从三列中选择不同的值，并使用MySQL在同一列中显示
为此，在单个MySQL查询中多次使用UNION。让我们首先创建一个表-
create table DemoTable -> ( -> Value1 int, -> Value2 int, -> Value3 int -> );
使用插入命令在表中插入一些记录-
insert into DemoTable values(20,null,null); insert into DemoTable values(20,null,null); insert into DemoTable values(20,null,null); insert into DemoTable values(10,null,null); insert into DemoTable values(80,20,100); insert into DemoTable values(10,null,null);
使用select语句显示表中的所有记录-
select *from DemoTable;
这将产生以下输出-
+--------+--------+--------+ | Value1 | Value2 | Value3 | +--------+--------+--------+ | 20 | NULL | NULL | | 20 | NULL | NULL | | 20 | NULL | NULL | | 10 | NULL | NULL | | 80 | 20 | 100 | | 10 | NULL | NULL | +--------+--------+--------+ 6 rows in set (0.00 sec)
以下是从3列中选择不同值到1的查询-
select *from -> ( -> select Value1 as AllValue from DemoTable -> union -> select Value2 as AllValue from DemoTable -> union -> select Value3 as AllValue from DemoTable -> ) tbl where AllValue IS NOT NULL -> order by AllValue;
这将产生以下输出-
+----------+ | AllValue | +----------+ | 10 | | 20 | | 80 | | 100 | +----------+ 4 rows in set (0.00 sec)