In this section I have given definition of several join terms. In another section inshallah I will demonstrate example of joins in oracle.
What is Join?
•In a straightforward a join is a query that combines rows from two or more tables, views, or materialized views.
•Oracle Database performs a join whenever multiple tables appear in the FROM clause of the query.
•In join if any two of these tables have a column name in common, then you must qualify all references to these columns throughout the query with table names to avoid ambiguity.
•Most join queries contain at least one join condition, either in the FROM clause or in the WHERE clause.
•The join condition compares two columns, each from a different table.
•To execute a join, Oracle Database combines pairs of rows, each containing one row from each table, for which the join condition evaluates to TRUE.
•To execute a join of three or more tables, Oracle first joins two of the tables based on the join conditions comparing their columns and then joins the result to another table based on join conditions containing columns of the joined tables and the new table. Oracle continues this process until all tables are joined into the result.
•The optimizer determines the order in which Oracle joins tables based on the join conditions, indexes on the tables, and, any available statistics for the tables.
•WHERE clause that contains a join condition can also contain other conditions that refer to columns of only one table. These conditions can further restrict the rows returned by the join query.
Types of Joins in Oracle:
An equijoin is a join with a join condition containing an equality operator. An equijoin combines rows that have equivalent values for the specified columns.
A self join is a join of a table to itself. This table appears twice in the FROM clause and is followed by table aliases that qualify column names in the join condition. To perform a self join, Oracle Database combines and returns rows of the table that satisfy the join condition.
If two tables in a join query have no join condition, then Oracle Database returns their Cartesian product. Oracle combines each row of one table with each row of the other. A Cartesian product always generates many rows and is rarely useful.
An inner join is a join of two or more tables that returns only those rows that satisfy the join condition. It is also called simple join.
An outer join extends the result of a simple join. An outer join returns all rows that satisfy the join condition and also returns some or all of those rows from one table for which no rows from the other satisfy the join condition.
Outer join can be categorizes into three.
a)Left Outer Join: To write a query that performs an outer join of tables A and B and returns all rows from A (a left outer join), use the LEFT [OUTER] JOIN syntax in the FROM clause, or apply the outer join operator (+) to all columns of B in the join condition in the WHERE clause. For all rows in A that have no matching rows in B, Oracle Database returns null for any select list expressions containing columns of B.
b)Right Outer Join:To write a query that performs an outer join of tables A and B and returns all rows from B (a right outer join), use the RIGHT [OUTER] JOIN syntax in the FROM clause, or apply the outer join operator (+) to all columns of A in the join condition in the WHERE clause. For all rows in B that have no matching rows in A, Oracle returns null for any select list expressions containing columns of A.
c)Full Outer Join:To write a query that performs an outer join and returns all rows from A and B, extended with nulls if they do not satisfy the join condition (a full outer join), use the FULL [OUTER] JOIN syntax in the FROM clause.
An antijoin returns rows from the left side of the predicate for which there are no corresponding rows on the right side of the predicate. That is, it returns rows that fail to match (NOT IN) the subquery on the right side.
A semijoin returns rows that match an EXISTS subquery without duplicating rows from the left side of the predicate when multiple rows on the right side satisfy the criteria of the subquery.
Semijoin and antijoin transformation cannot be done if the subquery is on an OR branch of the WHERE clause.