在MySQL查询中如何在CASE条件下使用count？
在MySQL中为此使用CASEWHEN，并在COUNT()方法内设置CASE条件以进行计数。让我们首先创建一个表-
mysql> create table DemoTable1374 -> ( -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, -> Name varchar(20), -> Score int -> );
使用插入命令在表中插入一些记录-
mysql> insert into DemoTable1374(Name,Score) values('Chris',45);
mysql> insert into DemoTable1374(Name,Score) values('David',78);
mysql> insert into DemoTable1374(Name,Score) values('Bob',45);
mysql> insert into DemoTable1374(Name,Score) values('Mike',75);
mysql> insert into DemoTable1374(Name,Score) values('Carol',45);
mysql> insert into DemoTable1374(Name,Score) values('Adam',89);使用select语句显示表中的所有记录-
mysql> select * from DemoTable1374;
这将产生以下输出-
+----+-------+-------+ | Id | Name | Score | +----+-------+-------+ | 1 | Chris | 45 | | 2 | David | 78 | | 3 | Bob | 45 | | 4 | Mike | 75 | | 5 | Carol | 45 | | 6 | Adam | 89 | +----+-------+-------+ 6 rows in set (0.00 sec)
以下是CASEWHEN的查询以设置条件和计数-
mysql> select count( case when Score=45 then 1 else NULL end) as SpecificCondition from DemoTable1374;
这将产生以下输出-
+-------------------+ | SpecificCondition | +-------------------+ | 3 | +-------------------+ 1 row in set (0.00 sec)