How can I count the number of times each employee ID appears in a dataset?

In SQL you can use a GROUP BY clause to aggregate the rows by employee_id and then count the occurrences. For example:

SELECT employee_id, COUNT(*) AS duplicate_count

FROM employees

GROUP BY employee_id

HAVING COUNT(*) > 1;

This query returns only those employee IDs that appear more than once, along with the number of times they appear. In Python with pandas you can achieve the same with:

duplicate_counts = df['employee_id'].value_counts()

print(duplicate_counts[duplicate_counts > 1])

Both approaches give you a quick view of which employee IDs are duplicated and how many times.