WHERE Clause

The WHERE clause in SQL is used to filter the rows returned by a query.

By default, when you run a SELECT query, MySQL returns all rows in the table. If you want to retrieve only the rows that meet certain conditions, you use a WHERE clause.

Why Use WHERE?

• To find specific records in a table.
• To limit results to only those that match a condition.
• To avoid retrieving unnecessary data.
• It can be used with SELECT, UPDATE, and DELETE queries.

Basic Syntax

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

Example Table: students

Example 1: Simple Numeric Condition

Select students who are older than 20:

SELECT * FROM students
WHERE age > 20;

Example 2: Exact Match (String Condition)

Select student named ‘Alice Smith’:

SELECT * FROM students
WHERE name = 'Alice Smith';

Example 3: Date Comparison

Select students who enrolled after June 20, 2025:

SELECT * FROM students
WHERE enrollment_date > '2025-06-20';

Example 4: Multiple Conditions with AND

Select students who are older than 20 AND enrolled before June 22, 2025:

SELECT * FROM students
WHERE age > 20 AND enrollment_date < '2025-06-22';

Example 5: Multiple Conditions with OR

Select students who are 19 years old OR named ‘Dan Green’:

SELECT * FROM students
WHERE age = 19 OR name = 'Dan Green';

Operators You Can Use in WHERE

Important Notes

• Text and dates should be written inside single quotes '...'.
• You can combine multiple conditions using AND and OR.
• Parentheses () can be used to control the order of evaluation when combining conditions.

Summary

• WHERE helps filter the data returned by a SELECT query.
• It can also be used with UPDATE and DELETE queries.
• You can use various operators and combine multiple conditions.
• Learning to use WHERE effectively is an important skill when working with SQL.

If you’d like, I can continue with the next section: ORDER BY Clause (sql_order_by), or any other section you prefer! Just let me know.