The WHERE Clause

Definition: The WHERE clause is used to filter records. It ensures that only the data meeting a specific condition is retrieved from the database.

Learning Outcome: Filtering is a major milestone in learning SQL. While SELECT shows you columns, WHERE allows you to drill down into specific rows, making it possible to find exact information within thousands of records.

Syntax

The WHERE clause is placed immediately after the FROM statement. It requires a condition (a logical test) to evaluate each row.

SELECT column1, column2
FROM table_name
WHERE condition;

Example: Numeric Filtering

If you want to find all students in your database who are older than 20, you would use the "greater than" operator (>):

SELECT *
FROM students
WHERE age > 20;

Explanation

• Filtering Logic: The database engine looks at the age column for every row. If the number is 21 or higher, the row is included in the results. If it is 20 or lower, the row is hidden.
• The Result: This query returns only the specific students who meet the criteria, providing a targeted subset of your data.

Common Comparison Operators

Key Notes

• Text vs Numbers: Remember that text values must be inside single quotes ('Bengaluru'), but numbers do not need quotes (25).
• Multiple Conditions: You can combine multiple filters using AND or OR (e.g., WHERE age > 20 AND city = 'Hassan').
• Performance: Using a WHERE clause is much more efficient than selecting all data and filtering it manually in another application like Excel.