express+mysql异步调用返回的问题

Question

加上return语句就报错，不加就ok，这是为什么？

router.post('/aaaa', upload.any(), function (req, res, next) {
    sql.getConnection(function (err, connection) {
        if (err) throw err;
        connection.query({
            sql: "某个sql更新语句",
            timeout: 40000, // 40s
            values: [111, 0, 10]
        }, function (error, results, fields) {
            if (error) throw error;
            res.send(JSON.stringify({
                resultCode: '00000',
                resultMsg: '用户信息查询成功',
                data: results
            }));
        });
        connection.release();
    });
    //return;
});