mysql - 以相反的方式合并

Question

你好，我有这个场景

我的 table

 create table foo(
    id int,
    num int1,
    stage enum('a','b','c'),
    unique(id,stage)
);

这里是一段数据

INSERT INTO `foo` (`id`, `num`, `stage`) VALUES
(1, 1, 'a'),
(1, 2, 'b'),
(1, 3, 'c'),
(2, 1, 'a'),
(2, 2, 'b'),
(2, 3, 'c'),
(3, 1, 'a'),
(3, 2, 'b'),
(4, 1, 'a');

Notes on table

an id that has a state of c must have a prior state of a,b triggers no problem

查询为

I did this useing distinct with scalar correlated subquery

SELECT DISTINCT
id, IFNULL((
SELECT num
FROM foo f
WHERE f.id = foo.id AND f.stage = 'a'),'') `a`, IFNULL((
SELECT num
FROM foo f
WHERE f.id = foo.id AND f.stage = 'b'),'') `b`, IFNULL((
SELECT num
FROM foo f
WHERE f.id = foo.id AND f.stage = 'c'),'') `c`
FROM foo 

[Bottom line] I want a better fasten to do this

谢谢

Answer 1

SELECT   id,
         MAX(IF(stage = 'a', num, NULL)) AS `a`
         MAX(IF(stage = 'b', num, NULL)) AS `b`
         MAX(IF(stage = 'c', num, NULL)) AS `c`
FROM     foo
GROUP BY id