Select column based on whether a specific row in another table exists - sql-server

Question

Question is similar to this one How to write a MySQL query that returns a temporary column containing flags for whether or not an item related to that row exists in another table
Except that I need to be more specific about which rows exists
I have two tables: 'competitions' and 'competition_entries'
Competitions:
ID | NAME | TYPE
--------------------------------
1 | Example | example type
2 | Another | example type
Competition Entries
ID | USERID | COMPETITIONID
---------------------------------
1 | 100 | 1
2 | 110 | 1
3 | 110 | 2
4 | 120 | 1
I want to select the competitions but add an additional column which specifies whether the user has entered the competition or not. This is my current SELECT statement
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
My desired result set from setting the #userid parameter to 110 is this
ID | NAME | TYPE | ENTERED
-------------------------------------
1 | Example | example type | 1
2 | Another | example type | 1
But instead I get this
ID | NAME | TYPE | ENTERED
-------------------------------------
1 | Example | example type | 0
1 | Example | example type | 1
1 | Example | example type | 0
2 | Another | example type | 1
Because it's counting the entries for all user ids

Answer 1

Fixing your query
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]
An alternative is to rewrite it using EXISTS which is pretty much the same but may be easier to understand.
BTW, using single quotes on the column name is deprecated. Use square brackets.
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