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SQL: Joins

Learn how to use SQL joins with syntax, visual illustrations, and examples.

Description

SQL JOINS are used to retrieve data from multiple tables. A SQL JOIN is performed whenever two or more tables are joined in a SQL statement.

There are 4 different types of SQL joins:

  • SQL INNER JOIN (or sometimes called simple join)
  • SQL LEFT OUTER JOIN (or sometimes called LEFT JOIN)
  • SQL RIGHT OUTER JOIN (or sometimes called RIGHT JOIN)
  • SQL FULL OUTER JOIN (or sometimes called FULL JOIN)

So let's discuss SQL JOIN syntax, look at visual illustrations of SQL JOINS, and explore SQL JOIN examples.

SQL INNER JOIN (simple join)

Chances are, you've already written a SQL statement that uses an SQL INNER JOIN. It is the most common type of SQL join. SQL INNER JOINS return all rows from multiple tables where the join condition is met.

Syntax

The syntax for the SQL INNER JOIN is:

SELECT columns
FROM table1 
INNER JOIN table2
ON table1.column = table2.column;

Visual Illustration

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

SQL

The SQL INNER JOIN would return the records where table1 and table2 intersect.

Example

Here is an example of a SQL INNER JOIN:

SELECT s.supplier_id, s.supplier_name, od.order_date
FROM suppliers AS s
INNER JOIN order_details AS od
ON s.supplier_id = od.supplier_id;

This SQL 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 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 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 SQL 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 SQL 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;

SQL LEFT OUTER JOIN

Another type of join is called a 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 SQL 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 SQL LEFT OUTER JOIN returns the shaded area:

SQL

The SQL 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 a SQL 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 SQL 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(+);

SQL RIGHT OUTER JOIN

Another type of join is called a SQL 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 SQL 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 SQL RIGHT OUTER JOIN returns the shaded area:

SQL

The SQL 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 a SQL 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 SQL 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;

SQL FULL OUTER JOIN

Another type of join is called a SQL 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 SQL 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 SQL FULL OUTER JOIN returns the shaded area:

SQL

The SQL FULL OUTER JOIN would return the all records from both table1 and table2.

Example

Here is an example of a SQL 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 SQL 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.