如果查询为空，MySQL 返回 0

Question

此查询应从表中选择一些统计值:

(SELECT MAX(`id`) as `count` FROM `stats` LIMIT 1)
UNION (SELECT COUNT(*) as `count` FROM `stats` WHERE DATE(`time`) >= (NOW() - INTERVAL 1 DAY) LIMIT 1)
UNION (SELECT COUNT(*) as `count` FROM `stats` WHERE DATE(`time`) >= (NOW() - INTERVAL 7 DAY) LIMIT 1)
UNION (SELECT COUNT(*) as `count` FROM `stats` WHERE DATE(`time`) >= (NOW() - INTERVAL 30 DAY) LIMIT 1)
UNION (SELECT DATE_FORMAT(MIN(`time`), '%D of %M, %Y') as `count` FROM `stats` LIMIT 1)

我试过了，但没用

(SELECT MAX(`id`) as `count` FROM `stats` LIMIT 1)
UNION IFNULL((SELECT COUNT(*) as `count` FROM `stats` WHERE DATE(`time`) >= (NOW() - INTERVAL 1 DAY) LIMIT 1), '0')
UNION IFNULL((SELECT COUNT(*) as `count` FROM `stats` WHERE DATE(`time`) >= (NOW() - INTERVAL 7 DAY) LIMIT 1), '0')
UNION IFNULL((SELECT COUNT(*) as `count` FROM `stats` WHERE DATE(`time`) >= (NOW() - INTERVAL 30 DAY) LIMIT 1), '0')
UNION IFNULL((SELECT DATE_FORMAT(MIN(`time`), '%D of %M, %Y') as `count` FROM `stats` LIMIT 1), '0')

Answer 1

您必须对 IFNULL 执行 SELECT:

SELECT MAX(`id`) as `count` FROM `stats` LIMIT 1
UNION 
SELECT IFNULL((SELECT COUNT(*) as `count` 
               FROM `stats` 
               WHERE DATE(`time`) >= (NOW() - INTERVAL 1 DAY) LIMIT 1), 0)
UNION 
SELECT IFNULL((SELECT COUNT(*) as `count` 
               FROM `stats` 
               WHERE DATE(`time`) >= (NOW() - INTERVAL 7 DAY) LIMIT 1), 0)
UNION 
SELECT IFNULL((SELECT COUNT(*) as `count` 
               FROM `stats` 
               WHERE DATE(`time`) >= (NOW() - INTERVAL 30 DAY) LIMIT 1), 0)
UNION 
SELECT IFNULL((SELECT DATE_FORMAT(MIN(`time`), '%D of %M, %Y') as `count` 
               FROM `stats` LIMIT 1), 0)

您还必须使用 0 而不是“0”，并去掉第一个查询周围的括号。