MySQL查詢與

Question

我有兩個表l & C它們由兩個字段l.mID和c.mID的C表包含三種類型的記錄cs, ls, & vs與我的查詢鏈接列計數我需要從L和作爲查詢有一部分選擇所有記錄三欄顯示cs, ls & vs的總數;

到目前爲止，我使用的是外部聯接：

SELECT l.*, COUNT(*) FROM l LEFT OUTER JOIN c ON c.mID = l.mID WHERE l.mID = 2 

但是，這只是一個附加柱採用了全總的任何幫助將是巨大的感謝。

Answer 1

您可以在計數使用CASE聲明：

select 
    l.* 
    ,COUNT(CASE WHEN c.type = 'cs' then 1 END) as cscount 
    ,COUNT(CASE WHEN c.type = 'ls' then 1 END) as lscount 
    ,COUNT(CASE WHEN c.type = 'vs' then 1 END) as vscount 
from l 
left outer join c on c.mID = l.mID 
where l.mID = 2 

Answer 2

select 
    l.* 
    ,sum(c.type = 'cs') as cscount 
    ,sum(c.type = 'ls') as lscount 
    ,sum(c.type = 'vs') as vscount 
from l 
left outer join c on c.mID = l.mID 
where l.mID = 2