JOINS in Oracle SQL – PLSQL

As the name suggests a join in Oracle SQL / PLSQL is used to join or combine 2 or more table rows. A join can be performed whenever we have more than one table in the FROM clause of any SQL SELECT statement.

Different kinds of JOINS are:

1. Inner Join
2. Outer Join

1. Inner JOIN:

Inner join is the most common type of join used in SQL queries. An INNER JOIN returns all the rows from multiple tables whenever the join condition is met. It’s also known as “EQUI JOIN”

Suppose have a table named ‘employee’ in the database as shown below.

Employee_ID	Employee_Name	Salary	Department
101	Emp A	10000	Sales
102	Emp B	20000	IT
103	Emp C	28000	IT
104	Emp D	30000	Support
105	Emp E	32000	Sales

Also we have one more table ‘comm’ as shown below:

Emp_ID	Commission_Percent
102	20
103	20
104
105	10
106	15
107	25

Let’s join ‘employee’ and ‘comm’ tables using INNER JOIN as:

SELECT employee_id
       ,employee_name
       ,salary
       ,department
       ,c.commission_percent
FROM employee e
     ,comm c
WHERE e.employee_id = c.emp_id;

The result of the above query will be:

Employee_ID	Employee_Name	Salary	Department	Commission_Percent
102	Emp B	20000	IT	20
103	Emp C	28000	IT	20
104	Emp D	30000	Support
105	Emp E	32000	Sales	10

Here we can see that the record for employee_id = ‘101’ from ‘employee’ table and records for emp_id = ‘106’ and ‘107’ from ‘comm’ table have not been fetched by the SELECT statement as they are not available in both the tables.

Also note that we have created alias names for tables ‘employee’ and ‘comm’ as ‘e’ and ‘c’ respectively.

2. Outer JOIN:

The outer join returns all the rows from one table and only those rows from the second table where the condition of the join is met.
Let’s take an example for easy understanding:

Assume that we have a new table namely ‘employee’ as shown below:

Employee_ID	Employee_Name	Salary	Department
101	Emp A	10000	Sales
102	Emp B	20000	IT
103	Emp C	28000	IT
104	Emp D	30000	Support
105	Emp E	32000	Sales

Also we have one more table ‘comm’ as shown below:

Emp_ID	Commission_Percent
102	20
103	20
104
105	10
106	15
107	25

If we write a query for OUTER JOIN as:

SELECT employee_id
       ,employee_name
       ,salary
       ,department
       ,c.commission_percent
FROM employee e
     ,comm c
WHERE e.employee_id = c.emp_id(+);

Once we have run the above code following will be the result:

Employee_ID	Employee_Name	Salary	Department	Commission_Percent
101	Emp A	10000	Sales	10
102	Emp B	20000	IT	20
103	Emp C	28000	IT	20
104	Emp D	30000	Support
105	Emp E	32000	Sales	10

Here we can see that the records for ‘employee_id = 101’ from ‘employee’ table are also fetched by query but the same was not available in ‘comm’ table.

Hence the above query returns all the rows from ‘employee’ table but returns only those rows from ‘comm’ table where the condition for join is met i.e. where ‘empoloyee_id’ from ‘employee’ table = ‘emp_id’ from ‘comm’ table.

If we change our query as shown below:

SELECT c.emp_id
       ,employee_name
       ,salary
       ,department
       ,c.commission_percent
FROM employee e
     ,comm c
WHERE e.employee_id(+) = c.emp_id;

We get the following output:

Emp_ID	Employee_Name	Salary	Department	Commission_Percent
102	Emp B	20000	IT	20
103	Emp C	28000	IT	20
104	Emp D	30000	Support
105	Emp E	32000	Sales	10
106				15
107				25

Here we can see that the records for ‘emp_id = 106’ and ‘107’ from ‘comm’ table are also fetched by query but the same was not available in ‘employee’ table.

Hence the above query returns all the rows from ‘comm’ table but returns only those rows from ‘employee’ table where the condition for join is met i.e. where ‘empoloyee_id’ from ‘employee’ table = ‘emp_id’ from ‘comm’ table.