Introduction to the 'WHERE' Keyword in SQL

Overview

In SQL, the 'WHERE' keyword is used to filter data from a table based on specified conditions. It allows you to retrieve only the rows that meet certain criteria. The 'WHERE' clause is a powerful tool that enhances the functionality and flexibility of SQL queries.

Syntax

The basic syntax of the 'WHERE' clause in a SELECT statement is as follows:

SELECT column1, column2, ...
FROM table_name
WHERE condition;

Usage

The 'WHERE' clause is typically used in conjunction with the 'SELECT' statement, but it can also be used with other SQL statements such as 'UPDATE', 'DELETE', and 'INSERT'. Here are some common use cases:

1. Retrieve Rows Based on a Single Condition:

SELECT * FROM employees
WHERE department = 'Sales';

This query will return all the rows from the 'employees' table where the department is 'Sales'.

2. Filter Rows Using Multiple Conditions:

SELECT * FROM orders
WHERE status = 'Delivered' AND total_price > 1000;

This query will retrieve all the rows from the 'orders' table where the status is 'Delivered' and the total price is greater than 1000.

3. Use Comparison Operators:

SELECT * FROM students
WHERE age >= 18 AND grade < 12;

This query will fetch all the rows from the 'students' table where the age is greater than or equal to 18 and the grade is less than 12.

4. Handle NULL Values:

SELECT * FROM customers
WHERE city IS NULL;

This query will return all the rows from the 'customers' table where the city column contains NULL values.

5. Use Wildcards in WHERE Clause:

SELECT * FROM products
WHERE product_name LIKE 'Apple%';

This query will retrieve all the rows from the 'products' table where the product_name starts with 'Apple'.

Conclusion

The 'WHERE' keyword is a fundamental part of SQL that allows you to filter data based on specified conditions. It provides a powerful mechanism to extract the required information from database tables. By combining the 'WHERE' clause with other SQL keywords, you can create complex queries to meet your specific requirements. Mastering the usage of the 'WHERE' keyword will greatly enhance your ability to manipulate data in SQL databases.