feat(backend/db): add util functions

`sortToSQL` and unimplemented `filterToSQL`

backend/db/utils.go

@@ -0,0 +1,53 @@
+package db
+
+import (
+	"fmt"
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+// Convert "filter" URL param to SQL "WHERE" condition
+func filterToSQL(filter string) (sql string, statusCode int, err error) {
+	// filterTokens := strings.Split(string(filter), ";")
+	sql = "(true)"
+	return
+}
+
+// Convert "sort" URL param to SQL "ORDER BY"
+func sortToSQL(sort string) (sql string, statusCode int, err error) {
+	if sort == "" {
+		return
+	}
+	sortOptions := strings.Split(sort, ",")
+	sql = " ORDER BY "
+	for i, sortOption := range sortOptions {
+		sortOrder := sortOption[:1]
+		sortColumn := sortOption[1:]
+		// parse sorting order marker
+		switch sortOrder {
+		case "+":
+			sortOrder = "ASC"
+		case "-":
+			sortOrder = "DESC"
+		default:
+			err = fmt.Errorf("invalid sorting order mark: %q", sortOrder)
+			statusCode = http.StatusBadRequest
+			return
+		}
+		// validate sorting column
+		var n int
+		n, err = strconv.Atoi(sortColumn)
+		if err != nil || n < 0 {
+			err = fmt.Errorf("invalid sorting column: %q", sortColumn)
+			statusCode = http.StatusBadRequest
+			return
+		}
+		// add sorting option to query
+		if i > 0 {
+			sql += ","
+		}
+		sql += fmt.Sprintf("%s %s NULLS LAST", sortColumn, sortOrder)
+	}
+	return
+}