SQL Query to Select Data from Tables Using Join and Where

In SQL, the JOIN clause combines data from multiple tables based on a common column, while the WHERE clause filters the results based on specific conditions. Together, they allow us to retrieve relevant data efficiently from related tables. This article will guide us in using SQL JOIN and WHERE clauses to efficiently select and filter data, with practical examples to help us master these essential SQL techniques.

What is a Query in SQL?

A SQL query is a request to retrieve or manipulate data from a database. SQL queries help developers and data analysts to interact with data stored in relational database systems, such as MySQL. Queries can be used for a variety of purposes, including:

• Retrieving data for analysis or reporting.
• Updating records in the database.
• Deleting or modifying data based on certain conditions.
• Performing calculations or aggregations on data.

What is Join in SQL?

A JOIN in SQL is a command that allows you to combine rows from two or more tables based on a related column between them. SQL JOINs are essential for querying multiple tables that are related by a common key or column.

What is the Where Clause in SQL?

The WHERE clause is used to filter records in SQL queries based on specified conditions. It helps to limit the results based on criteria such as numeric ranges, text patterns, or specific field values.

• Retrieve records where employees' salaries are greater than 6,00,000.
  
• Filter data to only return customers from a specific city or those who made purchases within a certain date range.

Step-by-Step Guide to SQL Join and WHERE Clause Operations

To begin using SQL JOIN and WHERE clauses to select data from multiple tables, we first need to set up a database with some sample tables. Below is a structured approach to help you get started with these operations.

Step 1: Create a Database

Start by creating a new database where you will store your tables:

CREATE DATABASE geeksforgeeks;

Step 2: Select the Database

Once the database is created, enter it to start using it:

USE geeksforgeeks;

Step 3: Create the employee Table

Create a table1 as employee in the database where we will perform our operations - 

CREATE TABLE employee ( ID int(10),
            Name varchar(55),
            Email varchar(100),
            Department int(10)
            );

Step 4: Create the dept Table

Create another table2 as dept where we will store the data of employees of the second company- 

CREATE TABLE dept     ( ID int(10),
            Name varchar(55),
            hodId int(10),
            profit int(20)
            );

Step 5: View Table Schemas

View the schema of the table to ensure the table is correct - 

> DESC employee;
> DESC dept;

Step 6: Insert Data into the employee Table

Insert sample employee data into the employee table:

INSERT INTO employee VALUES(1, "Devesh", "[email protected]", 1);
INSERT INTO employee VALUES(2, "Mayank", "[email protected]", 1);
INSERT INTO employee VALUES(3, "Aditya", "[email protected]", 2);
INSERT INTO employee VALUES(4, "Divyanshi", "[email protected]", 2);
INSERT INTO employee VALUES(5, "Megha", "[email protected]", 3);
INSERT INTO employee VALUES(6, "Himanshi", "[email protected]", 3);
INSERT INTO employee VALUES(7, "Tanishka", "[email protected]", 4);
INSERT INTO employee VALUES(8, "Jatin", "[email protected]", 4);

Step 7: Insert Data into the dept Table

Now, insert data into the dept table:

INSERT INTO dept VALUES(1, "Computer Science", 1, 100000);
INSERT INTO dept VALUES(2, "Electrical", 2, 45000);
INSERT INTO dept VALUES(3, "Biotechnology", 3, 30000);
INSERT INTO dept VALUES(4, "Architecture", 4, 15000);

Step 8: Query the Data Using JOIN and WHERE Clauses

Now, let's perform some queries using JOIN and WHERE clauses to filter and combine data from the two tables.

Example 1: Retrieve Employees Who Are HODs of Departments

This query joins the employee and dept tables and retrieves employees whose IDs match the hodId of the departments.

Query:

SELECT employee.ID, employee.Name, employee.Email
FROM employee 
JOIN dept
WHERE
employee.ID = dept.hodId;

Output

Example 2: Retrieve Employees from Departments with Profit Greater Than 45,000

Here we perform a LEFT JOIN to get all employees from departments where the profit is greater than 45,000.

Query:

SELECT * 
FROM employee
LEFT JOIN dept
ON
employee.Department = dept.ID
WHERE 
employee.Name IN
(SELECT Name FROM employee WHERE dept.profit > 45000);

Output

Example 3: Retrieve All Data Using FULL JOIN (Cross Join)

A FULL JOIN retrieves all records from both the employee and dept tables. Shows all records from both tables, even if there’s no match in one of them.

Query:

SELECT *
FROM employee 
FULL JOIN dept
WHERE
dept.id > 0;

Output

Example 4: Retrieve Employees from Departments with Profit Greater Than 5,000

This query shows employees from departments whose total profit is greater than 5,000. This query will Display department IDs, names, and HOD IDs for departments with profits greater than 5,000.

Query:

SELECT DISTINCT dept.ID, dept.Name, dept.hodId
FROM dept
JOIN employee
ON
dept.ID = employee.Department
WHERE
hodId IN
(SELECT hodId FROM dept WHERE hodId > 0);

Output

Conclusion

By understanding how to use SQL JOINs and the WHERE clause, we can easily combine data from multiple tables based on specific conditions. Optimizing SQL queries ensures faster and more accurate results. Remember, different types of JOINs allow us to retrieve various relationships between tables, and the WHERE clause enables filtering of data for precise results.