How transpose a table using SQL?

问题

I have a table like this:

id | P  | C | A |  B
--------------------
1  |100 |3  |a1 | b1
2  |101 |3  |a2 | b2
3  |102 |3  |a3 | b3
4  |103 |3  |a4 | b4
5  |100 |4  |a5 | b5 
6  |101 |4  |a6 | b6 
7  |102 |4  |a7 | b7
8  |103 |4  |a8 | b8

I want to get a new transposed structure like this:

 P  |_3A | _3B |_4A | _4B   
-------------------------
100 | a1 |  b1 | a5 | b5
101 | a2 |  b2 | a6 | b6  
102 | a3 |  b3 | a7 | b7
103 | a4 |  b4 | a8 | b8

As you can see ,new field names have been extracted from C field in the original table. Is there any way to do this using SQL?

回答1:

Postgres has some advanced functionality in terms of arrays and crosstab. However, a database independent way of doing this is by using aggregation:

select t.p,
       max(case when c = 3 then a end) as a3,
       max(case when c = 3 then b end) as b3,
       max(case when c = 4 then a end) as a4,
       max(case when c = 4 then b end) as b4
from atable t
group by t.p;

This will work in both SQLite and Postgres (and just about any other database).

回答2:

Test Data

DECLARE @TABLE TABLE (id INT, P INT, C INT, A VARCHAR(2), B VARCHAR(2))
INSERT INTO @TABLE VALUES 
(1  ,100 ,3  ,'a1','b1'),
(2  ,101 ,3  ,'a2','b2'),
(3  ,102 ,3  ,'a3','b3'),
(4  ,103 ,3  ,'a4','b4'),
(5  ,100 ,4  ,'a5','b5'),
(6  ,101 ,4  ,'a6','b6'),
(7  ,102 ,4  ,'a7','b7'),
(8  ,103 ,4  ,'a8','b8')

Query

SELECT * FROM 
 (
    SELECT P , Vals , '_' + CAST(C AS VARCHAR(10)) + N  AS Cols
    FROM @TABLE 
        UNPIVOT (Vals FOR N IN (A, B))up
 )A
 PIVOT (MAX(Vals)
        FOR Cols 
        IN ([_3A],[_3B],[_4A],[_4B])
        )p

Result

╔═════╦═════╦═════╦═════╦═════╗
║  P  ║ _3A ║ _3B ║ _4A ║ _4B ║
╠═════╬═════╬═════╬═════╬═════╣
║ 100 ║ a1  ║ b1  ║ a5  ║ b5  ║
║ 101 ║ a2  ║ b2  ║ a6  ║ b6  ║
║ 102 ║ a3  ║ b3  ║ a7  ║ b7  ║
║ 103 ║ a4  ║ b4  ║ a8  ║ b8  ║
╚═════╩═════╩═════╩═════╩═════╝

来源：https://stackoverflow.com/questions/26827699/how-transpose-a-table-using-sql

标签

sql

postgresql

transpose