Problem

Given a table with two columns, write a SQL query to find the most recent date for each unique value in the first column.
Example input:
+------------+-------------+
|    date    | unique_value |
+------------+-------------+
| 2019-01-01 | 1            |
| 2019-01-02 | 2            |
| 2019-01-03 | 1            |
| 2019-01-04 | 2            |
+------------+-------------+
Example output:
+------------+-------------+
|    date    | unique_value |
+------------+-------------+
| 2019-01-03 | 1            |
| 2019-01-04 | 2            |
+------------+-------------+

Solution

SELECT date, unique_value
FROM table
WHERE date = (SELECT MAX(date) FROM table WHERE unique_value = table.unique_value)

/*

The solution above is optimal because it uses a subquery to find the most recent date for each unique value.

*/