sql - 将两列 c1,c2 组合为表中的 c3

Question

c1  c2

10  30
20  40

C1，C2 是两列，我希望使用 sql 查询得到如下所示的输出。

C3

10
20
30
40

Answer 1

使用UNION（隐式不同）或UNION ALL：

SELECT c1 AS c3 FROM tablename
UNION ALL
SELECT c2       FROM tablename

Answer 2

您可以使用UNION, 没有ALL关键字，查询的结果只是唯一值。如果要保留重复，请使用UNION ALL.

SELECT C1 as C3 FROM TABLENAME
UNION
SELECT C2 as C3 FROM TABLENAME

Answer 3

您将需要使用UNION ALL查询来获取结果：

select c1 as c3
from yourtable
union all
select c2 as c3
from yourtable

请参阅带有演示的 SQL Fiddle

这种类型的查询会将数据从多列取消透视到多行。

根据您使用的数据库产品，有些产品具有UNPIVOT为您进行数据转换的功能。例如，如果您使用 SQL Server 或 Oracle，则可以使用：

select c3
from yourtable
unpivot
(
  c3 for col in (c1, c2)
) u
order by c3

请参阅带有演示的 SQL Fiddle

Answer 4

这将保留按表排序

SELECT 1 AS ordering, c1 AS c3
FROM mytable
UNION
SELECT 2 AS ordering, c2 AS c3
FROM mytable
ORDER BY ordering, c3

如果你不能忽略排序列，你可以把它变成一个子查询

SELECT c3
FROM (
    SELECT 1 AS ordering, c1 AS c3
    FROM mytable
    UNION
    SELECT 2 AS ordering, c2 AS c3
    FROM mytable) a
ORDER BY ordering, c3