mysql case when then 的大概用法

select 

id, name,

# case代码块用于排序
# 函数开始
CASE
    # 判断条件  (相当于if)
    WHEN

        # 条件
        `start_time` > '20200727' and `status`= 1 

        # 符合条件 赋值
        THEN
            2


    WHEN 
        start_time <= '20200727' AND end_time > '20200727' and `status` = 1 
        THEN
            1

    when 
        end_time <= '20200727' or `status` = 2
        then
            3
    ELSE
        4

# 函数结束
END
    # 别名
    as `order_column`'


from tablename where order_column in (1, 2, 3)