Mysql如果结果为空返回默认值

Question

如果初始条目不存在，我试图让 mysql 返回一组基本值。

这是我尝试过的:

SELECT 
IFNULL(count( mcr.rating ), 1) as rating_count,
IFNULL(avg( mcr.rating ), 5) as rating_average,
IFNULL(sum( mcr.rating  ), 5) as rating_value,

IFNULL(mck.slug, '-') as rating_slug,
IFNULL(mck.name, '-') as rating_name,
IFNULL(mck.criteria_id, '-') as id,
IFNULL(mck.category, '-') as category

FROM microstock_criteria_key AS mck
LEFT JOIN microstock_collective_reviews AS mcr ON mck.criteria_id = mcr.criteria_id
WHERE mcr.agency_id =5
GROUP BY mck.criteria_id
ORDER BY mck.priority ASC 

但我仍然得到:

MySQL returned an empty result set (i.e. zero rows). (Query took 0.0004 sec)

当我应该(如果我理解的话)获得我在 IFNULL 语句中设置的默认值时。

那么如何正确完成此操作以在可能为空的结果上返回默认结果？

Answer 1

由于您使用评论表的 Agency_id 进行过滤，因此我认为您需要 RIGHT JOIN。

SELECT 
IFNULL(count( mcr.rating ), 1) as rating_count,
IFNULL(avg( mcr.rating ), 5) as rating_average,
IFNULL(sum( mcr.rating  ), 5) as rating_value,

IFNULL(mck.slug, '-') as rating_slug,
IFNULL(mck.name, '-') as rating_name,
IFNULL(mck.criteria_id, '-') as id,
IFNULL(mck.category, '-') as category

FROM microstock_criteria_key AS mck
RIGHT JOIN microstock_collective_reviews AS mcr ON mck.criteria_id = mcr.criteria_id
WHERE mcr.agency_id =5
GROUP BY mck.criteria_id
ORDER BY mck.priority ASC