Table schema:
CREATE TABLE EMPLOYEE (EmployeeId VARCHAR); CREATE TABLE CUSTOMER (LastName VARCHAR, SupportRepId VARCHAR)

Question: Please show the employee last names that serves no more than 20 customers.
SQL: SELECT T1.LastName FROM CUSTOMER AS T1 JOIN EMPLOYEE AS T2 ON T1.SupportRepId = T2.EmployeeId GROUP BY T1.SupportRepId HAVING COUNT(*) <= 20