Problem

Given two tables, parent and child, write a SQL query to find all the parents who have at least one child.
Example input:
parent
id
1
2
3
4
child
id
parent_id
1
1
2
2
3
3
4
4
5
4
Example output:
parent
id
1
2
3
4

Solution

SELECT DISTINCT parent.id
FROM parent
JOIN child ON parent.id = child.parent_id;

/*

The above solution is optimal because it uses a JOIN to combine the two tables and then uses DISTINCT to remove any duplicate rows.

*/