add password check query

Relates #2

Author: Nicos

Queries/check_user_password.js

@@ -0,0 +1,14 @@
+const databaseConnection = require("../database/db_connections.js");
+
+const check_user_password = (username, cb) => {
+  databaseConnection.query(
+    `SELECT password FROM users WHERE username=$1`,
+    [username],
+    (err, res) => {
+      if (err) cb(err);
+      else cb(null, res);
+    }
+  );
+};
+
+module.exports = check_user_password;