Joins in ORACLE

What are different types of Joins in ORACLE?

Oracle JOINS are used to retrieve data from multiple tables. An Oracle JOIN is performed whenever two or more tables are joined in a SQL statement. There are different types of Oracle joins:

1. ORACLE Cross Join(Or Sometimes called Cartesian Join)
2. ORACLE INNER JOIN(Or Sometimes called Simple Join)
3. ORACLE OUTER JOIN  

OUTER Join is further divided into :

• LEFT OUTER JOIN (Or Sometimes called LEFT JOIN)
• RIGHT OUTER JOIN (Or Sometimes called RIGHT JOIN)
• FULL OUTER JOIN (Or Sometimes called FULL JOIN)

Cross Join (Cartesian join)

Cross join will give m*n rows where- m (Number of rows in left table) and n (Number of rows in right table)

Select * from Employee Cross Join Department;

Or

Select * from Employee, Department;

INNER JOIN (SIMPLE JOIN)

It is the most common type of join. Oracle INNER JOINS return all rows from multiple tables where the join condition is met.

Syntax

The syntax for the INNER JOIN in Oracle/PL SQL is:

SELECT Columns

FROM Table1 

INNER JOIN Table2

ON Table1.Column = Table2.Column;

 Visual Illustration

In this visual diagram, the Oracle INNER JOIN returns the shaded area:

The Oracle INNER JOIN would return the records where Table1 and Table2 intersect.

Example

Here is an example of an Oracle INNER JOIN:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date

FROM suppliers 

INNER JOIN orders

ON suppliers.supplier_id = orders.supplier_id;

This Oracle INNER JOIN example would return all rows from the suppliers and orders tables where there is a matching supplier_id value in both the suppliers and orders tables.

Let's look at some data to explain how the INNER JOINS works:

We have a table called suppliers with two fields (supplier_id and supplier_name). It contains the following data:

Supplier_id	Supplier_name
10000	IBM
10001	Hewlett Packard
10002	Microsoft
10003	NVIDIA

We have another table called orders with three fields (order_id, supplier_id, and order_date). It contains the following data:

Order_id	Supplier_id	Order_date
500125	10000	2003/05/12
500126	10001	2003/05/13
500127	10004	2003/05/14

If we run the Oracle SELECT statement (that contains an INNER JOIN) below:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date

FROM suppliers

INNER JOIN orders

ON suppliers.supplier_id = orders.supplier_id;

Our result set would look like this:

supplier_id	Name	order_date
10000	IBM	2003/05/12
10001	Hewlett Packard	2003/05/13

The rows for Microsoft and NVIDIA from the supplier table would be omitted, since the supplier_id's 10002 and 10003 do not exist in both tables. The row for 500127 (order_id) from the orders table would be omitted, since the supplier_id 10004 does not exist in the suppliers table.

Old Syntax

As a final note, it is worth mentioning that the Oracle INNER JOIN example above could be rewritten using the older implicit syntax as follows (but we still recommend using the INNER JOIN keyword syntax):

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date

FROM suppliers, orders

WHERE suppliers.supplier_id = orders.supplier_id;

OUTER JOINS- Are the following:

LEFT OUTER JOIN

Another type of join is called an Oracle LEFT OUTER JOIN. This type of join returns all rows from the LEFT-hand table specified in the ON condition and only those rows from the other table where the joined fields are equal (join condition is met).

Syntax

The syntax for the Oracle LEFT OUTER JOIN is:

SELECT columns

FROM Table1

LEFT [OUTER] JOIN Table2

ON Table1.column = Table2.column;

In some databases, the LEFT OUTER JOIN keywords are replaced with LEFT JOIN.

Visual Illustration

In this visual diagram, the Oracle LEFT OUTER JOIN returns the shaded area:

The Oracle LEFT OUTER JOIN would return the all records from Table1 and only those records from Table2 that intersect with Table1.

Example

Here is an example of an Oracle LEFT OUTER JOIN:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date

FROM suppliers

LEFT OUTER JOIN orders

ON suppliers.supplier_id = orders.supplier_id;

This LEFT OUTER JOIN example would return all rows from the suppliers table and only those rows from the orders table where the joined fields are equal.

If a supplier_id value in the suppliers table does not exist in the orders table, all fields in the orders table will display as <null> in the result set.

Let's look at some data to explain how LEFT OUTER JOINS work:

We have a table called suppliers with two fields (supplier_id and supplier_name). It contains the following data:

supplier_id	supplier_name
10000	IBM
10001	Hewlett Packard
10002	Microsoft
10003	NVIDIA

We have a second table called orders with three fields (order_id, supplier_id, and order_date). It contains the following data:

order_id	supplier_id	order_date
500125	10000	2003/05/12
500126	10001	2003/05/13

If we run the SELECT statement (that contains a LEFT OUTER JOIN) below:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date

FROM suppliers

LEFT OUTER JOIN orders

ON suppliers.supplier_id = orders.supplier_id;

Our result set would look like this:

supplier_id	supplier_name	order_date
10000	IBM	2003/05/12
10001	Hewlett Packard	2003/05/13
10002	Microsoft	<null>
10003	NVIDIA	<null>

The rows for Microsoft and NVIDIA would be included because a LEFT OUTER JOIN was used. However, you will notice that the order_date field for those records contains a <null> value.

Old Syntax

As a final note, it is worth mentioning that the LEFT OUTER JOIN example above could be rewritten using the older implicit syntax that utilizes the outer join operator (+) as follows (but we still recommend using the LEFT OUTER JOIN keyword syntax):

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date

FROM suppliers, orders

WHERE suppliers.supplier_id = orders.supplier_id(+);

Another Example-

RIGHT OUTER JOIN

Another type of join is called an Oracle RIGHT OUTER JOIN. This type of join returns all rows from the RIGHT-hand table specified in the ON condition and only those rows from the other table where the joined fields are equal (join condition is met).

Syntax

The syntax for the Oracle RIGHT OUTER JOIN is:

SELECT columns

FROM Table1

RIGHT [OUTER] JOIN Table2

ON Table1.Column = Table2.Column;

In some databases, the RIGHT OUTER JOIN keywords are replaced with RIGHT JOIN.

Visual Illustration

In this visual diagram, the Oracle RIGHT OUTER JOIN returns the shaded area:

The Oracle RIGHT OUTER JOIN would return the all records from Table2 and only those records from Table1 that intersect with Table2.

Example

Here is an example of an Oracle RIGHT OUTER JOIN:

SELECT orders.order_id, orders.order_date, suppliers.supplier_name

FROM suppliers

RIGHT OUTER JOIN orders

ON suppliers.supplier_id = orders.supplier_id;

This RIGHT OUTER JOIN example would return all rows from the orders table and only those rows from the suppliers table where the joined fields are equal.

If a supplier_id value in the orders table does not exist in the suppliers table, all fields in the suppliers table will display as <null> in the result set.

Let's look at some data to explain how RIGHT OUTER JOINS work:

We have a table called suppliers with two fields (supplier_id and supplier_name). It contains the following data:

supplier_id	supplier_name
10000	Apple
10001	Google

We have a second table called orders with three fields (order_id, supplier_id, and order_date). It contains the following data:

order_id	supplier_id	order_date
500125	10000	2013/08/12
500126	10001	2013/08/13
500127	10002	2013/08/14

If we run the SELECT statement (that contains a RIGHT OUTER JOIN) below:

SELECT orders.order_id, orders.order_date, suppliers.supplier_name

FROM suppliers

RIGHT OUTER JOIN orders

ON suppliers.supplier_id = orders.supplier_id;

Our result set would look like this:

order_id	order_date	supplier_name
500125	2013/08/12	Apple
500126	2013/08/13	Google
500127	2013/08/14	<null>

The row for 500127 (order_id) would be included because a RIGHT OUTER JOIN was used. However, you will notice that the supplier_name field for that record contains a <null> value.

Old Syntax

As a final note, it is worth mentioning that the RIGHT OUTER JOIN example above could be rewritten using the older implicit syntax that utilizes the outer join operator (+) as follows (but we still recommend using the RIGHT OUTER JOIN keyword syntax):

SELECT orders.order_id, orders.order_date, suppliers.supplier_name

FROM suppliers, orders

WHERE suppliers.supplier_id(+) = orders.supplier_id;

ANOTHER EXAMPLE-

FULL OUTER JOIN

Another type of join is called an Oracle FULL OUTER JOIN. This type of join returns all rows from the LEFT-hand table and RIGHT-hand table with nulls in place where the join condition is not met.

Syntax

The syntax for the Oracle FULL OUTER JOIN is:

SELECT columns

FROM Table1

FULL [OUTER] JOIN Table2

ON Table1.Column = Table2.Column;

In some databases, the FULL OUTER JOIN keywords are replaced with FULL JOIN.

Visual Illustration

In this visual diagram, the Oracle FULL OUTER JOIN returns the shaded area:

The Oracle FULL OUTER JOIN would return the all records from both Table1 and Table2.

Example

Here is an example of an Oracle FULL OUTER JOIN:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date

FROM suppliers

FULL OUTER JOIN orders

ON suppliers.supplier_id = orders.supplier_id;

This FULL OUTER JOIN example would return all rows from the suppliers table and all rows from the orders table and whenever the join condition is not met, <nulls> would be extended to those fields in the result set.

If a supplier_id value in the suppliers table does not exist in the orders table, all fields in the orders table will display as <null> in the result set. If a supplier_id value in the orders table does not exist in the suppliers table, all fields in the suppliers table will display as <null> in the result set.

Let's look at some data to explain how FULL OUTER JOINS work:

We have a table called suppliers with two fields (supplier_id and supplier_name). It contains the following data:

supplier_id	supplier_name
10000	IBM
10001	Hewlett Packard
10002	Microsoft
10003	NVIDIA

We have a second table called orders with three fields (order_id, supplier_id, and order_date). It contains the following data:

order_id	supplier_id	order_date
500125	10000	2013/08/12
500126	10001	2013/08/13
500127	10004	2013/08/14

If we run the SELECT statement (that contains a FULL OUTER JOIN) below:

SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date

FROM suppliers

FULL OUTER JOIN orders

ON suppliers.supplier_id = orders.supplier_id;

Our result set would look like this:

supplier_id	supplier_name	order_date
10000	IBM	2013/08/12
10001	Hewlett Packard	2013/08/13
10002	Microsoft	<null>
10003	NVIDIA	<null>
<null>	<null>	2013/08/14

The rows for Microsoft and NVIDIA would be included because a FULL OUTER JOIN was used. However, you will notice that the order_date field for those records contains a <null> value.

The row for supplier_id 10004 would be also included because a FULL OUTER JOIN was used. However, you will notice that the supplier_id and supplier_name field for those records contain a <null> value.

Old Syntax

As a final note, it is worth mentioning that the FULL OUTER JOIN example above could not have been written in the old syntax without using a UNION query.

ANOTHER EXAMPLE-

What will be the result of different joins on below two tables?

TableA		TableB
ColA		ColB
1		1
1		1
1		1
1		1

In all the joins result will be same as the column values are same in both the tables. Each value from TableA will match with the TableB and fetch the result set. In this case result set will give m*n (16) rows in all type of joins.

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