Question

我写的这句话在所有情况下都能正常工作，除了一个案例

SELECT p.priv_no FROM osqs_privileges p,osqs_users_privileges up 
WHERE up.priv_no = p.priv_no AND up.user_no = 54 AND up."GRANT" = 'Y' 
    UNION 
SELECT p.priv_no FROM 
osqs_privileges p,osqs_groups_privileges gp,osqs_users_groups ug,osqs_users_privileges up 
    WHERE gp.priv_no = p.priv_no AND ug.grp_no = gp.grp_no AND ug.user_no = 54 
AND gp.priv_no NOT IN 
    (SELECT priv_no FROM osqs_users_privileges WHERE user_no = 54 AND "GRANT" = 'N');

在osqs_users_privileges没有行的情况下，此语句（SELECT priv_no FROM osqs_users_privileges WHERE user_no = 54 AND“GRANT”='N'）返回0行，即使其他语句返回数据，所有脚本也返回0行。为什么呢？

Answer 1

这是由于NOT IN子句中NULL的行为造成的。

例如，

select 'yes' where 3 in (1, 2, 3, null)  -- Return result
select 'yes' where 3 not in (1, 2, null) -- Returns an empty set

为什么呢？因为3 <> null会产生UNKNOWN。

条件为空

评估为UNKNOWN的条件几乎就像FALSE。对于   例如，一个带有WHERE子句中的条件的SELECT语句   计算到UNKNOWN不返回任何行。但是，一个条件评估   UNKNOWN与UNKNOWN的进一步操作不同于FALSE   条件评估将评估为UNKNOWN。因此，不要错   计算结果为TRUE，但NOT UNKNOWN计算结果为UNKNOWN。

Ref

Answer 2

您需要使用OUTER JOIN：

SELECT p.priv_no FROM osqs_privileges p,osqs_users_privileges up 
WHERE up.priv_no = p.priv_no AND up.user_no = 54 AND up."GRANT" = 'Y' 
    UNION 
SELECT p.priv_no 
FROM osqs_privileges p
LEFT JOIN osqs_groups_privileges gp on osqs_users_groups ug on gp.priv_no = p.priv_no
LEFT JOIN osqs_users_privileges up on ug.grp_no = gp.grp_no
WHERE ug.user_no = 54 
AND gp.priv_no NOT IN 
    (SELECT priv_no FROM osqs_users_privileges WHERE user_no = 54 AND "GRANT" = 'N');

Answer 3

这是正确的命令：

    SELECT p.priv_no FROM osqs_privileges p,osqs_users_privileges up 
WHERE up.priv_no = p.priv_no AND up.user_no = 55 AND up."GRANT" = 'Y' 
    UNION 
SELECT p.priv_no 
FROM osqs_privileges p 
LEFT JOIN osqs_groups_privileges gp on gp.priv_no = p.priv_no 
LEFT JOIN osqs_users_groups ug on ug.grp_no = gp.grp_no 
LEFT JOIN osqs_users_privileges up on up.priv_no = p.priv_no 
WHERE ug.user_no = 55 
AND gp.priv_no NOT IN 
    (SELECT priv_no FROM osqs_users_privileges WHERE user_no = 55 AND "GRANT" = 'N') ;

这个sql语句有什么问题？

3 个答案: