Question

我有两张桌子：

CREATE TABLE Users(
    UserId UNIQUEIDENTIFIER NOT NULL PRIMARY KEY,
    UserName NVARCHAR(200) NOT NULL,
    -- some other columns
    CurrentSubscriptionId UNIQUEIDENTIFIER NULL,
)

CREATE TABLE Subscriptions(
    SubscriptionId UNIQUEIDENTIFIER NOT NULL PRIMARY KEY,
    UserId UNIQUEIDENTIFIER NOT NULL,
    -- some other columns
)

我有一个存储过程，用于检索＆＃34;用户＆＃34;与他们的＆＃34;当前订阅＆＃34;：

SELECT List-Of-Columns FROM Users LEFT OUTER JOIN Subscriptions ON
   Users.CurrentSubscriptionId=Subscriptions.SubscriptionId
WHERE SomeConditionOnUsers

关键是我只需要来自＆＃34;订阅的数据＆＃34;对于存储过程的某些调用，而不是其他调用。所以我要在存储过程中添加一个@includeSubscriptionData位参数（这是伪代码，我必须单独列出每个有条件的列）：

SELECT List-Of-Columns-FromUsers,
    CASE WHEN @includeSubscriptionData THEN Columns-From-Subscriptions ELSE NULL END
FROM Users LEFT OUTER JOIN Subscriptions ON
   Users.CurrentSubscriptionId=Subscriptions.SubscriptionId
WHERE SomeConditionOnUsers

希望避免加入。所以我先尝试了这个：

DECLARE @includeSubscriptionData BIT;
SET @includeSubscriptionData=0;
SELECT List-Of-Columns-FromUsers,
    CASE WHEN @includeSubscriptionData THEN SubscriptionId ELSE NULL END
FROM Users LEFT OUTER JOIN Subscriptions ON
   Users.CurrentSubscriptionId=Subscriptions.SubscriptionId
WHERE SomeConditionOnUsers

该计划表明仍然可以访问＆＃34;订阅＆＃34;。

当该位设置为零时，如何摆脱对第二个表的访问？

Answer 1

您是否尝试过将其加入加入条件？

DECLARE @includeSubscriptionData BIT;
SET @includeSubscriptionData=0;
SELECT List-Of-Columns-FromUsers,
    SubscriptionId
FROM Users LEFT OUTER JOIN Subscriptions ON
   Users.CurrentSubscriptionId=Subscriptions.SubscriptionId and @includeSubscriptionData = 1
WHERE SomeConditionOnUsers
OPTION(RECOMPILE)

Answer 2

方法1 - 控制流程

IF @includeSubscriptionData = 1
  BEGIN
    SELECT list
         , of
         , columns
    FROM   Users
     INNER
      JOIN Subscriptions
        ON Users.CurrentSubscriptionId = Subscriptions.SubscriptionId
    WHERE  SomeConditionOnUsers
    ;
  END
ELSE
  BEGIN
    SELECT list
         , of
         , columns
    FROM   Users
    ;
  END
;

方法2 - 修改了连接

SELECT list
     , of
     , columns
FROM   Users
 LEFT
  JOIN Subscriptions
    ON Users.CurrentSubscriptionId = Subscriptions.SubscriptionId
   AND @includeSubscriptionData = 1
WHERE  SomeConditionOnUsers

Answer 3

您可以使用IF / ELSE。

DECLARE @includeSubscriptionData BIT;
SET @includeSubscriptionData=0;

IF @includeSubscriptionData == 0
    BEGIN
        SELECT List-Of-Columns-FromUsers    
        FROM Users WHERE SomeConditionOnUsers
    END
ELSE
    BEGIN
        SELECT List-Of-Columns-FromUsers    
        FROM Users LEFT OUTER JOIN Subscriptions ON Users.CurrentSubscriptionId=Subscriptions.SubscriptionId
        WHERE SomeConditionOnUsers
    END

如何根据某些条件完成SQL JOIN？

3 个答案:

方法1 - 控制流程

方法2 - 修改了连接