QiutingWang / LeetcodeSQL.github.io

LeetcodeSQL Summarize

• Ranking Problem: 排名函数必须有包含 ORDER BY 的 OVER 语句。

  • ROW_NUMBER()
    • Get a unique sequential number for each row in the specified data
    • By default, it sorts the data in ascending order and starts assigning ranks for each row. -select ROW_NUMBER() OVER(order by column_name desc) as row_num from table_name
  • RANK()
    • Specify rank for each row in the result set对查询出来的记录进行排名
    • select RANK() OVER(order by column_name) as rank from tablename
    • Rank函数考虑到了over子句中排序字段值相同的情况
    • DENSE_RANK()
      • Specify a unique rank number within the partition as per the specified column value.
      • If we have duplicate values,SQL assigns different ranks to those rows as well.dense_rank函数在生成序号时是连续的,dense_rank函数出现相同排名时，将不跳过相同排名号
      • select DENSE_RANK() OVER(order by column_name) as den_rank from tablename
  • NTILE()
    • Distribute the number of rows in the specified (N) number of groups.Specify the value for the desired number of groups.
    • select NTILE(4) OVER(order by columnname desc) as ntile from tablename
    • 对序号进行分组处理，将有序分区中的行分发到指定数目的组中

• Consecutive Problems

  • LAG()
    • Access to a value stored in a different row above the current row.
    • lead(expr, offset, default) Over(Order by columnname) ,default values: offset = 1, default = NULL
    • SELECT seller_name, sale_value, LAG(sale_value) OVER(ORDER BY sale_value) as previous_sale_value FROM sale;
  • LEAD()
    • Accesses a value stored in a row below.
    • SELECT seller_name, sale_value, LEAD(sale_value) OVER(ORDER BY sale_value) as next_sale_value FROM sale;
    • We can also use lag and lead for comparing differences

• Datetime

  • https://www.sqlshack.com/sql-convert-date-functions-and-formats/
  • Datediff(string enddate,string startdate) 日期比较函数：结束日期-开始日期
  • Weekofyear(string date) 日期转周数：得出日期在当前的周数
  • Date_add(string startdate, int days) & Date_sub(string startdate, int days) 得到开始日期后增加/减少的日期
  • year(string date) month(string date) day(string date) hour(string date) minute(string date) second(string date) 返回日期中的年/月/日/小时/秒

• Distribution Functions

  • percent_rank() over (order by columnname)
    • the percentile ranking number of a row—a value in [0, 1] interval: (rank-1) / (total number of rows - 1)
    • cume_dist() over (order by columnname)
    • the cumulative distribution of a value within a group of values

• Logical Order in Operations - Select,From, Where, Group by, Having, Order By, Limit

• Convert data format

  • CAST
    • CAST ( expression AS data_type [ ( length ) ] )
  • Convert
    • CONVERT ( data_type [ ( length ) ] , expression [ , style ] )