如何在 MySQL 查询中使用 CASE 条件进行计数?
mysqlmysqli database
在 MySQL 中使用 CASE WHEN 进行此操作，并在 COUNT() 方法中设置 CASE 条件进行计数。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 -> ); Query OK, 0 rows affected (0.61 sec)
使用 insert 命令在表中插入一些记录 −
mysql> insert into DemoTable1374(Name,Score) values('Chris',45);
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable1374(Name,Score) values('David',78);
Query OK, 1 row affected (0.10 sec)
mysql> insert into DemoTable1374(Name,Score) values('Bob',45);
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable1374(Name,Score) values('Mike',75);
Query OK, 1 row affected (0.10 sec)
mysql> insert into DemoTable1374(Name,Score) values('Carol',45);
Query OK, 1 row affected (0.09 sec)
mysql> insert into DemoTable1374(Name,Score) values('Adam',89);
Query OK, 1 row affected (0.18 sec)
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 CASE WHEN 设置条件和计数的查询 −
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)
