fix: add implementation for buildGroupBy

Signed-off-by: Muhammad Aaqil <aaqilcs102@gmail.com>

Author: aaqilniz

lib/sql.js

@@ -1260,6 +1260,16 @@ SQLConnector.prototype.buildFields = function(model, data, excludeIds) {
   return this._buildFieldsForKeys(model, data, keys, excludeIds);
 };
 
+/**
+ * Build a part of query for group by
+ * @param {String[]} groupBy Array of column names to be grouped by
+ * @returns a part of query for group by as a string
+ */
+SQLConnector.prototype.buildGroupBy = function(groupBy) {
+  const groupByColumns = Object.keys(groupBy);
+  return 'GROUP BY ' + groupByColumns.join(',');
+};
+
 /**
  * Build an array of fields for the database operation from data array
  * @param {String} model Model name