AND, OR and NOT · StackMonsters

One condition is often not enough. "Members from Canada" is two conditions: the country is CA, and the customer is a member.

AND, OR, NOT

Keyword	The row stays when
`a AND b`	both conditions are true
`a OR b`	at least one condition is true
`NOT a`	the condition is false

sql

SELECT name, city
FROM customers
WHERE country = 'CA' AND is_member = true;

customers

id	name	country	is_member
1	Ava Chen	CA	true
2	Liam Patel	CA	false
3	Sofia Rossi	IT	true
8	Omar Haddad	CA	true

country = 'CA' AND is_member = true: both must be true

NOT country = 'CA' keeps the same rows as country <> 'CA'.

A condition with AND and OR is not read from left to right. AND is done first:

text

country = 'US' OR country = 'CA' AND is_member = true
           is read as
country = 'US' OR (country = 'CA' AND is_member = true)

That keeps every US customer, and the Canadian members. To get the members from the US or Canada, put the OR in parentheses:

customers

id	name	country	is_member
1	Ava Chen	CA	true
2	Liam Patel	CA	false
4	Noah Kim	US	false
5	Mia Johnson	US	true
8	Omar Haddad	CA	true

Without parentheses: Noah Kim stays, but he is not a member

customers

id	name	country	is_member
1	Ava Chen	CA	true
2	Liam Patel	CA	false
4	Noah Kim	US	false
5	Mia Johnson	US	true
8	Omar Haddad	CA	true

(country = 'US' OR country = 'CA') AND is_member = true

Parentheses are done first. Use them every time a condition has both AND and OR. The query is then clear to every reader.