1.选择客户表前三用户
    SELECT *
    FROM customers
    LIMIT 3

    2.选择客户表从第六个开始，三个用户
    SELECT *
    FROM customers
    LIMIT 6, 3

    3.选择客户表中积分最高的三个用户
    SELECT *
    FROM customers
    ORDER BY points DESC
    LIMIT 3