Introduction
The WHERE clause in SQL is used to filter records in a table based on a specified condition. It allows you to retrieve only the rows that meet certain criteria, making your queries more precise and efficient.
Syntax
SELECT column1, column2
FROM table_name
WHERE condition;
- SELECT specifies the columns to retrieve.
- FROM specifies the table to query.
- WHERE defines the condition that the rows must satisfy.
Example 1: Basic Filtering
Retrieve all employees from the “Employees” table who work in the “Sales” department:
SELECT FirstName, LastName, Department
FROM Employees
WHERE Department = 'Sales';
Example 2: Using Comparison Operators
You can use comparison operators such as =, >, <, >=, <=, <>:
SELECT FirstName, LastName, Salary
FROM Employees
WHERE Salary > 50000;
Example 3: Using Logical Operators
Combine multiple conditions using AND, OR, and NOT:
SELECT FirstName, LastName, Department, Salary
FROM Employees
WHERE Department = 'Sales' AND Salary > 50000;
Example 4: Using Pattern Matching
Use LIKE to search for a specific pattern:
SELECT FirstName, LastName
FROM Employees
WHERE FirstName LIKE 'J%';
This retrieves all employees whose first name starts with “J”.
Example 5: Using NULL Values
Check for NULL values with IS NULL or IS NOT NULL:
SELECT FirstName, LastName, ManagerID
FROM Employees
WHERE ManagerID IS NULL;
Key Points to Remember
- The WHERE clause always comes after the FROM clause.
- Conditions can include columns, constants, and expressions.
- Use parentheses
()to group conditions for complex queries. - Filtering reduces the number of rows returned, improving query performance.