基於另一個表中的特定行是否存在

Question

問題選擇列是與此類似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

除了我需要更具體的瞭解這行存在

我有兩個表：「比賽」和 'competition_entries'

比賽：

ID | NAME  | TYPE 
-------------------------------- 
1  | Example | example type 
2  | Another | example type 

參賽作品

ID | USERID | COMPETITIONID 
--------------------------------- 
1  | 100  | 1 
2  | 110  | 1 
3  | 110  | 2 
4  | 120  | 1 

我想選擇比賽，但添加一個額外的列，指定用戶是否參加比賽。這是我目前的SELECT語句

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 

我期望的結果從@userid參數設置爲110設置是這樣的

ID | NAME | TYPE   | ENTERED 
------------------------------------- 
1  | Example | example type | 1 
2  | Another | example type | 1 

而是我得到這個

ID | NAME | TYPE   | ENTERED 
------------------------------------- 
1  | Example | example type | 0 
1  | Example | example type | 1 
1  | Example | example type | 0 
2  | Another | example type | 1 

因爲它計算的條目爲所有用戶標識

Answer 1

修復您的查詢

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] 

另一種方法是用EXISTS重寫它，它幾乎相同但可能更容易理解。
順便說一句，在列名上使用單引號已被棄用。使用方括號。

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