Problem

Given a table of data with columns for name, email, and signup date, write a SQL query to find the most recent signup for each email address.
Example input:
name | email | signup_date
John | john@example.com | 2019-01-01
John | john@example.com | 2019-01-02
Jane | jane@example.com | 2019-01-03
Example output:
name | email | signup_date
John | john@example.com | 2019-01-02
Jane | jane@example.com | 2019-01-03

Solution

SELECT name, email, MAX(signup_date)
FROM table
GROUP BY email;

/*

The solution above is optimal because it uses the MAX function to find the most recent signup date for each email address.

*/