在 SQL 的 IN 子句中为不匹配的值返回空行

Question

如果在 IN 子句中找到值，我有一个返回记录的查询。匹配的记录从 0 到 1 不等，因此是查询的结果。但我想将计数设为 5，即使其余行填充为 NULL。有什么办法可以做到这一点？ 如，对于给定的查询

SELECT categoryID, categoryName
FROM category
WHERE categoryID
IN ( 52, 58, 60 , 62 , 64) 

我只有前 3 个 id 的值，因此结果只给了我 3 行。但我希望结果计数为 5，其余不满意的行填充空值以进行计数。 当前结果：

CategoryID  CategoryName
52           ABC
58           XYZ
60           DEF

需要的结果

CategoryID  CategoryName
52           ABC
58           XYZ
60           DEF
NULL         NULL
NULL         NULL

Answer 1

将LEFT JOIN 与列出您想要的所有类别ID 的合成表一起使用：

SELECT c.categoryID, c.categoryName
FROM (SELECT 52 AS categoryID
      UNION SELECT 58 UNION SELECT 60 UNION SELECT 62 UNION SELECT 64) AS a
LEFT JOIN category AS c ON a.categoryID = c.categoryID

另一种方法是创建一个包含所有可能的类别 ID 的表。然后使用：

SELECT c.categoryID, c.categoryName
FROM allCategories AS a
LEFT JOIN category AS c ON a.categoryID = c.categoryID
WHERE a.categoryID IN (52, 58, 60 , 62 , 64)

Answer 2

在末尾添加 5 个空行，只选择前 5 行。

SELECT categoryID, categoryName FROM (
SELECT categoryID, categoryName, 1 as ord 
FROM category
WHERE categoryID
IN ( 52, 58, 60 , 62 , 64)
UNION ALL
SELECT NULL as categoryID, NULL as categoryName, 0 as ord 
UNION ALL
SELECT NULL as categoryID, NULL as categoryName, 0 as ord 
UNION ALL
SELECT NULL as categoryID, NULL as categoryName, 0 as ord  
UNION ALL
SELECT NULL as categoryID, NULL as categoryName, 0 as ord  
UNION ALL
SELECT NULL as categoryID, NULL as categoryName, 0 as ord  )t 
ORDER BY ord desc LIMIT 0,5;