同一张表的多个左连接是否更好更快？

Question

SELECT
    E.Id, 
    E.Name, 
    CASE EL.Application_Level WHEN 33 then 1 ELSE 0 END As MyDisplayName,
    CASE EL1.Application_Level WHEN 34 then 1 ELSE 0 END As FoodTag
FROM
    Entity E 
    INNER JOIN dbo.Countries CN ON cn.id=E.COUNTRY_ID  
    LEFT JOIN dbo.Entity_Levels EL ON E.ID = EL.Entity_ID AND EL.Application_Level = 33
    LEFT JOIN dbo.Entity_Levels EL1 ON E.ID = EL1.Entity_ID AND EL1.Application_Level = 34

我想使用 OR 条件而不是使用同一个表的两个连接，因为将添加更多应用程序级别，我不想添加更多连接。

无论如何我可以使用或条件和一个连接并将多条记录合并到一条记录。

Answer 1

由于您没有显示任何示例数据，这只是一个猜测，但看起来您可以通过仅对 ENTITY_LEVELS 表的 1 个连接来执行此类操作：

SELECT
E.Id, 
max(E.Name) as Name,
max(case when EL.application_level = 33 then 1 else 0 end) as MyDisplayName,
max(case when EL.application_level = 34 then 1 else 0 end) as FoodTag,
max(case when EL.application_level = 35 then 1 else 0 end) as RandomOtherThing
from entity E
INNER JOIN dbo.Countries CN ON cn.id=E.COUNTRY_ID  
LEFT JOIN dbo.Entity_Levels EL ON E.ID = EL.Entity_ID 
group by E.Id

Answer 2

SELECT E.Id
    ,E.Name
    ,CASE WHEN EL.Application_Level = 33
             THEN 1
         ELSE 0
     END AS MyDisplayName
    ,CASE WHEN EL.Application_Level = 34
            THEN 1
         ELSE 0
     END AS FoodTag
FROM Entity E WITH(NOLOCK) 
INNER JOIN dbo.Countries CN WITH(NOLOCK) ON CN.id=E.COUNTRY_ID
LEFT JOIN dbo.Entity_Levels EL WITH(NOLOCK) ON E.ID=EL.Entity_ID