乘以两列不同的表......一个空

Question

在这个连接查询，我乘两个不同表中的列，premium & points。我希望发生的是，如果没有在premium表联接的列那么乘数为1。

这里是查询

SELECT parent.*,(premiumtable.bid * pointstable.points) as total FROM strings parent 
     LEFT JOIN premium premiumtable on parent.sid=premiumtable.sid AND premiumtable.paid='1' 
     LEFT JOIN points pointstable on parent.sid=pointstable.`string-id` 
     WHERE parent.category=:category AND (parent.submittype='0' OR parent.submittype='3') GROUP BY parent.id ORDER BY total LIMIT 5 

因此，如果没有什么是对premiumtable，而不是premiumtable.bid * pointstable.points加入了它将是1 * pointstable.points。该premiumtable.bid充当乘数，如果它不存在，我愿意total等于pointstable.points

Answer 1

尝试使用COALESCE：

SELECT parent.*, 
     (COALESCE(premiumtable.bid,1) * pointstable.points) as total 
FROM strings parent 
LEFT JOIN premium premiumtable 
    on parent.sid=premiumtable.sid AND premiumtable.paid='1' 
LEFT JOIN points pointstable 
    on parent.sid=pointstable.`string-id` 
WHERE parent.category=:category 
AND (parent.submittype='0' OR parent.submittype='3') 
GROUP BY parent.id 
ORDER BY total LIMIT 5 

Answer 2

使用case声明：

SELECT parent.*, 
     case when premiumtable.bid is null 
      then pointstable.points 
      else premiumtable.bid * pointstable.points 
     end as total 
FROM strings parent 
LEFT JOIN premium premiumtable on parent.sid=premiumtable.sid AND premiumtable.paid='1' 
LEFT JOIN points pointstable on parent.sid=pointstable.`string-id` 
WHERE parent.category=:category 
AND (parent.submittype='0' OR parent.submittype='3') 
GROUP BY parent.id 
ORDER BY total 
LIMIT 5