数据库相关-底层原理-手写sql，sql优化

手写sql：常见的group by ，计数，去重，
1.字段出现次数：

SELECT
  *
FROM
  payment_flow
WHERE
  flow_id in (
    select
      flow_id
    from
      payment_flow
    group by
      flow_id
    having
      count(flow_id) > 1
  )
  and DIRECTION = '1'
ORDER by
  id DESC

2.按照多字段是否重复的条件查询对应数据：
https://blog.youkuaiyun.com/anya/article/details/6407280；
3.binlog-redolog-undolog相关：
https://mp.weixin.qq.com/s/oIlMz0VQTOVtKaWktYffqQ