Multiple mainstream SQL queries only take the latest one of the data

Business scenario ：

      ORACLE You can see ：https://blog.csdn.net/qq_25223941/article/details/109358683

      MYSQL You can see ：https://blog.csdn.net/qq_25223941/article/details/109358496

      When there are multiple pieces of data for a certain type , But I just want to take the latest one . When we can determine which type, we use simple sql You can solve it .

      ORACLE-SQL：

      Such as : select * from ( select * from t_table a where a.tpye=? order by a.time desc ) where rownum=1;

      MYSQL-SQL:

       Such as ： select * from t_table a where a.type=?  order by time desc limit1;

      But when we're not sure which type , When full table scanning or multi data scanning is required , It needs to be used oracle The unique function in solves .

     ORACLE-SQL：

      Such as ：select * from (select  a.type,row_number() over(partition by a.type order by a.time desc) as rn from t_table a where xxxx) where rn=1;

     MYSQL-SQL：

      Such as ：select a.type,max(id) from( select * from t_table  order by time desc ) a group by a.type;

      perhaps ： select * from t_table order by time desc limit 1;

    among partition by The following field means to distinguish and group according to this field ,order by Sort ,row_number() over This means to get the number of rows according to the conditions inside ,

    Summarize the meaning of this function ： according to type Field grouping by time After sorting the fields , Access to this type How much data exists in the table （ How many are there means rn How many lines ）rn=1 It means that taking the first row will get the latest data .