Problem

Write a SQL query that returns the names of all employees who have worked on at least two projects.
Example input:
EMPLOYEES
+----+----------+
| id | name |
+----+----------+
| 1 | John |
| 2 | Jane |
| 3 | Joe |
+----+----------+
PROJECTS
+----+--------------+
| id | name |
+----+--------------+
| 1 | Project A |
| 2 | Project B |
| 3 | Project C |
+----+--------------+
EMPLOYEE_PROJECTS
+----+-------------+-------------+
| id | employee_id | project_id |
+----+-------------+-------------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 2 | 2 |
| 4 | 2 | 3 |
| 5 | 3 | 1 |
+----+-------------+-------------+
Example output:
+----------+
| name |
+---------- +
| John |
| Jane |
+----------+

Solution

SELECT name
FROM EMPLOYEES
WHERE id IN (
    SELECT employee_id
    FROM EMPLOYEE_PROJECTS
    GROUP BY employee_id
    HAVING COUNT(*) >= 2
)