用一条单独的MySQL查询在两个表中找到最高和最低的值？

要从两个表中找到最高和最低值，请使用MAX()和MIN()。由于结果需要从两个表中显示，因此需要使用UNION。让我们首先创建一个表——

mysql> create table DemoTable1
(
  UniqueId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
  Score1 int
);
Query OK, 0 rows affected (0.76 sec)

使用插入命令将一些记录插入表中——

mysql> insert into DemoTable1(Score1) values(56);
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable1(Score1) values(76);
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable1(Score1) values(65);
Query OK, 1 row affected (0.09 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——

+----------+--------+
| UniqueId | Score1 |
+----------+--------+
|        1 |     56 |
|        2 |     76 |
|        3 |     65 |
+----------+--------+
3 rows in set (0.00 sec)

以下是创建第二个表的查询——

mysql> create table DemoTable2
(
  UniqueId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
  Score2 int
);
Query OK, 0 rows affected (0.93 sec)

使用插入命令将一些记录插入表中——

mysql> insert into DemoTable2(Score2) values(67);
Query OK, 1 row affected (0.68 sec)
mysql> insert into DemoTable2(Score2) values(94);
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable2(Score2) values(98);
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——

+----------+--------+
| UniqueId | Score2 |
+----------+--------+
|        1 |     67 |
|        2 |     94 |
|        3 |     98 |
+----------+--------+
3 rows in set (0.00 sec)

以下是查找2个表中最高和最低值的查询——

mysql> select max(commonValue) AS Highest_Value,min(commonValue) AS Lowest_Value from
(
  select Score1 as commonValue from DemoTable1
  union
  select Score2 as commonValue from DemoTable2
) tbl;

这将产生以下输出——

+---------------+--------------+
| Highest_Value | Lowest_Value |
+---------------+--------------+
|            98 |           56 |
+---------------+--------------+
1 row in set (0.04 sec)