What are the last names of employees who serve at most 20 customers?
CREATE TABLE playlist (
    playlistid number,
    name text
)

CREATE TABLE album (
    albumid number,
    title text,
    artistid number
)

CREATE TABLE playlisttrack (
    playlistid number,
    trackid number
)

CREATE TABLE genre (
    genreid number,
    name text
)

CREATE TABLE invoice (
    invoiceid number,
    customerid number,
    invoicedate time,
    billingaddress text,
    billingcity text,
    billingstate text,
    billingcountry text,
    billingpostalcode text,
    total number
)

CREATE TABLE track (
    trackid number,
    name text,
    albumid number,
    mediatypeid number,
    genreid number,
    composer text,
    milliseconds number,
    bytes number,
    unitprice number
)

CREATE TABLE customer (
    customerid number,
    firstname text,
    lastname text,
    company text,
    address text,
    city text,
    state text,
    country text,
    postalcode text,
    phone text,
    fax text,
    email text,
    supportrepid number
)

CREATE TABLE artist (
    artistid number,
    name text
)

CREATE TABLE mediatype (
    mediatypeid number,
    name text
)

CREATE TABLE invoiceline (
    invoicelineid number,
    invoiceid number,
    trackid number,
    unitprice number,
    quantity number
)

CREATE TABLE employee (
    employeeid number,
    lastname text,
    firstname text,
    title text,
    reportsto number,
    birthdate time,
    hiredate time,
    address text,
    city text,
    state text,
    country text,
    postalcode text,
    phone text,
    fax text,
    email text
)
SELECT T1.lastname FROM customer AS T1 JOIN employee AS T2 ON T1.supportrepid = T2.employeeid GROUP BY T1.supportrepid HAVING COUNT(*) <= 20
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: What are the last names of employees who serve at most 20 customers? ### Input: CREATE TABLE playlist (
    playlistid number,
    name text
)

CREATE TABLE album (
    albumid number,
    title text,
    artistid number
)

CREATE TABLE playlisttrack (
    playlistid number,
    trackid number
)

CREATE TABLE genre (
    genreid number,
    name text
)

CREATE TABLE invoice (
    invoiceid number,
    customerid number,
    invoicedate time,
    billingaddress text,
    billingcity text,
    billingstate text,
    billingcountry text,
    billingpostalcode text,
    total number
)

CREATE TABLE track (
    trackid number,
    name text,
    albumid number,
    mediatypeid number,
    genreid number,
    composer text,
    milliseconds number,
    bytes number,
    unitprice number
)

CREATE TABLE customer (
    customerid number,
    firstname text,
    lastname text,
    company text,
    address text,
    city text,
    state text,
    country text,
    postalcode text,
    phone text,
    fax text,
    email text,
    supportrepid number
)

CREATE TABLE artist (
    artistid number,
    name text
)

CREATE TABLE mediatype (
    mediatypeid number,
    name text
)

CREATE TABLE invoiceline (
    invoicelineid number,
    invoiceid number,
    trackid number,
    unitprice number,
    quantity number
)

CREATE TABLE employee (
    employeeid number,
    lastname text,
    firstname text,
    title text,
    reportsto number,
    birthdate time,
    hiredate time,
    address text,
    city text,
    state text,
    country text,
    postalcode text,
    phone text,
    fax text,
    email text
) ### Response: SELECT T1.lastname FROM customer AS T1 JOIN employee AS T2 ON T1.supportrepid = T2.employeeid GROUP BY T1.supportrepid HAVING COUNT(*) <= 20