What is a join?

A join is a data operation where usually two tables get combined into one. There are essentially three types of join, full join/ outer join, left/right outer join and inner join. A full join is when every entry from the first table is connected with every entry from the second table resulting in all possible combinations.

An inner join is when every is when every entry from the first table gets combined with every entry of the second table that matches some condition, usually that the value of a specific field of the first entry is the same as the value of a specific field of the second entry. Entries from the first table for which no matching entry from the second table can be found get deleted.

A left/right join is the same as an inner join, but entries from the first(left join) or the second(right join) table dont get deleted even if no match is found for them.