SELECT
Shift1.employee_id,
COUNT(*) AS overlapping_shifts
FROM EmployeeShifts AS Shift1
INNER JOIN EmployeeShifts AS Shift2
USING (employee_id)
WHERE
Shift1.start_time < Shift2.start_time
AND Shift2.start_time < Shift1.end_time
GROUP BY 1
ORDER BY 1;
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