Module 1: SQL Foundations•45 min

Filtering with WHERE

Progress Tracking

Now It Gets Useful

So far, your queries return every row in a table. That's fine for small datasets, but in the real world, you're usually looking for something specific. Which employees are in the HR department? What orders were placed in March? Which users signed up this week?

This is what WHERE is for. It filters your results to only the rows that match certain conditions.

Basic WHERE Syntax

WHERE goes after FROM and before ORDER BY. You specify a condition, and SQL returns only the rows where that condition is true.

SQL

SELECT column1, column2
FROM table_name
WHERE condition;

Get all employees in the HR department:

SQL

SELECT
  first_name,
  last_name,
  department
FROM techcorp_workforce
WHERE department = 'HR';

Filtering Text Values

When filtering text (strings), wrap the value in single quotes:

SQL

WHERE department = 'Admin'
WHERE first_name = 'Monika'
WHERE order_details = 'Shoes'

Single Quotes for Text Values

SQL uses single quotes for text values. Double quotes are for column aliases and identifiers. Mixing them up is a common source of errors.

Practice: Basic WHERE

Let's practice on a real question.

Table: los_angeles_restaurant_health_inspections

serial_number	activity_date	facility_name	score	grade	service_code	service_description	employee_id	facility_address	facility_city	facility_id	facility_state	facility_zip	owner_id	owner_name	pe_description	program_element_pe	program_name	program_status	record_id
DAQHRSETQ	2017-06-08	MARGARITAS CAFE	93	A	1	ROUTINE INSPECTION	EE0000006	5026 S CRENSHAW BLVD	LOS ANGELES	FA0023656	CA	90043	OW0004133	BAZAN, ASCENCION	RESTAURANT (61-150) SEATS HIGH RISK	1638	MARGARITAS CAFE	ACTIVE	PR0011718
DA2GQRJOS	2017-03-07	LAS MOLENDERAS	97	A	1	ROUTINE INSPECTION	EE0000997	2635 WHITTIER BLVD	LOS ANGELES	FA0160416	CA	90023	OW0125379	MARISOL FEREGRINO	RESTAURANT (0-30) SEATS HIGH RISK	1632	LAS MOLENDERAS	INACTIVE	PR0148504
DAMQTA46T	2016-03-22	SANDRA'S TAMALES	93	A	1	ROUTINE INSPECTION	EE0001049	5390 WHITTIER BLVD	LOS ANGELES	FA0171769	CA	90022-4032	OW0178828	SANDRA'S TAMALES INC.	RESTAURANT (0-30) SEATS MODERATE RISK	1631	SANDRA'S TAMALES	ACTIVE	PR0164225
DAXMBTIRZ	2018-02-12	CAFE GRATITUDE	97	A	1	ROUTINE INSPECTION	EE0000828	639 N LARCHMONT BLVD STE #102	LOS ANGELES	FA0058921	CA	90004	OW0005704	CAFE GRATITUDE LARCHMONT LLC	RESTAURANT (61-150) SEATS HIGH RISK	1638	CAFE GRATITUDE	ACTIVE	PR0019854
DAK8TBMS0	2015-09-10	THE WAFFLE	90	A	1	ROUTINE INSPECTION	EE0000709	6255 W SUNSET BLVD STE #105	LOS ANGELES	FA0051830	CA	90028	OW0035796	THE WAFFLE, LLC	RESTAURANT (61-150) SEATS HIGH RISK	1638	THE WAFFLE	ACTIVE	PR0010922

Tables: los_angeles_restaurant_health_inspections

Comparison Operators

The equals sign = is just one way to compare values. Here are all the comparison operators:

= equals — example: WHERE department = 'HR' finds matches
<> or != not equal — example: WHERE department <> 'HR' excludes HR
> greater than — example: WHERE salary > 100000 finds high earners
< less than — example: WHERE salary < 50000 finds lower salaries
>= greater than or equal — example: WHERE salary >= 100000
<= less than or equal — example: WHERE salary <= 100000

<> and != Are the Same

Both <> and != mean "not equal." They work the same way. Use whichever your team prefers, but be consistent.

Filtering Numbers

Numbers don't need quotes.

Table: techcorp_workforce

id	first_name	last_name	department	salary	phone_number	joining_date
1	Sarah	Mitchell	HR	95000	555-0101	2021-03-15
2	Michael	Chen	HR	88000	555-0102	2022-06-01
3	Emily	Rodriguez	HR	82500		2021-09-20
4	David	Park	HR	80000	555-0104	2023-01-10
5	Lisa	Thompson	HR	65000		2021-04-05

Tables: techcorp_workforce

What about employees earning exactly 130,000?

Filtering Dates

Dates need single quotes, just like text. The format depends on your database, but 'YYYY-MM-DD' works in most systems:

Table: techcorp_workforce

id	first_name	last_name	department	salary	phone_number	joining_date
1	Sarah	Mitchell	HR	95000	555-0101	2021-03-15
2	Michael	Chen	HR	88000	555-0102	2022-06-01
3	Emily	Rodriguez	HR	82500		2021-09-20
4	David	Park	HR	80000	555-0104	2023-01-10
5	Lisa	Thompson	HR	65000		2021-04-05

Tables: techcorp_workforce

Date Comparison is Chronological

Date comparisons use chronological order. '2023-01-15' > '2023-01-01' is true because January 15 comes after January 1.

Filtering the Orders Table

Let's practice with the orders table:

Table: orders

id	cust_id	order_date	order_details	total_order_cost
1	3	2019-03-04	Coat	100
2	3	2019-03-01	Shoes	80
3	3	2019-03-07	Skirt	30
4	7	2019-02-01	Coat	25
5	7	2019-03-10	Shoes	80

Tables: orders

Or orders for a specific product:

Tables: orders

Common Mistakes

Using double quotes for text values

SQL

-- This will give an error (or behave unexpectedly):
WHERE department = "HR"

-- Correct:
WHERE department = 'HR'

Forgetting quotes around text

SQL

-- This will give an error:
WHERE department = HR
-- SQL thinks HR is a column name, not a value

-- Correct:
WHERE department = 'HR'

Adding quotes around numbers

SQL

-- This might work, but it's wrong:
WHERE salary > '100000'
-- You're comparing text, not numbers

-- Correct:
WHERE salary > 100000

Case sensitivity

This one depends on your database. In PostgreSQL, text comparisons are case-sensitive by default:

SQL

WHERE department = 'HR'  -- matches 'HR'
WHERE department = 'hr'  -- does NOT match 'HR'

We'll cover case-insensitive matching later with LOWER() and ILIKE.

Using aliases in WHERE

Column aliases defined in SELECT aren't available in WHERE. SQL processes WHERE before SELECT.

SQL

-- This will give an error:
SELECT salary * 0.1 AS bonus
FROM techcorp_workforce
WHERE bonus > 5000;
-- Can't use alias here!

-- Correct: repeat the calculation
SELECT salary * 0.1 AS bonus
FROM techcorp_workforce
WHERE salary * 0.1 > 5000;

Key Takeaways

WHERE filters rows based on conditions.
Text values need single quotes, numbers don't.
Use =, <>, >, <, >=, <= to compare values.
Date comparisons work chronologically.
Watch out for case sensitivity with text.

What's Next

One condition is useful, but real questions often need multiple conditions — like finding HR employees earning over 80,000, or finding orders from January or February. In the next lesson, you'll learn to combine conditions with AND, OR, and NOT.

Next upContinue →

Logical Operators: AND, OR, NOT

45 min