从具有空值和非空值的多列中获取最大值？

为此，您可以使用COALESCE()。要获得最大值，请GREATEST()在MySQL中使用。让我们首先创建一个表-

mysql> create table DemoTable
   -> (
   -> Value1 int,
   -> Value2 int,
   -> Value3 int
   -> );

使用插入命令在表中插入一些记录-

mysql> insert into DemoTable values(NULL,80,76);
mysql> insert into DemoTable values(NULL,NULL,100);
mysql> insert into DemoTable values(56,NULL,45);
mysql> insert into DemoTable values(56,120,90);

使用select语句显示表中的所有记录-

mysql> select *from DemoTable;

这将产生以下输出-

+--------+--------+--------+
| Value1 | Value2 | Value3 |
+--------+--------+--------+
|   NULL |     80 |     76 |
|   NULL |   NULL |    100 |
|     56 |   NULL |     45 |
|     56 |    120 |     90 |
+--------+--------+--------+
4 rows in set (0.00 sec)

以下是从多个列中获取最大值的查询-

mysql> select greatest(coalesce(Value1,0),coalesce(Value2,0),coalesce(Value3,0)) from DemoTable;

这将产生以下输出-

+--------------------------------------------------------------------+
| greatest(coalesce(Value1,0),coalesce(Value2,0),coalesce(Value3,0)) |
+--------------------------------------------------------------------+
|                                                                 80 |
|                                                                100 |
|                                                                 56 |
|                                                                120 |
+--------------------------------------------------------------------+
4 rows in set (0.00 sec)