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MySQL查询在两个单独的条件下选择一个计数？

2023-06-16 16:37:03 396

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)

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MySQL查询在两个单独的条件下选择一个计数？

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