MySQL的加入：獲得與數器件爲每種類型的

Question

我有兩個表：

1）部分

part_id | part_name 

2）職位

post_id | part_id | type | title | content 

在職位表後有一個三種類型，所以在類型列我把價值（1或2或3）

我想得到表（與計數和加入）像這樣

part_id | part_name | count_posts_type1 | count_posts_type2 | count_posts_type3 

如何用mysql做到這一點？

例如：

Answer 1

如果您擁有有限的類型列表，請執行此操作。

然後左連接得到沒有帖子的部分結果（並且合併結果爲0而不是null），在這裏我們繼續。

select 
pa.part_id, pa.part_name, 
coalesce(sum(case when po.type = 1 then 1 else 0 end), 0) as count_posts_type1, 
coalesce(sum(case when po.type = 2 then 1 else 0 end), 0) as count_posts_type2, 
coalesce(sum(case when po.type = 3 then 1 else 0 end), 0) as count_posts_type3 
from parts pa 
left join posts po on po.part_id = pa.part_id 
group by pa.part_id, pa.part_name 

Answer 2

你試過類似（只是開玩笑，我知道你嘗試過什麼）是這樣的：是一個相當普遍的方式，當使用帶有案件總和

SELECT 
    parts.part_id, 
    parts.part_name, 
    COUNT(t1.post_id) AS count_posts_type1, 
    COUNT(t2.post_id) AS count_posts_type2, 
    COUNT(t3.post_id) AS count_posts_type3 
FROM posts 
    RIGHT JOIN part ON posts.part_id=parts.part_id 
    INNER JOIN (SELECT post_id FROM posts WHERE type=1) as t1(post_id) ON posts.post_id = t1.post_id 
    INNER JOIN (SELECT post_id FROM posts WHERE type=1) as t2(post_id) ON posts.post_id = t2.post_id 
    INNER JOIN (SELECT post_id FROM posts WHERE type=1) as t3(post_id) ON posts.post_id = t3.post_id