MySQL 如何在单个查询中计算两个表中的行数

首先我们创建一张表 –

mysql> create table DemoTable1
(
   Name varchar(40)
);
Query OK, 0 rows affected (0.81 sec)

然后使用insert命令在表中插入一些记录 –

mysql> insert into DemoTable1 values('Chris');
Query OK, 1 row affected (0.48 sec)
mysql> insert into DemoTable1 values('Robert');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable1 values('Mike');
Query OK, 1 row affected (0.10 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 –

+--------+
| Name   |
+--------+
| Chris  |
| Robert |
| Mike   |
+--------+
3 rows in set (0.00 sec)

以下是创建第二张表的查询 –

mysql> create table DemoTable2
(
   Score int
);
Query OK, 0 rows affected (1.16 sec)

然后使用insert命令在表中插入一些记录 –

mysql> insert into DemoTable2 values(86);
Query OK, 1 row affected (0.22 sec)
mysql> insert into DemoTable2 values(98);
Query OK, 1 row affected (0.08 sec)
mysql> insert into DemoTable2 values(75);
Query OK, 1 row affected (0.14 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 –

+-------+
| Score |
+-------+
| 86    |
| 98    |
| 75    |
+-------+
3 rows in set (0.00 sec)

以下是在一个查询中计算两个表的行数的查询 –

mysql> select
   (select count(*) from DemoTable1) AS FirstTableCount,
   (select count(*) from DemoTable2) AS SecondTableCount
   from dual;

这将产生以下输出 –

+-----------------+------------------+
| FirstTableCount | SecondTableCount |
+-----------------+------------------+
| 3               | 3                |
+-----------------+------------------+
1 row in set (0.00 sec)

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