### Database Schema
CREATE TABLE `Album` (
  `AlbumId` integer NOT NULL
,  `Title` varchar(160) NOT NULL
,  `ArtistId` integer NOT NULL
,  PRIMARY KEY (`AlbumId`)
,  CONSTRAINT `FK_AlbumArtistId` FOREIGN KEY (`ArtistId`) REFERENCES `Artist` (`ArtistId`) ON DELETE NO ACTION ON UPDATE NO ACTION
)

CREATE TABLE `Artist` (
  `ArtistId` integer NOT NULL
,  `Name` varchar(120) DEFAULT NULL
,  PRIMARY KEY (`ArtistId`)
)

CREATE TABLE `Customer` (
  `CustomerId` integer NOT NULL
,  `FirstName` varchar(40) NOT NULL
,  `LastName` varchar(20) NOT NULL
,  `Company` varchar(80) DEFAULT NULL
,  `Address` varchar(70) DEFAULT NULL
,  `City` varchar(40) DEFAULT NULL
,  `State` varchar(40) DEFAULT NULL
,  `Country` varchar(40) DEFAULT NULL
,  `PostalCode` varchar(10) DEFAULT NULL
,  `Phone` varchar(24) DEFAULT NULL
,  `Fax` varchar(24) DEFAULT NULL
,  `Email` varchar(60) NOT NULL
,  `SupportRepId` integer DEFAULT NULL
,  PRIMARY KEY (`CustomerId`)
,  CONSTRAINT `FK_CustomerSupportRepId` FOREIGN KEY (`SupportRepId`) REFERENCES `Employee` (`EmployeeId`) ON DELETE NO ACTION ON UPDATE NO ACTION
)

CREATE TABLE `Employee` (
  `EmployeeId` integer NOT NULL
,  `LastName` varchar(20) NOT NULL
,  `FirstName` varchar(20) NOT NULL
,  `Title` varchar(30) DEFAULT NULL
,  `ReportsTo` integer DEFAULT NULL
,  `BirthDate` datetime DEFAULT NULL
,  `HireDate` datetime DEFAULT NULL
,  `Address` varchar(70) DEFAULT NULL
,  `City` varchar(40) DEFAULT NULL
,  `State` varchar(40) DEFAULT NULL
,  `Country` varchar(40) DEFAULT NULL
,  `PostalCode` varchar(10) DEFAULT NULL
,  `Phone` varchar(24) DEFAULT NULL
,  `Fax` varchar(24) DEFAULT NULL
,  `Email` varchar(60) DEFAULT NULL
,  PRIMARY KEY (`EmployeeId`)
,  CONSTRAINT `FK_EmployeeReportsTo` FOREIGN KEY (`ReportsTo`) REFERENCES `Employee` (`EmployeeId`) ON DELETE NO ACTION ON UPDATE NO ACTION
)

CREATE TABLE `Genre` (
  `GenreId` integer NOT NULL
,  `Name` varchar(120) DEFAULT NULL
,  PRIMARY KEY (`GenreId`)
)

CREATE TABLE `Invoice` (
  `InvoiceId` integer NOT NULL
,  `CustomerId` integer NOT NULL
,  `InvoiceDate` datetime NOT NULL
,  `BillingAddress` varchar(70) DEFAULT NULL
,  `BillingCity` varchar(40) DEFAULT NULL
,  `BillingState` varchar(40) DEFAULT NULL
,  `BillingCountry` varchar(40) DEFAULT NULL
,  `BillingPostalCode` varchar(10) DEFAULT NULL
,  `Total` decimal(10,2) NOT NULL
,  PRIMARY KEY (`InvoiceId`)
,  CONSTRAINT `FK_InvoiceCustomerId` FOREIGN KEY (`CustomerId`) REFERENCES `Customer` (`CustomerId`) ON DELETE NO ACTION ON UPDATE NO ACTION
)

CREATE TABLE `InvoiceLine` (
  `InvoiceLineId` integer NOT NULL
,  `InvoiceId` integer NOT NULL
,  `TrackId` integer NOT NULL
,  `UnitPrice` decimal(10,2) NOT NULL
,  `Quantity` integer NOT NULL
,  PRIMARY KEY (`InvoiceLineId`)
,  CONSTRAINT `FK_InvoiceLineInvoiceId` FOREIGN KEY (`InvoiceId`) REFERENCES `Invoice` (`InvoiceId`) ON DELETE NO ACTION ON UPDATE NO ACTION
,  CONSTRAINT `FK_InvoiceLineTrackId` FOREIGN KEY (`TrackId`) REFERENCES `Track` (`TrackId`) ON DELETE NO ACTION ON UPDATE NO ACTION
)

CREATE TABLE `MediaType` (
  `MediaTypeId` integer NOT NULL
,  `Name` varchar(120) DEFAULT NULL
,  PRIMARY KEY (`MediaTypeId`)
)

CREATE TABLE `Playlist` (
  `PlaylistId` integer NOT NULL
,  `Name` varchar(120) DEFAULT NULL
,  PRIMARY KEY (`PlaylistId`)
)

CREATE TABLE `PlaylistTrack` (
  `PlaylistId` integer NOT NULL
,  `TrackId` integer NOT NULL
,  PRIMARY KEY (`PlaylistId`,`TrackId`)
,  CONSTRAINT `FK_PlaylistTrackPlaylistId` FOREIGN KEY (`PlaylistId`) REFERENCES `Playlist` (`PlaylistId`) ON DELETE NO ACTION ON UPDATE NO ACTION
,  CONSTRAINT `FK_PlaylistTrackTrackId` FOREIGN KEY (`TrackId`) REFERENCES `Track` (`TrackId`) ON DELETE NO ACTION ON UPDATE NO ACTION
)

CREATE TABLE `Track` (
  `TrackId` integer NOT NULL
,  `Name` varchar(200) NOT NULL
,  `AlbumId` integer DEFAULT NULL
,  `MediaTypeId` integer NOT NULL
,  `GenreId` integer DEFAULT NULL
,  `Composer` varchar(220) DEFAULT NULL
,  `Milliseconds` integer NOT NULL
,  `Bytes` integer DEFAULT NULL
,  `UnitPrice` decimal(10,2) NOT NULL
,  PRIMARY KEY (`TrackId`)
,  CONSTRAINT `FK_TrackAlbumId` FOREIGN KEY (`AlbumId`) REFERENCES `Album` (`AlbumId`) ON DELETE NO ACTION ON UPDATE NO ACTION
,  CONSTRAINT `FK_TrackGenreId` FOREIGN KEY (`GenreId`) REFERENCES `Genre` (`GenreId`) ON DELETE NO ACTION ON UPDATE NO ACTION
,  CONSTRAINT `FK_TrackMediaTypeId` FOREIGN KEY (`MediaTypeId`) REFERENCES `MediaType` (`MediaTypeId`) ON DELETE NO ACTION ON UPDATE NO ACTION
)

### Question
What are the last names of employees who serve at most 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