PHP & MySQL · Lesson 9 of 11

WHERE

Filter database rows with precise predicates.

Lesson

WHERE restricts which rows a query reads or changes. Combine conditions deliberately and use parentheses when AND and OR are mixed. Bind values rather than interpolating them, and ensure the filter reflects the intended authorization boundary. This query returns only published lessons in a specific course. Further guidance Use migrations to version schema changes, constraints to protect relationships, and prepared statements for every dynamic value. Scope writes with a precise condition, check affected rows, and use transactions when multiple statements must succeed together. Store credentials outside source control. Practice task: Create a fresh example for WHERE. Predict the result before running it, then change one input and explain why the result changes. Test a normal case and one boundary or invalid case where it applies. Compare your output with the example and note one rule you would use in a real project.

Code examples

WHERE

SQL

SELECT title
FROM lessons
WHERE tutorial_id = 3
  AND is_published = 1
ORDER BY sort_order;

Expected output

Published lesson titles for tutorial 3

Key points

Filter database rows with precise predicates.
Use prepared statements for values and constraints for data integrity.
Scope writes carefully and verify their result.

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