Comparison Operators

Definition: Comparison operators are symbols used in SQL filters to determine the relationship between two values. They are the building blocks of the WHERE clause, allowing the database to evaluate whether a record should be included in your results.

Usage: Beginner SQL tutorials introduce these early because they allow you to move beyond simply viewing data to actually asking specific questions, such as "Which students are from a specific city?" or "Which products are out of stock?"

Common Comparison Operators

These operators are used to compare a column's value against a specific criteria:

Practical Example

To retrieve all student records for those living specifically in Bengaluru, you use the Equal to operator:

SELECT *
FROM students
WHERE city = 'Bengaluru';

Explanation

• Filtering by Text: When comparing text (like 'Bengaluru'), always wrap the value in single quotes.
• Filtering by Number: When comparing numbers (like age > 18), do not use quotes.
• Logical Evaluation: For every row in the students table, the database asks: "Is the city equal to Bengaluru?" If the answer is True, the row is displayed. If False, the row is hidden.

Key Notes

• Standardization: While != is widely accepted in modern SQL, <> is the official ISO standard for "not equal to." Both usually work, but <> is more portable across different database types.
• Null Values: Operators like = or != do not work on NULL (empty) values. To find empty data, you must use IS NULL or IS NOT NULL.
• Case Sensitivity: Depending on the database settings (Collation), 'bengaluru' might be treated differently than 'Bengaluru'. It is best practice to match the case exactly as it is stored.