JOIN statement not working with where clause - sql

Question

I'm trying to do
select *
from buildings
where levels = 3
join managers;
but it says error at join. I want to match the the id of the building with the id in the managers table so I think I want natural join.

Answer 1

If you put the JOIN in the right place (between FROM and WHERE) and it were legal to write JOIN without ON then the result would be a cross join - a cartesian product that you then filter in WHERE.
That's a perfectly valid thing to do, though not probably what you intended in this case. It can be achieved by adding comma-separate tables to the FROM clause, eg:
FROM buildings, managers
It's generally better style to write an explicit inner join when you intend to join two tables on a condition:
SELECT *
FROM buildings b INNER JOIN managers m ON (b.manager_id = m.manager_id)
WHERE b.levels = 3;
... because it makes it clear to someone else reading the statement that the ON clause is a join condition, and the bl.levels=3 is a filter. The SQL implementation generally doesn't care, and quite likely transforms the above into:
SELECT *
FROM buildings b, managers m
WHERE b.levels = 3 AND b.manager_id = m.manager_id;
internally anyway, but it's easier (IMO) to understand complex queries with many joins when they're written using explicit join syntax.
There's another way to write what you want, but it's dangerous and IMO shouldn't be used:
SELECT *
FROM buildings bl
NATURAL JOIN managers m
WHERE bl.levels = 3;
That JOINs on any columns that're named the same. It's a nightmare to debug, you have to look up the table structures to understand what it does, it breaks if someone renames a column, and it's just painful. Do not use. See Table expressions in the PostgreSQL manual.
More acceptable is the USING syntax also discussed above:
SELECT *
FROM buildings bl
INNER JOIN managers m USING (manager_id)
WHERE bl.levels = 3;
which matches the columns named manager_id in both columns and JOINs on them, combining them into a single column, but unlike NATURAL JOIN does so explicitly and without scary magic.
I still prefer to write INNER JOIN ... ON (...) but it's reasonable to use USING and, IMO, never reasonable to use NATURAL.
Test table structures were:
create table managers ( manager_id integer primary key );
create table buildings (
manager_id integer references manager(manager_id),
levels integer
);

Answer 2

JOIN operator applied to the tables, you should provide it in the FROM clause. If you do so you will get a new error claiming that there is no JOIN condition, because you have to provide the JOIN condition after the ON clause (it is not optional):
SELECT *
FROM buildings b JOIN managers m ON b.managerid = m.id
WHERE b.levels = 3

Answer 3

if you want to do that, you have to write something like:
select *
from bulidings b
join managers m on b.id=m.id
where levels = 3
The request seems odd however, I believe that one of them should have a foreign key into the other. So a more appropriate query would be:
select *
from buidings b
join managers m on b.id = m.building_id //// or b.manager_id = m.id
where levels = 3

Answer 4

I think which query you are written is wrong.
So please try this
select * from buildings as bl
join managers as mn on bl.F_ManagerID = mn.ManagerID
where bl.levels = 3
I think this will help you...