mysql内连接返回null值，

Question

SELECT `mpeda_fish`.`id`, `mpeda_fish`.`fish` as analysis, sum(mpeda_fishdetails.quantity) as qty
FROM (`mpeda_fishdetails`)
INNER JOIN `mpeda_scientificfish` ON `mpeda_scientificfish`.`id` = `mpeda_fishdetails`.`scientificfish`
INNER JOIN `mpeda_fish` ON `mpeda_fish`.`id` = `mpeda_scientificfish`.`fish`
INNER JOIN `mpeda_fishcatch` ON `mpeda_fishcatch`.`id` = `mpeda_fishdetails`.`fishcatch`
INNER JOIN `mpeda_harbour` ON `mpeda_harbour`.`id` = `mpeda_fishcatch`.`harbour`
WHERE `mpeda_fishcatch`.`status` =  1
ORDER BY `mpeda_fishdetails`.`id` ASC

此查询获取2列空值，一列获取数据，为什么？

Answer 1

您使用SUM（）函数。为了获得有意义的结果，您应该有一个group by子句。

SELECT `mpeda_fish`.`id`, `mpeda_fish`.`fish` as analysis, sum(mpeda_fishdetails.quantity) as qty
FROM (`mpeda_fishdetails`)
INNER JOIN `mpeda_scientificfish` ON `mpeda_scientificfish`.`id` = `mpeda_fishdetails`.`scientificfish`
INNER JOIN `mpeda_fish` ON `mpeda_fish`.`id` = `mpeda_scientificfish`.`fish`
INNER JOIN `mpeda_fishcatch` ON `mpeda_fishcatch`.`id` = `mpeda_fishdetails`.`fishcatch`
INNER JOIN `mpeda_harbour` ON `mpeda_harbour`.`id` = `mpeda_fishcatch`.`harbour`
WHERE `mpeda_fishcatch`.`status` =  1
GROUP BY `mpeda_fish`.`id`, `mpeda_fish`.`fish`
ORDER BY `mpeda_fishdetails`.`id` ASC