ```SQL SELECT COUNT(A.id NOT NULL) FROM B LEFT JOIN A ON (B.id = A.B_id) ```