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MySQL中在含有GROUP BY的SELECT语句中显示COUNT()为0的结果
mysql> select * from t1;+-------+------+| level | name |+-------+------+| 1 | a || 1 | b || 1 | c || 2 | a || 2 | b || 4 | a || 5 | b |+-------+------+7 rows in set (0.00 sec)mysql> select * from t2;+------+| id |+------+| 1 || 2 || 3 || 4 || 5 |+------+5 rows in set (0.02 sec)mysql> select t2.id as level,count(case when name is not null then 1 end) as count -> from t2 left join t1 on t2.id = t1.level -> group by t2.id;+-------+-------+| level | count |+-------+-------+| 1 | 3 || 2 | 2 || 3 | 0 || 4 | 1 || 5 | 1 |+-------+-------+5 rows in set (0.00 sec)
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