add check username query

Relates to #2

Author: Nicos

Queries/check_username.js

@@ -0,0 +1,14 @@
+const databaseConnection = require("../database/db_connections.js");
+
+const check_username_exists = (username, cb) => {
+  databaseConnection.query(
+    `SELECT CASE WHEN EXISTS(SELECT username FROM users WHERE username = $1) THEN CAST (true AS BOOLEAN) ELSE CAST (false AS BOOLEAN) END`,
+    [username],
+    (err, res) => {
+      if (err) cb(err);
+      else cb(null, res.rows);
+    }
+  );
+};
+
+module.exports = check_user_exists;