Query where foreign key column can be NULL

问题

I want to get data if orgid = 2 or if there is no row at all for the uid. orgid is an integer. The closest thing I could think of is to do IS NULL but I'm not getting data for the uid that doesn't have an orgid row. Any idea?

select u.uid,u.fname,u.lname from u 
inner join u_org on u.uid = u_org.uid 
inner join login on u.uid = login.uid 
where u_org.orgid=2 or u_org.orgid is NULL 
and login.access != 4;

Basically the OR is if u_org.orgid row doesn't exist.

回答1:

If there is "no row at all for the uid", and you JOIN like you do, you get no row as result. Use LEFT [OUTER] JOIN instead:

SELECT u.uid, u.fname, u.lname
FROM   u 
LEFT   JOIN u_org o ON u.uid = o.uid 
LEFT   JOIN login l ON u.uid = l.uid 
WHERE (o.orgid = 2 OR o.orgid IS NULL)
AND    l.access IS DISTINCT FROM 4;

Also, you need the parenthesis I added because of operator precedence. (AND binds before OR).

I use IS DISTINCT FROM instead of != in the last WHERE condition because, again, login.access might be NULL, which would not qualify.

However, since you only seem to be interested in columns from table u to begin with, this alternative query would be more elegant:

SELECT u.uid, u.fname, u.lname
FROM   u
WHERE (u.uid IS NULL OR EXISTS (
   SELECT 1
   FROM   u_org o
   WHERE  o.uid = u.uid
   AND    o.orgid = 2
   ))
AND NOT EXISTS (
   SELECT 1
   FROM   login l
   WHERE  l.uid = u.uid
   AND    l.access = 4
   );

This alternative has the additional advantage, that you always get one row from u, even if there are multiple rows in u_org or login.

来源：https://stackoverflow.com/questions/13618078/query-where-foreign-key-column-can-be-null

标签

sql

postgresql

null

left-join