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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  

id 1, email
id 2, email
id 3, email

query output:
email, 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

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

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