三左連接在行返回意想不到的結果

Question

所以，我有這樣的SQL查詢：

select 
    o.id, 
    o.name, 
    d.name, 
    w.day, 
    o.suspended 
from resource o 
    left join resource d on d.vehicle_ID = o.ID 
    left join week_days_availability_to_resource aw on d.ID=aw.resource_ID 
    left join week_days_availability w on aw.calendar_ID=w.ID 
where o.IS_DELETED <> true 
     and (d.IS_DELETED is null or d.IS_DELETED <> true) 
     and o.marketId = 98 
     and (w.day = 3 or w.day is null) 
     and (o.suspended <> true or o.id in (-1)) 

它返回我需要的所有行 - excep只是一個。表resource o中有一行與resource d連接，然後連接week_days_availability_to_resource（表中有一些行，resource_ID等於d.ID）。問題是week_days_availability中沒有行，day等於三，所以它不返回任何內容。但我期望mysql返回值day字段這一行返回 - 因爲我認爲這是left join實際上是什麼。好像我錯了。
任何想法？

Answer 1

我認爲你需要將where條件移入on條款。

from resource o 
    left join resource d on d.vehicle_ID = o.ID 
    left join week_days_availability_to_resource aw on d.ID=aw.resource_ID 
    left join week_days_availability w on aw.calendar_ID=w.ID and w.day = 3 
where o.IS_DELETED <> true 
     and (d.IS_DELETED is null or d.IS_DELETED <> true) 
     and o.marketId = 98 
     and (w.day = 3 or w.day is null) 
     and (o.suspended <> true or o.id in (-1)) 

的問題是，w.day可能等於其他一些值，比如4 left join條件（如最初寫的），但它不符合條件where。通過將條件移入on子句中，沒有匹配，您將獲得NULL。