Module 6: SQL Combining Conditions with AND & OR

Module 6

Combining Conditions with AND & OR

A step-by-step, hands-on SQL practice lesson — with a live query playground

SELECT * FROM employees WHERE department = 'Sales' AND status = 'Active';

So far every filter you've written has checked one thing at a time. Real questions are rarely that simple — "Sales employees who are also active," or "anyone in Sales or Marketing." This lesson covers AND and OR, the two keywords that let you combine multiple conditions into one precise query — plus the single most common mistake beginners make when mixing them.

Contents

1. Introduction
2. Requirements
3. Practice: Step by Step
4. Common Problems & Solutions
5. Conclusion
6. Homework & Quiz
7. Live SQL Playground
8. Related Lessons
9. FAQ

Introduction

What You'll Learn — and Why It Matters

🎯 Objective

By the end of this lesson, you will be able to:

• Use AND to require that every condition in a query is true.
• Use OR to match rows where at least one condition is true.
• Combine AND and OR correctly using parentheses.
• Avoid the classic mistake of mixing them without grouping, which silently changes your results.

⚠️ What to Avoid

Common mistakes

• Mixing AND and OR in one condition without parentheses — MySQL evaluates AND before OR, which can quietly return the wrong rows.
• Assuming OR narrows results like AND does — OR actually widens them.
• Writing WHERE department = 'Sales' AND 'HR' instead of repeating the column name on both sides.

💡 Important to Know

Before you start

Think of AND as narrowing a search — every extra condition makes the result set smaller or the same size, never bigger. OR does the opposite — every extra condition can only make the result set bigger or the same size. When you mix both in one query, wrap the OR part in parentheses so it's evaluated as one unit.

Requirements

What You'll Need Before Starting

Operating SystemWindows, macOS, or Linux — this lesson works identically on all of them.

Database ServerMySQL installed and running. See How to Install MySQL if you haven't set it up yet.

Optional but recommendedXAMPP with phpMyAdmin, for a visual way to run queries. See How to Install XAMPP.

Prior knowledgeComfortable with SELECT, aliases, and basic WHERE filtering from Module 1.

No installation needed for the live playground further down this page — it runs entirely in your browser.

Practice

Step-by-Step Practice

Follow along in order — each step builds on the one before it.

1 Open MySQL (CLI or phpMyAdmin)

Open a terminal and connect with the MySQL command-line client, or open phpMyAdmin at http://localhost/phpmyadmin/.

mysql -u root -p

The basic syntax patterns for this lesson look like this:

SELECT columns FROM table WHERE condition1 AND condition2;
SELECT columns FROM table WHERE condition1 OR condition2;

2 Create a Database and Verify It

CREATE DATABASE module6_practice;
SHOW DATABASES;
USE module6_practice;

Now create a table and verify it was created:

CREATE TABLE employees (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  department VARCHAR(50),
  salary INT,
  status VARCHAR(20)
);
SHOW TABLES;
DESCRIBE employees;

3 Insert Sample Data

INSERT INTO employees (id, name, department, salary, status) VALUES
(1, 'Sokha', 'Sales', 2800, 'Active'),
(2, 'Maria', 'Marketing', 3200, 'Active'),
(3, 'Wei', 'Sales', 3000, 'Inactive'),
(4, 'Aiden', 'IT', 3500, 'Active'),
(5, 'Nadia', 'HR', 2600, 'Active'),
(6, 'Liam', 'Marketing', 2100, 'Inactive');

4 Filter with AND

Both conditions must be true — Sales and active.

SELECT name, department, status FROM employees
WHERE department = 'Sales' AND status = 'Active';

5 Filter with OR

Either condition can be true — Sales or Marketing.

SELECT name, department FROM employees
WHERE department = 'Sales' OR department = 'Marketing';

6 AND with a Numeric Condition

Combine a text condition with a numeric one.

SELECT name, salary FROM employees
WHERE status = 'Active' AND salary > 2700;

7 Mixing AND and OR — the Right Way

Active employees who are in either Sales or Marketing. Without the parentheses, this query would return every active Sales employee, plus every Marketing employee regardless of status — not what we want.

SELECT name, department, status FROM employees
WHERE status = 'Active' AND (department = 'Sales' OR department = 'Marketing');

8 Combine with Column Selection and an Alias

Bring in what you learned in Module 2.

SELECT name AS employee_name, salary AS monthly_pay
FROM employees
WHERE department = 'Sales' AND salary >= 2800;

9 Combine with Sorting

Filter first with AND/OR, then sort what's left.

SELECT name, department, salary FROM employees
WHERE status = 'Active' AND (department = 'Sales' OR department = 'IT')
ORDER BY salary DESC;

Troubleshooting

Common Problems and Solutions

Problem 1: Mixing AND and OR returns unexpected extra rows

You wrote WHERE status = 'Active' AND department = 'Sales' OR department = 'Marketing' and got inactive Marketing employees too.

SolutionMySQL evaluates AND before OR, so this reads as "(Active AND Sales) OR Marketing" — not what you meant. Add parentheses: WHERE status = 'Active' AND (department = 'Sales' OR department = 'Marketing').

Problem 2: Trying to match multiple values with one column and AND

You wrote WHERE department = 'Sales' AND department = 'HR' and got zero rows.

SolutionA single row's department can't equal two different values at once. Use OR instead — WHERE department = 'Sales' OR department = 'HR' — or the IN operator, covered in a future module.

Problem 3: An alias from SELECT doesn't work inside WHERE

You aliased salary AS monthly_pay and then tried WHERE monthly_pay > 3000 AND department = 'IT', and got an "unknown column" error.

SolutionSame rule as previous modules — WHERE can't see aliases created in the same SELECT. Use the real column name, salary, inside WHERE.

Problem 4: Too many AND conditions return zero rows

You stacked five AND conditions and nothing matched, even though you expected some results.

SolutionEvery AND you add narrows the result further — it's easy to over-filter by accident. Comment out conditions one at a time to find which one is excluding everything, or double-check for typos in column values.

Problem 5: A row you expected is silently missing

A row where department is NULL doesn't show up in either department = 'Sales' OR department = 'HR' or its opposite.

SolutionThis is expected — NULL never satisfies =, !=, AND, or OR comparisons directly. If you need to include those rows, add OR department IS NULL explicitly.

Conclusion

What You Should Take Away

You should now be comfortable combining multiple conditions in a single query using AND to narrow results and OR to widen them — and, most importantly, you know to reach for parentheses the moment you mix the two together.

In Module 7, we'll build on this with shortcuts like IN and BETWEEN that make long chains of OR and comparison conditions much easier to write and read.

Homework

Homework & Quiz

📝 Homework Task

Using the employees table from this lesson, write your own query that:

• Selects name, department, and salary.
• Finds employees who are Active AND earn 2600 or more.
• Sorts the results by salary, highest first.

Try it yourself first — then check it against the live playground below.

🧪 Quiz: Test Your Understanding (10 Questions)

Pick one answer per question, then click Submit to see your score instantly.

1. With AND, how many conditions must be true for a row to match?

At least one Every single one None of them Exactly two

2. With OR, how many conditions must be true for a row to match?

At least one Every single one None of them Exactly zero

3. Which operator does MySQL evaluate first when AND and OR appear together?

OR Whichever comes first in the query AND They run at the same time

4. What fixes unexpected results when mixing AND and OR?

Removing the OR Wrapping the OR condition in parentheses Using a comma instead Putting AND in quotes

5. WHERE department = 'Sales' AND department = 'HR' returns:

All Sales and HR employees A syntax error Zero rows Only Sales employees

6. Which is the correct way to match Sales or HR employees?

WHERE department = 'Sales' OR department = 'HR' WHERE department = 'Sales' AND 'HR' WHERE department = 'Sales', 'HR' WHERE department == 'Sales' OR 'HR'

7. Adding more AND conditions to a query tends to:

Return more rows Return the same or fewer rows Always cause an error Have no effect on results

8. Does a row with a NULL department match "department = 'Sales' OR department = 'HR'"?

Yes, always Only with AND No, never Only if it's the first row

9. Can WHERE reference an alias created in the same SELECT statement?

No Yes, always Only with parentheses Only with numeric aliases

10. Which clause would you add to sort filtered results by salary, highest first?

GROUP BY salary ORDER BY salary ASC ORDER BY salary DESC SORT salary

0 / 10

Try It Live

🧪 Live SQL Playground

A real SQLite database running entirely in your browser (via WebAssembly) — nothing is sent to a server. Practice combining AND/OR on the employees table below, or pick an example query to get started.

employees id INTEGER PK
name TEXT
department TEXT
salary INTEGER
status TEXT

— Load an example query — List all employees Sales AND Active Sales OR Marketing Active AND salary over 2700 Active AND (Sales OR Marketing) Active in Sales/IT, sorted by salary

SELECT * FROM employees;

Loading SQL engine…

Run a query to see results here.

Related Lessons

Continue the Series

Module 1: Mastering the SQL SELECT Statement (A Step-by-Step Guide)

How to Install XAMPP Step by Step for Beginners

How to Install MySQL Step by Step for Beginners

FAQ

Frequently Asked Questions

1. What's the core difference between AND and OR?

AND requires every listed condition to be true for a row to appear. OR only requires at least one of them to be true.

2. Why do AND and OR give different results when mixed without parentheses?

Because MySQL evaluates AND before OR by default, so an unparenthesized mix can group conditions differently than you intended. Parentheses make the grouping explicit and predictable.

3. Can I combine more than two conditions?

Yes — you can chain as many AND and OR conditions as you need, as long as you group them clearly with parentheses when mixing the two.

4. Is there a shortcut for many OR conditions on the same column?

Yes — the IN operator, for example WHERE department IN ('Sales', 'HR', 'IT'), replaces a long chain of OR conditions on the same column with something shorter and easier to read.

5. Does the order of AND/OR conditions affect the result?

No, as long as the grouping (parentheses) stays the same, reordering the individual conditions doesn't change which rows match.

6. Can I use AND/OR with numeric comparisons too?

Yes — you can freely mix equality, numeric comparisons like > and <, and text conditions within the same AND/OR expression.

7. What happens if none of my conditions match any rows?

MySQL simply returns an empty result set — no error, just zero rows. This is completely normal.

8. Does NULL work normally inside AND/OR conditions?

No — NULL represents an unknown value, so it can never satisfy a direct comparison inside AND or OR. You need an explicit IS NULL check to include those rows.

9. Can I use WHERE with a column that isn't in my SELECT list?

Yes. WHERE can reference any real column in the table, whether or not you're displaying it in your results.

10. Is there a NOT operator to reverse a condition?

Yes — NOT flips a condition's result, for example WHERE NOT department = 'Sales' matches every department except Sales. It combines with AND and OR the same way.

Part of the CodeMend SQL Module series. Finished Module 6? Move on to Module 7 for IN, BETWEEN, and LIKE shortcuts.