Question

Lets say I have a table called user_table:

user_table
 id
 first_name
 last_name
 gender
 email
 created_at

How would I write a query listing emails used by more than one user?

EDIT:

Can the same results be achieved by use LEFT JOIN instead of GROUP BY?

Answer 1

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

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Alternatively, you can use an INNER JOIN:

SELECT DISTINCT u1.email 
FROM user_table u1
INNER JOIN user_table u2
  ON u1.email = u2.email AND u1.id < u2.id