Sql server 根据另一个表中是否存在特定行来选择列
这个问题和这个类似 但我需要更具体地说明存在哪些行 我有两张表格:“竞赛”和“竞赛项目” 比赛:Sql server 根据另一个表中是否存在特定行来选择列,sql-server,tsql,Sql Server,Tsql,这个问题和这个类似 但我需要更具体地说明存在哪些行 我有两张表格:“竞赛”和“竞赛项目” 比赛: ID | NAME | TYPE -------------------------------- 1 | Example | example type 2 | Another | example type 参赛作品 ID | USERID | COMPETITIONID --------------------------------- 1
ID | NAME | TYPE
--------------------------------
1 | Example | example type
2 | Another | example type
参赛作品
ID | USERID | COMPETITIONID
---------------------------------
1 | 100 | 1
2 | 110 | 1
3 | 110 | 2
4 | 120 | 1
我想选择比赛,但添加一个附加列,指定用户是否已参加比赛。这是我当前的SELECT语句
SELECT
c.[ID],
c.[NAME],
c.[TYPE],
(CASE
WHEN e.ID IS NOT NULL AND e.USERID = @userid THEN 1
ELSE 0
END
) AS 'ENTERED'
FROM competitions AS c
LEFT OUTER JOIN competition_entries AS e
ON e.COMPETITIONID = c.ID
我将@userid
参数设置为110
的期望结果集如下
ID | NAME | TYPE | ENTERED
-------------------------------------
1 | Example | example type | 1
2 | Another | example type | 1
但是我得到了这个
ID | NAME | TYPE | ENTERED
-------------------------------------
1 | Example | example type | 0
1 | Example | example type | 1
1 | Example | example type | 0
2 | Another | example type | 1
因为它正在计算所有用户ID的条目修复您的查询
SELECT
c.[ID],
c.[NAME],
c.[TYPE],
MAX(CASE
WHEN e.ID IS NOT NULL AND e.USERID = @userid THEN 1
ELSE 0
END
) AS 'ENTERED'
FROM competitions AS c
LEFT OUTER JOIN competition_entries AS e ON e.COMPETITIONID = c.ID
GROUP BY
c.[ID],
c.[NAME],
c.[TYPE]
另一种方法是使用几乎相同但更容易理解的EXISTS重写它。顺便说一句,不推荐在列名上使用单引号。使用方括号
SELECT
c.[ID],
c.[NAME],
c.[TYPE],
CASE WHEN EXISTS (
SELECT *
FROM competition_entries AS e
WHERE e.COMPETITIONID = c.ID
AND e.USERID = @userid) THEN 1 ELSE 0 END [ENTERED]
FROM competitions AS c
是的,使用第二个就可以了,而且更容易理解,我知道这是沿着这些思路的东西。谢谢你的点名提示。