Simple web API that access sqlite database

This is a very simple web API for a school assignment, as a part of a bigger system. The web API returns a list of courses or a single course as json. I doubt that I will get any feedback on the code since Java is taught in the course so I am posting it here.

I am a beginner in Racket but have experience with C-style languages (C, C#, Python, etc).

• I want to know how I can improve my solution and to be more idiomatic?
• My function for converting the row-result from the database query to json (rows-result->json) is very messy, how can improve this? I have added example results (mock-row-result) from the database query that can be used as an argument.

The web api has two end points:

• GET /course/{courseid}
• GET /course with optional parameters: CourseCode, StudyPeriod

The database is a single table in an sqlite-database with the rows:

• id - int
• AppCode - text
• CourseCode - text
• StudyPeriod - text
• Name - text

Code:

#lang racket
(require web-server/servlet
         web-server/servlet-env
         web-server/dispatch
         db
         json
         net/url-structs)

(define PORT 8081)
(define DATABASE "paraplyet.db")

; Database connection
(define paraplyet-db
  (sqlite3-connect #:database DATABASE))

; Convert results from database to json, messy
(define (rows-result->json result)
  (define column-names
    (map (lambda (r) (string->symbol (cdr (first r)))) (rows-result-headers result)))
  (define rows-lists (map vector->list (rows-result-rows result)))
  ; Create a list of hashmaps to be able to use jsexpr->string
  (define result-hashmaps
    (map (lambda (row) (make-hash (map cons column-names row))) rows-lists))
  (jsexpr->string result-hashmaps))

; Mock results from database query, can be used for the function rows-result->json
(define mock-row-result (rows-result
                         '(((name . "AppCode") (decltype . "TEXT"))
                           ((name . "CourseCode") (decltype . "TEXT"))
                           ((name . "StudyPeriod") (decltype . "TEXT"))
                           ((name . "Name") (decltype . "TEXT")))
                         '(#("LTU-37067" "D0004N" "HT2018" "Databaser I")
                           #("LTU-17045" "D0020N" "HT2018" "Utveckling av informationssystem"))))    


; Return list of courses with optional filtering the the parameters CourseCode and StudyPeriod
(define (get-courses req)
  (define params (make-hash (url-query (request-uri req))))
  (define courses (query paraplyet-db "SELECT AppCode, CourseCode, StudyPeriod, Name
                                       FROM Course
                                       WHERE CourseCode = UPPER(COALESCE($1, CourseCode)) AND
                                             StudyPeriod = UPPER(COALESCE($2, StudyPeriod))"
                         (hash-ref params 'CourseCode sql-null)
                         (hash-ref params 'StudyPeriod sql-null)))

  ; Respond with json
  (response/xexpr (rows-result->json courses)
                  #:mime-type (string->bytes/utf-8 "application/json")))

; Return a specific course
(define (get-course req app-code)
  (define courses (query paraplyet-db "SELECT AppCode, CourseCode, StudyPeriod, Name
                                       FROM Course
                                       WHERE AppCode = $1" app-code))

  ; Return 404 if course is missing, otherwise return JSON
  (if (empty? (rows-result-rows courses))
      (response/xexpr "404: Resource not found" #:code 404)
      (response/xexpr (rows-result->json courses)
                      #:mime-type (string->bytes/utf-8 "application/json"))))

; Function to respond with 404
(define (not-found req)
  (response/xexpr "404: Not Found"
                  #:code 404))


; Definition of routing
; /course                  Get many courses (filter with parameters)
; /course/{AppCode}        Get course with specific AppCode
(define-values (go _)
  (dispatch-rules
   [("course" (string-arg)) #:method "get" get-course]
   [("course")              #:method "get" get-courses]
   [else not-found]))

(define (start-server)
  (serve/servlet go
                 #:port PORT
                 #:servlet-regexp #rx""
                 #:launch-browser? #f
                 ))
(start-server)