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)

更新于:30-9-2019

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