如何創建一個查詢以獲得列中的細目作爲結果中的列？

Question

原始數據

________________ 
|col1 |col2|col3| 
________________ 
|A | X | 1 | 
________________ 
|A | Y | 5 | 
________________ 
|B | X | 2 | 
________________ 
|B | Y | 6 | 
________________ 
|C | Y | 7 | 
________________ 

期望的輸出

_______________ 
|col1| X | Y | 
_______________ 
|A | 1 | 5 | 
_______________ 
|B | 2 | 6 | 
_______________ 
|C |NULL| 7 | 
_______________ 

Answer 1

交叉表（）的查詢看起來像這樣：

SELECT col1 
    , col2 AS "X" 
    , col3 AS "Y" 
FROM crosstab(
     'SELECT col1 , col2, col3 
     FROM table 
     ORDER BY 1' 
     ,$$VALUES ('X'::text), ('Y')$$ 
) AS ct (
    col1 int 
, col2 numeric -- use actual data type! 
, col3 numeric); 

解釋和鏈接在這個有關的答案： PostgreSQL Crosstab Query

Answer 2

一種方法是有條件的聚合：對於你的例子

select col1, 
     max(case when col2 = 'X' then col3 end) as x, 
     max(case when col2 = 'Y' then col3 end) as y 
from t 
group by col1;