Friday, 23 August 2013

SQL: Joins


An SQL join is used to combine rows from multiple tables. An SQL join is performed whenever two or more tables are joined in an SQL statement.
The different kinds of SQL joins are:
  • INNER JOIN (or sometimes called simple join)
  • LEFT OUTER JOIN (or sometimes called LEFT JOIN)
  • RIGHT OUTER JOIN (or sometimes called RIGHT JOIN)
  • FULL OUTER JOIN (or sometimes called FULL JOIN)
Let's take a look at each of them.

INNER JOIN (simple join)

Chances are, you've already written an SQL statement that uses an 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.
       The syntax for the INNER JOIN is:
SELECT columns
FROM table1 
INNER JOIN table2
ON table1.column = table2.column;

Illustration of an INNER JOIN

An INNER JOIN returns the shaded area:


  Here is an example of an 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 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_idsupplier_name
10000IBM
10001Hewlett Packard
10002Microsoft
10003NVIDIA

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

order_idsupplier_idorder_date
500125100002003/05/12
500126100012003/05/13
500127100042003/05/14

       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_idnameorder_date
10000IBM2003/05/12
10001Hewlett Packard2003/05/13

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.

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).

       The syntax for the 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.

Illustration of a LEFT OUTER JOIN

A LEFT OUTER JOIN returns the shaded area:
 Here is an example of a 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 name). It contains the following data:

supplier_idsupplier_name
10000IBM
10001Hewlett Packard
10002Microsoft
10003NVIDIA

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

order_idsupplier_idorder_date
500125100002003/05/12
500126100012003/05/13

       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_idsupplier_nameorder_date
10000IBM2003/05/12
10001Hewlett Packard2003/05/13
10002Microsoft<null>
10003NVIDIA<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 LEFT OUTER JOIN 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(+);

RIGHT OUTER JOIN

Another type of join is called a 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).

       The syntax for the 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.

Illustration of a RIGHT OUTER JOIN

A RIGHT OUTER JOIN returns the shaded area:

   
Here is an example of a 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 name). It contains the following data:

supplier_idsupplier_name
10000Apple
10001Google

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

order_idsupplier_idorder_date
500125100002013/08/12
500126100012013/08/13
500127100022013/08/14

        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_idorder_datesupplier_name
5001252013/08/12Apple
5001262013/08/13Google
5001272013/08/14<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.

FULL OUTER JOIN

Another type of join is called a 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.
       
       The syntax for the 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.

Illustration of a FULL OUTER JOIN

A FULL OUTER JOIN returns the shaded area:


 Here is an example of a 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 name). It contains the following data:

supplier_idsupplier_name
10000IBM
10001Hewlett Packard
10002Microsoft
10003NVIDIA

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

order_idsupplier_idorder_date
500125100002013/08/12
500126100012013/08/13
500127100042013/08/14

        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_idsupplier_nameorder_date
10000IBM2013/08/12
10001Hewlett Packard2013/08/13
10002Microsoft<null>
10003NVIDIA<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.

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