（mysql）如何获取总和？

我要求和的和。如我所愿

如何创建一个列，将这三个表的sum列相加

首先，3太难了，所以我尝试了2张表，但我得到了一个错误

(select sum(serreq_totalamount)
                from servicerequirement
                    group by key_id)+
            (select sum(facreq_totalamount)
                from facilityrequirement
                    group by key_id)+
            (select sum(resorder_totalamount)
                from restaurantorder
                    group by key_id)

join也是如此

Error Code: 1242. Subquery returns more than 1 row  0.000 sec

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如果有人帮助我，我会很高兴的。

您必须进行查询并通过密钥id加入他们

然后你可以把它们加起来

Error Code: 1242. Subquery returns more than 1 row  0.000 sec

(select
(select sum(serreq_totalamount)
        from servicerequirement
            group by servicerequirement.key_id),
    (select sum(facreq_totalamount)
        from facilityrequirement
            group by facilityrequirement.key_id)
        from servicerequirement
    inner join facilityrequirement
    on servicerequirement.key_id=facilityrequirement.key_id);

Error Code: 1242. Subquery returns more than 1 row  0.000 sec

SELECT t1.key_id,s1+s2+s3
FROM
(select key_id,sum(serreq_totalamount) s1
                from servicerequirement
                    group by key_id) t1
INNER JOIN 
            (select key_id,sum(facreq_totalamount) s2
                from facilityrequirement
                    group by key_id) t2 ON t1.key_id = t2.key_id
INNER JOIN
            (select key_id,sum(resorder_totalamount)
                from restaurantorder
                    group by key_id) t3 ON t2.key_id = t3.key_id