Question

I have three tables:

Table User( userid username)

Table Key( userid keyid)

Table Laptop( userid laptopid)

i want all users who have either a key or a laptop, or both. How do i write the query so that it uses a join between table User and table Key, as well as a join between table User and table Laptop?

The main problem is that in the actual scenario, there are twelve or so table joins, sth like:

” select .. From a left join b on (…), c join d on (..),e,f,g where …”,

and i see that a could be joined to b, and a could also be joined to f. So assuming i can’t make the tables a,b, and f appear side-by-side, how do i write the sql query?

Answer 1

You can use multiple joins to combine multiple tables:

select *
from user u
left join key k on u.userid = k.userid
left join laptop l on l.userid = u.userid

A “left join” also finds users which do not have a key or a laptop. If you replace both with “inner join”, it would find only users with a laptop and a key.

When a “left join” does not find a row, it will return NULL in its fields. So you can select all users that have either a laptop or a key like this:

select *
from user u
left join key k on u.userid = k.userid
left join laptop l on l.userid = u.userid
where k.userid is not null or l.userid is not null

NULL is special, in that you compare it like “field is not null” instead of “field <> null”.

Added after your comment: say you have a table Mouse, that is related to Laptop, but not to User. You can join that like:

select *
from user u
left join laptop l on l.userid = u.userid
left join mouse m on m.laptopid = l.laptopid

If this does not answer your question, you gotta clarify it some more.