在单个MySQL查询中计算来自不同表的非NULL值

要计算来自不同表的值，语法如下−

Select
   (
   select count(yourColumnName) from yourTableName1) as anyAliasName1,
   (
   select count(yourColumnName) from yourTableName2) as anyAliasName2;

让我们先创建一个表−

mysql> create table DemoTable1
   -> (
   -> Id int
   -> );
Query OK, 0 rows affected (1.06 sec)

使用insert命令将数据插入到表中−

mysql> insert into DemoTable1 values(1);
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable1 values(NULL);
Query OK, 1 row affected (0.15 sec)
mysql> insert into DemoTable1 values(2);
Query OK, 1 row affected (0.34 sec)
mysql> insert into DemoTable1 values(3);
Query OK, 1 row affected (0.13 sec)

使用select语句显示表中的所有记录−

mysql> select * from DemoTable1;

这将产生以下输出−

+------+
| Id   |
+------+
|    1 |
| NULL |
|    2 |
|    3 |
+------+
4 rows in set (0.00 sec)

以下是创建第二个表的查询−

mysql> create table DemoTable2
   -> (
   -> Id int
   -> );
Query OK, 0 rows affected (0.65 sec)

使用insert命令将数据插入到表中−

mysql> insert into DemoTable2 values(10);
Query OK, 1 row affected (0.22 sec)
mysql> insert into DemoTable2 values(NULL);
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable2 values(NULL);
Query OK, 1 row affected (0.08 sec)

使用select语句显示表中的所有记录−

mysql> select * from DemoTable2;

这将产生以下输出−

+------+
| Id   |
+------+
|   10 |
| NULL |
| NULL |
+------+
3 rows in set (0.00 sec)

以下是获取来自不同表的计数的查询−

mysql> select
   -> (
   -> select count(Id) from DemoTable1) as CountFirstTableId,
   -> (
   -> select count(Id) from DemoTable2) as CountSecondTableId
   -> ;

这将产生以下输出−

+-------------------+--------------------+
| CountFirstTableId | CountSecondTableId |
+-------------------+--------------------+
|                 3 |                  1 |
+-------------------+--------------------+
1 row in set (0.00 sec)

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