SQL - Inner Join

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An SQL Join clause is used to combine multiple related tables in a database, based on common fields/columns.

There are two major types of joins: Inner Join and Outer Join. Other joins like Left Join, Right Join, Full Join etc. Are just subtypes of these two major joins. In this tutorial, we will only learn about the Inner Join.

The SQL Inner Join

The SQL Inner Join is a type of join that combines multiple tables by retrieving records that have matching values in both tables (in the common column).

It compares each row of the first table with each row of the second table, to find all pairs of rows that satisfy the join-predicate. When the join-predicate is satisfied, the column values from both tables are combined into a new table.

The Inner Join is also referred as Equijoin. It is the default join; i.e., even if the "Join"keyword is used instead of "Inner Join", tables are joined using matching records of common columns.

Explanation

Let us look at an example scenario to have a better understanding.

Suppose we have the information of employees in a company divided between two tables namely EmpDetails and Marital status. Where,

• EmpDetails table holds details like Employee ID, Name and Salary.

• MaritalStatus table holds the details Employee ID, Age, and Marital Status.

When we perform the Inner Join operation on these two tables based on the join-predicate EmpDetails.EmpID = MaritalStatus.EmpID, the resultant records hold the following info: ID, Name, Salary, Age and, Status of the matched records.

Syntax

Following is the basic syntax of SQL Inner Join −

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name = table2.column_name;

Example

Assume we have created a table named CUSTOMERS, which contains the personal details of customers including their name, age, address and salary etc., using the following query −

CREATE TABLE CUSTOMERS (
   ID INT NOT NULL,
   NAME VARCHAR (20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2),       
   PRIMARY KEY (ID)
);

Now insert values into this table using the INSERT statement as follows −

INSERT INTO CUSTOMERS VALUES
(1, 'Ramesh', 32, 'Ahmedabad', 2000.00 ),
(2, 'Khilan', 25, 'Delhi', 1500.00 ),
(3, 'Kaushik', 23, 'Kota', 2000.00 ),
(4, 'Chaitali', 25, 'Mumbai', 6500.00 ),
(5, 'Hardik', 27, 'Bhopal', 8500.00 ),
(6, 'Komal', 22, 'Hyderabad', 4500.00 ),
(7, 'Muffy', 24, 'Indore', 10000.00 );

The table will be created as −

Let us create another table ORDERS, containing the details of orders made and the date they are made on.

CREATE TABLE ORDERS (
   OID INT NOT NULL,
   DATE VARCHAR (20) NOT NULL,
   CUSTOMER_ID INT NOT NULL,
   AMOUNT DECIMAL (18, 2)
);

Using the INSERT statement, insert values into this table as follows −

INSERT INTO ORDERS VALUES
(102, '2009-10-08 00:00:00', 3, 3000.00),
(100, '2009-10-08 00:00:00', 3, 1500.00),
(101, '2009-11-20 00:00:00', 2, 1560.00),
(103, '2008-05-20 00:00:00', 4, 2060.00);

The table is displayed as follows −

Let us now combine these two tables using the Inner Join query as shown below −

SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
INNER JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

Output

The result of this query is obtained as follows −

Joining Multiple Tables Using Inner Join

Until now, we have only learnt how to join two tables using Inner Join. However, we can also join as many tables as possible, using Inner Join, by specifying the condition (with which these tables are to be joined).

Syntax

Following is the syntax to join more than two tables using Inner Join −

SELECT column1, column2, column3... 
FROM table1
INNER JOIN table2
ON condition_1
INNER JOIN table3
ON condition_2
....
....
INNER JOIN tableN
ON condition_N;

Note that, even in this case, only two tables can be joined together on a single condition. This process is done sequentially until all the tables are combined.

Example

Let us make use of the previous tables CUSTOMERS and ORDERS along with a new table EMPLOYEE. We will create the EMPLOYEE table using the query below −

CREATE TABLE EMPLOYEE (
   EID INT NOT NULL,
   EMPLOYEE_NAME VARCHAR (30) NOT NULL,
   SALES_MADE DECIMAL (20)
);

Now, we can insert values into this empty tables using the INSERT statement as follows −

INSERT INTO EMPLOYEE VALUES
(102, 'SARIKA', 4500),
(100, 'ALEKHYA', 3623),
(101, 'REVATHI', 1291),
(103, 'VIVEK', 3426);

The details of EMPLOYEE table can be seen below.

Using the following query, we can combine three tables CUSTOMERS, ORDERS and EMPLOYEE.

SELECT OID, DATE, AMOUNT, EMPLOYEE_NAME FROM CUSTOMERS
INNER JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
INNER JOIN EMPLOYEE
ON ORDERS.OID = EMPLOYEE.EID;

Output

The result of the inner join query above is shown as follows −

Inner Join with WHERE Clause

Clauses in SQL work with the purpose of applying constraints while retrieving data using SQL queries. There are various clauses that SQL uses to constraint the data; such as WHERE clause, GROUP BY clause, ORDER BY clause, UNION clause etc.

The WHERE clause is used to filter the data from tables. This clause specifies a condition to retrieve only those records that satisfy it.

Inner Join uses WHERE clause to apply more constraints on the data to be retrieved. For instance, while retrieving the employee records of an organization, if we only want to check the data of employees that earn more than 25000 in a month, we need to specify a WHERE condition (salary > 25000) to retrieve only those employee records.

Syntax

The syntax of Inner Join when used with WHERE clause is given below −

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name = table2.column_name
WHERE condition;

Example

In this example we are joining the tables CUSTOMERS and ORDERS using the inner join query and we are applying some constraints on the result using the WHERE clause.

Here, we are retrieving the ID and NAME from the CUSTOMERS table and DATE and AMOUNT from the ORDERS table where the amount paid is higher than 2000.

SELECT ID, NAME, DATE, AMOUNT FROM CUSTOMERS
INNER JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
WHERE ORDERS.AMOUNT > 2000.00;

Output

The resultant table after applying the where clause with inner join contains the rows that has AMOUNT values greater than 2000.00 −