MySQL 中使用多列进行多次 COUNT?
你可以使用聚合函数 SUM() 和 IF()。我们首先创建一个表−
mysql> create table DemoTable ( FirstName varchar(100), LastName varchar(100) ); Query OK, 0 rows affected (2.80 sec)
使用 insert 命令向表中插入一些记录 −
mysql> insert into DemoTable values('Adam','Smith'); Query OK, 1 row affected (0.25 sec) mysql> insert into DemoTable values('John','Smith'); Query OK, 1 row affected (0.36 sec) mysql> insert into DemoTable values('John','Doe'); Query OK, 1 row affected (1.38 sec) mysql> insert into DemoTable values('Bob','Doe'); Query OK, 1 row affected (0.16 sec) mysql> insert into DemoTable values('Sam','Smith'); Query OK, 1 row affected (0.25 sec)
使用 select 语句从表中显示所有记录 −
mysql> select *from DemoTable;
这将产生以下输出 −
+-----------+----------+ | FirstName | LastName | +-----------+----------+ | Adam | Smith | | John | Smith | | John | Doe | | Bob | Doe | | Sam | Smith | +-----------+----------+ 5 rows in set (0.00 sec)
以下是针对带有多列的 MySQL 多个 COUNT 查询。
mysql> select sum(if(FirstName='John',1,0)) as John_Count, sum(if(LastName='Smith',1,0)) as Smith_Count from DemoTable;
这将产生以下输出 −
+------------+-------------+ | John_Count | Smith_Count | +------------+-------------+ | 2 | 3 | +------------+-------------+ 1 row in set (0.00 sec)
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