Join Explanation

[Augury]

I have a co-worker who is not a programmer or database guy, and today he came and randomly asked me about joins in an sql based database. I tried to explain about inner joins and he just wasn't grasping it. Do any of you have an idea of how I can explain this or maybe no of a reference I can send him. The problem I have is I think I am assuming too much knowledge.

[felgall]

An inner join contains the records from both tables where a field or group of fields in one table matches the corresponding field(s) in the other table each matching combination will be output.

For example if table 1 contains

a book
b hair
c dog

and table 2 contains

a fairy
a hat
b money
d cat

then if we do an inner join on the first field from both tables we get

a book fairy
a book hat
b hair money

if instead we do an outer join we will get

a book fairy
a book hat
b hair money
c dog NULL
d NULL cat

the outer join adds records to the join for those entries in each table that don't have matches in the other.