Joining Tables
Joins enable a SELECT statement to retrieve data from two or more tables in a database. The tables are listed in the FROM clause. The optional ON clause can reduce the number of rows returned, and the WHERE clause can further reduce the number of rows returned.
From the information in a SELECT that describes a join, InterBase builds a table that contains the results of the join operation, the results table, sometimes also called a dynamic or virtual table.
InterBase supports two types of joins: inner joins and outer joins.
Inner joins link rows in tables based on specified join conditions and return only those rows that match the join conditions. If a joined column contains a NULL value for a given row, that row is not included in the results table. Inner joins are the more common type because they restrict the data returned and show a clear relationship between two or more tables.
Outer joins link rows in tables based on specified join conditions but return rows whether they match the join conditions or not. Outer joins are useful for viewing joined rows in the context of rows that do not meet the join conditions.