The most common JOIN type in relational database models is the SQL INNER JOIN. In practice, users use INNER JOINs, for example, if two database tables need to be connected using the same columns. Each record of one table is merged with a corresponding record of the other table. Data records that the database management system (DBMS) cannot find a match for in the other table remain hidden.
An SQL OUTER JOIN, on the other hand, not only outputs the data records of both tables that fulfill the selection condition (for example, the equality of the values of two columns), but also all other tuples of one table or the other.
With reference to the reading direction of the SQL syntax, there is a left and a right table. The corresponding operations are called LEFT OUTER JOIN and RIGHT OUTER JOIN. If, in addition to the data records that fulfill the selection condition, you want to output all data records in the left and right tables in database queries, this is a FULL OUTER JOIN.
The principle of the different JOIN types can be illustrated very well by quantity diagrams: