如何将 table 的前两列堆叠成一列，同时仅将第三列与第一列配对？

Question

前面问了一个很，这次的要求有点复杂。我有一个如下所示的 table：

| id | code_1 | code_2 | pair_1 |
|----|--------|--------|--------|
|  1 |     a1 |     a2 |     b1 |
|  2 |     a3 |     a4 | (null) |
|  3 |     a5 | (null) |     b2 |
|  4 |     a6 | (null) | (null) |
|  5 | (null) |     a7 |     b3 |
|  6 | (null) |     a8 | (null) |
|  7 | (null) | (null) |     b4 |
|  8 | (null) | (null) | (null) |

我想将 code_1 和 code_2 堆叠到一个列中，这可以通过使用来完成：

select id, code, pair_1 as pair
  from source
       unpivot (code for code_ in (code_1, code_2)) as unpvt;

，产生以下输出：

| id | code |   pair |
|----|------|--------|
|  1 |   a1 |     b1 |
|  1 |   a2 |     b1 |
|  2 |   a3 | (null) |
|  2 |   a4 | (null) |
|  3 |   a5 |     b2 |
|  4 |   a6 | (null) |
|  5 |   a7 |     b3 |
|  6 |   a8 | (null) |

，但我也想捕获 code_1 与 pair_1 配对的条件，即每当 code 来自 code_1 时，它将使用中的值pair_1 对于 pair；每当 code 来自 code_2 时，它将始终使用 null 作为 pair.

例如，给定原始 table，目标 table 应该是这样的：

| id | code |   pair |
|----|------|--------|
|  1 |   a1 |     b1 |
|  2 |   a2 | (null) |
|  3 |   a3 | (null) |
|  4 |   a4 | (null) |
|  5 |   a5 |     b2 |
|  6 |   a6 | (null) |
|  7 |   a7 | (null) |
|  8 |   a8 | (null) |

DDL:

http://sqlfiddle.com/#!18/743a5/16

create table source (
  id     int identity(1, 1) primary key,
  code_1 varchar(10),
  code_2 varchar(10),
  pair_1 varchar(10)
);
insert into source values
('a1', 'a2', 'b1'),
('a3', 'a4', null),
('a5', null, 'b2'),
('a6', null, null),
(null, 'a7', 'b3'),
(null, 'a8', null),
(null, null, 'b4'),
(null, null, null);


create table target (
  id   int identity(1, 1) primary key,
  code varchar(10),
  pair varchar(10)
);
insert into target values
('a1', 'b1'),
('a2', null),
('a3', null),
('a4', null),
('a5', 'b2'),
('a6', null),
('a7', null),
('a8', null);

Answer 1

Can't you do simple UNION ALL?

试试下面的查询

SELECT * FROM
(
SELECT id, code_1 as code, pair_1 as pair FROM source WHERE code_1 IS NOT NULL
UNION ALL 
SELECT id, code_2 as code, NULL as pair FROM source WHERE code_2 IS NOT NULL
) UnionTable
ORDER BY id

如何将 table 的前两列堆叠成一列，同时仅将第三列与第一列配对？

How do I stack the first two columns of a table into a single column, but also pair third column with the first column only?

sql

tsql

sql-server

unpivot