MySQL query and compare two different tables

Question

I'm very new to SQL queries, so forgive me if this is a really easy question.

I have 2 database tables HWC and collection , HWC.id is referenced in collection.col

HWC
- id (PRIMARY)
- stuff
- more stuff
- lots more stuff
- Year

collection
- id (PRIMARY)
- userId
- col

Question:

I want to query the collection table for a specific user to see what entries from HWC they are missing.

I don't even know where to start logically, I don't expect anyone to build the query for me, but pointing me in the correct direction would be very much appreciated.

Answer 1

You want items from the collect that the user is missing . This suggests a left outer join . In particular, you want to keep everything in the HWC table and find those things that are missing:

select hwc.*
from hwc left join
     collection c
     on hwc.id = c.col
where hwc.id is null and c.user_id = @UserId;

When learning SQL, students often learn this syntax:

select hwc.*
from hwc
where hwc.id not in (select c.col from collection c where c.user_id = @UserId);

This is perfectly good SQL. Some databases don't do a great job optimizing not in . And, it can behave unexpectedly when c.col is NULL . For these reasons, this is often rewritten as a not exists query:

select hwc.*
from hwc
where not exists (select 1
                  from collection c
                  where c.col = hwc.id and c.user_id = @UserId
                 );

I offer you these different alternatives because you are learning SQL. It is worth learning how all three work. In the future, you should find each of these mechanisms ( left join , not in , and not exists ) useful.

Answer 2

It sounds like you mean SQL JOINS.

SQL Joins Tutorial:

Lets say you want to Query your collection like so:

SELECT collection.userId, HWC.stuff
FROM collection 
INNER JOIN HWC ON collection.col = HWC.id

This will pick userId from collection, and stuff from HWC, where these ID's have relations.

Hope I helped, good luck!