person: every user has a different email right?
1 query later… person, now sad: oh no

This query uses HAVING to find all emails that are shared by more than one user:

SELECT email, COUNT(*)  
FROM users  
GROUP BY email  
HAVING COUNT(*) > 1

users:
id 1, email asdf@fake.com
id 2, email bob@builder.com
id 3, email asdf@fake.com

query output:
email asdf@fake.com, COUNT(*) 2

HAVING is like WHERE, but with 1 difference: HAVING filters rows AFTER grouping and WHERE filters rows BEFORE grouping.

Because of this, you can use aggregates (like COUNT (*)) in a HAVING clause but not with WHERE.

Here’s another HAVING example that finds months with more than $6.00 in income:

SELECT month  
FROM sales  
GROUP BY month  
HAVING SUM(price) > 6

sales:
month: Jan. item: catnip price: 5
month: Feb item: laser price: 8
month: March item: food price: 4
month: March item: food price: 3

query output:
month: Feb
month: March