If join returns null then join on default row(如果 join 返回 null,则在默认行上加入)
问题描述
我正在尝试将 2 个表连接到一行,如果该行为空,则将该行连接到默认行.
I am trying to join 2 tables on a row, and if that row is null, then join that row to the default row.
表 1:事件
EventID EventName
----------- -------------
1 January
2 February
3 March
4 April
表 2:菜单
MenuID EventID MenuVersion
---------- ----------- ---------------
1 1
2 3 2
3 4 4
4 4
我的尝试
SELECT * FROM Events
LEFT JOIN Menus
ON Events.EventID = Menus.EventID
我得到的输出
EventID EventName MenuID EventID MenuVersion
----------- ------------- --------- ---------- ---------------
1 January
2 February
3 March 2 3 2
4 April 3 4 4
在这种情况下,Menus
表的默认行是具有最高 MenuID
和空 EventID
的行.
The default row of the Menus
table in this case is the row with the highest MenuID
and a null EventID
.
我想要的输出
EventID EventName MenuID EventID MenuVersion
----------- ------------- --------- ---------- ---------------
1 January 4 4
2 February 4 4
3 March 2 3 2
4 April 3 4 4
推荐答案
交叉应用
默认行,并在没有行被left join
打开时使用其值.
Cross apply
the default row and use its values when no row is left join
ed on.
DECLARE @Events TABLE (EventId INT, EventName VARCHAR(12));
DECLARE @Menus TABLE (MenuId INT, EventId INT, MenuVersion INT);
INSERT INTO @Events (EventId, EventName)
VALUES
(1, 'January'),
(2, 'February'),
(3, 'March'),
(4, 'April');
INSERT INTO @Menus (MenuId, EventId, MenuVersion)
VALUES
(1, null, 1),
(2, 3, 2),
(3, 4, 4),
(4, null, 4);
SELECT E.EventId, E.EventName, COALESCE(M.MenuId, D.MenuId) MenuId, M.EventId, COALESCE(M.MenuVersion, D.MenuVersion) MenuVersion
FROM @Events E
LEFT JOIN @Menus M ON M.EventID = E.EventID
CROSS APPLY (SELECT TOP 1 * FROM @Menus WHERE EventId IS NULL ORDER BY MenuId DESC) D;
按要求返回:
EventId EventName MenuId EventId MenuVersion
1 January 4 NULL 4
2 February 4 NULL 4
3 March 2 3 2
4 April 3 4 4
注意:如果您将来使用 DDL/DML 语句提出这样的问题,您会得到更快的答复,因为它使人们不必全部输入.
Note: If you set out your questions like this in future with the DDL/DML statements you'll get a much faster response because it saves people from having to type it all in.
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