SQL IF语句错误

Question

create view Com as
select C.id,
IF(A.agree = null) THEN
0
else A.agree
end if
+
IF(GA.agree = null) THEN
0
else GA.agree
end if
FROM
comments C left join AGREES A on A.id_comment=C.id left join GAGREES GA on GA.id_comment=C.id

此代码有语法错误我该如何解决这个问题？

Answer 1

= null并不意味着什么。您必须使用is null

Answer 2

试试这个：

SELECT C.id,
       IFNULL(A.agree, 0) + IFNULL(GA.agre, 0)
FROM comments C 
    left join AGREES A on A.id_comment=C.id 
    left join GAGREES GA on GA.id_comment=C.id

Answer 3

您无法在此类查询中嵌套if块。如果您需要实现类似逻辑，那么您将使用case语句。但是，在此案例中，您只需在线使用coalesce()或isnull()。

select
 c.id
,isnull(a.agree,0) + isnull(ga.agree,0)
from Comments as c
left join Agrees as a
on a.id_comment = c.id 
left join GAgrees ga
on ga.id_comment = c.id

......或者（以及MYSQL需要的东西）：

select
 c.id
,coalesce(a.agree,0) + coalesce(ga.agree,0)
from Comments as c
left join Agrees as a
on a.id_comment = c.id 
left join GAgrees ga
on ga.id_comment = c.id

Answer 4

在MySQL中使用
ISNULL()或
IS NULL或
IFNULL