sql中coalesce()函数的使用

1.coalesce()函数的作用就是返回一个不为null的值

2.例如在统计已读消息和未读消息场景就可以使用

    SELECT
     msg.user_id ,
     coalesce(sum(case msg.read_flag when '1' then 1 else 0 end ),0) as `read`,
     coalesce(sum(case msg.read_flag when '0' then 1 else 0 end ),0) as unRead
    FROM
      cto_assist_message msg
    WHERE
      msg.user_id = #{id}
    AND msg.del_flag = '0'
    GROUP BY
      msg.user_id