Question

我有一个生成空集的mySQL查询。例如，

SELECT id 
FROM `my_table` 
WHERE type = 'old'
AND neighborhoods = 'Newport'

生成一个空集。如果查询生成一个空集，则可以让它返回：

id
---
0

谢谢。

-Laxmidi

Answer 1

您可以添加查询的COUNT部分

SELECT id, COUNT(id) as Total 
FROM `my_table` 
WHERE type = 'old'
AND neighborhoods = 'Newport'

这将返回至少一条Total=0

的记录

Answer 2

是，

SELECT COUNT(id) 
FROM `my_table` 
WHERE type = 'old'
AND neighborhoods = 'Newport'

将始终返回结果集。

Answer 3

如果您需要INCLUDE id，那么您可以使用LEFT OUTER来完成此任务。

SELECT a.id,
       IFNULL(b.cnt,0) as cnt
FROM
(
  SELECT distinct id, 'myjoin' as myjoin
  FROM `my_table` 
  WHERE id = [what you are looking for]
) a LEFT OUTER JOIN 
(
  SELECT COUNT(id) as cnt 
  FROM `my_table` 
  WHERE type = 'old'
  AND neighborhoods = 'Newport'
) b ON a.myjoin=b.myjoin

第一个查询返回你想要的id然后加入（LEFT OUTER）到一个固定值（在这种情况下称为'myjoin'），如果后一个查询返回空，将返回0。

MySQL Query生成一个空集

3 个答案: