首页 热点资讯 义务教育 高等教育 出国留学 考研考公
您的当前位置:首页正文

MySQL中在含有GROUPBY的SELECT语句中显示COUNT()为0的结果_MySQL

2020-11-09 来源:化拓教育网

bitsCN.com

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)

bitsCN.com
显示全文