在单个 MySQL 查询中实现多个 COUNT()
mysqlmysqli database更新于 2023/11/27 2:22:00
为此，请使用 CASE 语句。让我们首先创建一个表 −
mysql> create table DemoTable -> ( -> LastName varchar(100) -> ); Query OK, 0 rows affected (0.60 sec)
使用 insert 命令在表中插入一些记录 −
mysql> insert into DemoTable values('Smith');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable values('Brown');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable values('Brown');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable values('Smith');
Query OK, 1 row affected (0.15 sec)
mysql> insert into DemoTable values('Taylor');
Query OK, 1 row affected (0.15 sec)
mysql> insert into DemoTable values('Taylor');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable values('Miller');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable values('Miller');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable values('Miller');
Query OK, 1 row affected (0.12 sec)
使用 select 语句显示表中的所有记录 −
mysql> select *from DemoTable;
输出
这将产生以下输出 −
+----------+ | LastName | +----------+ | Smith | | Brown | | Brown | | Smith | | Taylor | | Taylor | | Miller | | Miller | | Miller | +----------+ 9 rows in set (0.00 sec)
以下是在一个查询中实现多个 COUNT() 方法的查询 −
mysql> select COUNT(CASE WHEN LastName='Smith' THEN 1 END) AS TotalFrequency, -> COUNT(CASE WHEN LastName='Brown' THEN 1 END) AS TotalFrequency, -> COUNT(CASE WHEN LastName='Taylor' THEN 1 END) AS TotalFrequency, -> COUNT(CASE WHEN LastName='Miller' THEN 1 END) AS TotalFrequency -> from DemoTable;
输出
这将产生以下输出 −
+----------------+----------------+----------------+----------------+ | TotalFrequency | TotalFrequency | TotalFrequency | TotalFrequency | +----------------+----------------+----------------+----------------+ | 2 | 2 | 2 | 3 | +----------------+----------------+----------------+----------------+ 1 row in set (0.00 sec)
