SQL JOIN Example

Joins can be quite perplexing initially hopefully this simple example will help. 

Joins are used to combine data from multiple tables (usually two tables)

There are three types of joins,

1. INNER

2. OUTER - FULL, LEFT, RIGHT

3. CROSS

For all examples I'll be using the below given tables and use the Name field as the condition.

                                                                         Table1

Id1	Name1
1	Padmika
2	Dissanayake
3	Sukitha
4	Pahan

                                                                        Table2

Id2	Name2
1	Fernando
2	Malaka
3	Padmika
4	Sukitha

1. INNER JOIN

Returns only the set of records that match the Name field in both Table 1 and Table2

SELECT * FROM Table1

INNER JOIN Table2

ON Table1.Name1 = Table2.Name2

Result,

Id1	Name1	Id2	Name2
1	Padmika	3	Padmika
3	Sukitha	4	Sukitha

* This gets executed in the following manner, first get the first record from Table 1 get its name field compare it to Table 2 first record name field if it’s a match return it, now compare with Table2 second record name field…  etc.

You will see that only the records that have a common name is returned

This is similar to the intersection of two sets (Table1 and Table2) based on Name.  

2. OUTER JOIN

2.1 FULL OUTER JOIN

Returns all records in Table 1 and Table 2, with matching records from both sides where available. If there is no match, the missing side will contain null.

SELECT * FROM Table1

FULL OUTER JOIN Table2

ON Table1.Name1 = Table2.Name2

Result,

Id1	Name1	Id2	Name2
1	Padmika	3	Padmika
2	Dissanayake	null	Null
3	Sukitha	4	Sukitha
4	Pahan	null	Null
null	Null	1	Fernando
null	Null	2	Malaka

This is similar to the union of two sets (Table1 and Table2) based on Name.

2.2 LEFT OUTER JOIN

 Returns the complete set of records from Table 1 (Left table), with the matching records for Name field (where available) in Table 2. If there is no match, the right side will contain null.

SELECT * FROM Table1

LEFT OUTER JOIN Table2

ON Table1.Name1 = Table2.Name2

Results,

Id1	Name1	Id2	Name2
1	Padmika	3	Padmika
2	Dissanayake	null	null
3	Sukitha	4	Sukitha
4	Pahan	null	null

Special case of the above result would be to select records that do not have any matching records from the right table (Table2)

To do this just add a ‘WHERE Table2.Id2 IS NULL’ clause to the above query.

Ex: You have a Customer table and an Order table (Has a CustomerID column) and want to find out which customers have not placed any orders  

2.3 RIGHT OUTER JOIN

Similar to the above but all the right table (Table2) records will return rather than the left table ones.

SELECT * FROM Table1

RIGHT OUTER JOIN Table2

ON Table1.Name1 = Table2.Name2

Results,

Id1	Name1	Id2	Name2
null	null	1	Fernando
null	null	2	Malaka
1	Padmika	3	Padmika
3	Sukitha	4	Sukitha

Note : When inspected carefully you will see that in this example,

OUTER JOIN result = LEFT JOIN result + RIGHT JOIN result – INNER JOIN result