MySQL查询在两个单独的条件下选择一个计数？
为此使用CASE语句。让我们首先创建一个表-
mysql> create table DemoTable -> ( -> StudentMarks int, -> isValid tinyint(1) -> );
使用插入命令在表中插入一些记录-
mysql> insert into DemoTable values(45,0); mysql> insert into DemoTable values(78,1); mysql> insert into DemoTable values(45,1); mysql> insert into DemoTable values(78,1); mysql> insert into DemoTable values(45,0); mysql> insert into DemoTable values(82,1); mysql> insert into DemoTable values(62,1);
使用select语句显示表中的所有记录-
mysql> select *from DemoTable;输出结果
+--------------+---------+ | StudentMarks | isValid | +--------------+---------+ | 45 | 0 | | 78 | 1 | | 45 | 1 | | 78 | 1 | | 45 | 0 | | 82 | 1 | | 62 | 1 | +--------------+---------+ 7 rows in set (0.00 sec)
这是在两个不同条件下选择计数的查询-
mysql> select StudentMarks, -> sum(case -> when StudentMarks=45 -> then case when isValid = 1 then 1 else 0 end -> else 1 end -> ) AS Freq -> from DemoTable -> group by StudentMarks;输出结果
+--------------+------+ | StudentMarks | Freq | +--------------+------+ | 45 | 1 | | 78 | 2 | | 82 | 1 | | 62 | 1 | +--------------+------+ 4 rows in set (0.00 sec)