Question

我有一个查询，如果在IN子句中找到值，则返回记录。匹配的记录从0到1不等，因此查询结果也是如此。但我希望将计数设为5，即使其余行填充NULL也是如此。有什么办法可以实现吗？至于给定的查询

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

我只有前3个ID的值，因此结果只给我3行。但我希望结果的计数为5，其余的非满足行填充空值以进行计数。目前的结果：

CategoryID  CategoryName
52           ABC
58           XYZ
60           DEF

所需的结果

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

Answer 1

将LEFT JOIN与合成表一起使用，列出所需的所有类别ID：

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;

在SQL中的IN子句中为非匹配值返回空行

2 个答案: