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SQL

数据科学岗位，需要好好刷SQL题。建议自己下载一个My SQL装到电脑上，模拟真实SQL环境来学习。

对product和metrics的业务了解能够帮助我们更好的准备SQL面试，SQL面试都是围绕着与business相关的metrics展开。

知识点

  • 书写顺序: SELECT -> DISTINCT -> FROM -> JOIN -> ON -> WHERE -> GROUP BY -> HAVING -> ORDER BY -> LIMIT

  • 执行逻辑顺序: FROM -> JOIN -> ON -> WHERE -> GROUP BY -> HAVING -> SELECT -> DISTINCT -> ORDER BY -> LIMIT

  • row number

  • explode

  • Windows function

  • frame clause

  • self-join

  • case when

  • aggregate function

  • WITH common_table_expression

  • rank

  • 数据库优化

    • 创建并使用正确的索引
    • 只返回需要的字段
    • 减少交互次数（批量提交）
    • 设置合理的Fetch Size（数据每次返回给客户端的条数）

  • where和having之后都是筛选条件

    • where在group by前， having在group by 之后
    • 聚合函数（avg、sum、max、min、count），不能作为条件放在where之后，但可以放在having之后

常见问题

  • What is the difference between union and union all? where and having?
  • Question 1: List out the top 3 names of the users who have the most purchase amount on '2018-01-01'
  • Question 2: Sort the table by timestamp for each user. Create a new column named "cum amount" which calculates the cumulative amount of a certain user of purchase on the same day.
  • Question 3: For each day, calculate the growth rate of purchase amount compared to the previous day. if no result for a previous day, show 'Null'.
  • Question 4: For each day, calculate a 30day rolling average purchase amount.
  • Question: what was the friend request acceptance rate for requests sent out on 2018-01-01?

实际

关键配置参数

  • innodb_buffer_pool_size、sync_binlog、innodb_log_file_siz
  • SQL explain 优化
  • python\scala 三个连续双引号表示跨行字符串，.format()

Reference