如何選擇使用UNION所有字段包括NULL？

Question

我使用的查詢：

SELECT idFavourite AS entity, 'favourite' AS type 
FROM favourite 
WHERE idCar = 1 AND idUser = 1 
UNION 
SELECT idLike AS entity, 'likes' AS type 
FROM `likes` 
WHERE idCar = 1 AND idUserLiked = 2 

如果第二個條件是不正確的，這個查詢只返回一個type。我如何總能得到兩個查詢的兩個type？

新建查詢：

SELECT likes.idLike AS likes, favourite.idFavourite AS fav FROM favourite 

LEFT JOIN `likes` ON (likes.idCar = favourite.idCar) AND (likes.idUserLiked = favourite.idUser) AND 
likes.idCar = 1 AND likes.idUserLiked = 2 

Answer 1

首先，使用union all除非你想承擔刪除重複的開銷（這是不可能在這種情況下）。你可以這樣做：

SELECT idFavourite AS entity, 'favourite' AS type 
FROM favourite 
WHERE idCar =1 AND idUser = 1 
UNION ALL 
SELECT idLike AS entity, 'likes' AS type 
FROM `likes` 
WHERE idCar = 1 AND idUserLiked = 2 
UNION ALL 
SELECT NULL, 'likes' 
FROM (SELECT 1 as x) t 
WHERE NOT EXISTS (SELECT 1 FROM likes WHERE idCar = 1 AND idUserLiked = 2); 

另外，如果你只希望從第二子查詢最一排，然後用聚合：

SELECT idFavourite AS entity, 'favourite' AS type 
FROM favourite 
WHERE idCar =1 AND idUser = 1 
UNION ALL 
SELECT MAX(idLike) AS entity, 'likes' AS type 
FROM `likes` 
WHERE idCar = 1 AND idUserLiked = 2; 

聚集查詢沒有GROUP BY總是返回一行。在這種情況下，如果WHERE子句過濾掉所有內容，則entity將爲NULL。