json_encode之后,json数据每组之间怎么加个逗号

[{"a":"1","b":"2","c":"3"}{"a":"4","b":"5","c":"6"}]怎么变为
[{"a":"1","b":"2","c":"3"},{"a":"4","b":"5","c":"6"}]
也就是每个数据中间加一个逗号

 
$result = $conn->query($sql);
if ($result->num_rows > 0) {
    // 输出数据
    $demo = [];
    while($row = $result->fetch_assoc()) {
            $demo[] = $row; 
        // echo "[{\"result\":\"1\"}]";
                                        //echo "[{\"result\":\"1\"}]";
    }
    echo json_encode($demo);
} else {
    echo "0 结果";
}
$conn->close();