max、min 是获取**列**的最大值、最小值。
而 greatest、least 函数可以用来分别获取**行**的最大值、最小值。
示例：greatest 函数
mysql> SELECT greatest(1,2)
+---------------+
| greatest(1,2) |
+---------------+
| 2 |
+---------------+
mysql> SELECT greatest(1,2,3)
+-----------------+
| greatest(1,2,3) |
+-----------------+
| 3 |
+-----------------+
mysql> SELECT greatest(1,2,3,null)
+----------------------+
| greatest(1,2,3,null) |
+----------------------+
| <null> |
+----------------------+
mysql> SELECT greatest(1,2,3,'abc')
+-----------------------+
| greatest(1,2,3,'abc') |
+-----------------------+
| 3 |
+-----------------------+
mysql> SELECT greatest(1,2,3,'abc',234)
+---------------------------+
| greatest(1,2,3,'abc',234) |
+---------------------------+
| 234 |
+---------------------------+
示例：least 函数
mysql> select least(1,2,3)
+--------------+
| least(1,2,3) |
+--------------+
| 1 |
+--------------+
mysql> select least(1,2,3, null)
+--------------------+
| least(1,2,3, null) |
+--------------------+
| <null> |
+--------------------+
示例：在表中使用 greatest、least 函数
建表：
create table `test_table` (
`num1` int,
`num2` int
) engine = InnoDB default charset = utf8mb4;
准备数据：
insert into test_table (num1, num2) values(1, 3);
insert into test_table (num1, num2) values(7, 5);
查询示例：
mysql> select num1, num2, greatest(num1, num2), least(num1, num2) from test_table;
+------+------+----------------------+-------------------+
| num1 | num2 | greatest(num1, num2) | least(num1, num2) |
+------+------+----------------------+-------------------+
| 1 | 3 | 3 | 1 |
| 7 | 5 | 7 | 5 |
+------+------+----------------------+-------------------+