问题
I have a query like this:
select a1.name, b1.info
from (select name, id, status
from table1 a) as a1
right outer join (select id, info
from table2 b) as b1 on (a1.id = b1.id)
I only want to include everything where a1.status=1 and since I'm using an outer join, I can't just add a where
constraint to table1, because all info from table2 that I want to be excluded will still be there, just without the name. I was thinking something like this:
select z1.name, z1.info
from ((select name, id, status
from table1 a) as a1
right outer join (select id, info
from table2 b) as b1 on (a1.id = b1.id)) as z1
where z1.status = 1
but I don't think that's legal.
EDIT: As described below, an outer join actually doesn't make sense for what I'm trying to do. What if, for example, I want all the data from table2 where status!=1 in table1, inclusive of all data where a corresponding ID does not at all exist in table1. Thus I would need an outer join of all data from table2, but still want to exclude those entries where the status=1.
Equivalent to this:
select z1.name, z1.info
from ((select name, id, status
from table1 a) as a1
right outer join (select id, info
from table2 b) as b1 on (a1.id = b1.id)) as z1
where z1.status != 1
回答1:
SELECT a1.Name, b1.Info
FROM table2 b1
JOIN table2 a1 ON b1.id= a1.id AND a1.status = 1
A right outer join does the exact same thing as a left outer join, with just the tables switched. You can filter on the join and it will still include the data from the initial table.
回答2:
Add the where
clause to the subquery
like this:
select a1.name, b1.info from
(
select name, id
from table1 a
where a.status = 1
) as a1
right outer join
(
select id, info
from table2 b
) as b1 on (a1.id=b1.id)
回答3:
select a1.name, b1.info from
(select name, id, status from table1 a WHERE status=1) as a1
right outer join
(select id, info from table2 b) as b1 on (a1.id=b1.id)
EDIT:
For your second scenario:
select a1.name, b1.info from
(select name, id, status from table1 a) as a1
right outer join
(select id, info from table2 b) as b1 on (a1.id=b1.id)
EXCEPT
select a1.name, b1.info from
(select name, id, status from table1 a WHERE status<>1) as a1
right outer join
(select id, info from table2 b) as b1 on (a1.id=b1.id)
That should work since you will get all the table2 data regardless.
EDIT 2:
OK, to get everything from table2 EXCEPT where there is a status ID in table 1, even if there is not an entry in table1, you need to use the EXCEPT
function, which will basically exclude a subset from a larger dataset.
来源:https://stackoverflow.com/questions/4071909/sql-alias-of-joined-tables