Joins in oracle apps_1

Types of Joins in Oracle:
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A)Equijoins
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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.

B)Self Joins
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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.

C)Cartesian Products
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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.

D)Inner Joins
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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.

E)Outer Joins
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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.

E)Antijoins
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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.

F)Semijoins
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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. 

 

Overview of Hash Joins
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•Hash joins are used for joining large data sets.

•The optimizer uses the smaller of two tables or data sources to build a hash table on the join key in memory.

•It then scans the larger table, probing the hash table to find the joined rows.

•This method is best used when the smaller table fits in available memory.

•The cost is then limited to a single read pass over the data for the two tables.