determining the staff's title when a certain transaction is done

问题

I have a transaction table with relevant data and username. and also, there is a another table that stores staff's title history by date. However, title history table doesnt have log of every single day.

Here is an example of my transaction table:

username    tran_date   amount
--------    ----------  ------
anderson    16/02/2019  1200
julie       17/02/2019  54000
williams    20/02/2019  32650

and staff's title log table:

username    date        title
--------    ----        ------
anderson    01/01/2019  assistant
anderson    06/04/2019  chief
williams    07/03/2019  chief
julie       15/02/2019  technician
julie       27/05/2019  chief technician

I would like to join with the title of staff on the closest day. as in;

username    tran_date   amount    title
--------    ----------  ------    ------
anderson    16/02/2019  1200      assistant
julie       17/02/2019  54000     technician
williams    20/02/2019  32650     chief

I have tried something blindly but nothing seems to logical :)

thanks for help!

回答1:

In SQL Server, you would express this using outer apply:

select t.username, t.tran_date, l.title
from transactions t outer apply
     (select top (1) l.*
      from log_table l
      where l.username = t.username and l.date <= t.tran_date
      order by l.date desc
     ) l ;

outer apply is SQL Servers equivalent to left join lateral.

来源：https://stackoverflow.com/questions/62127422/determining-the-staffs-title-when-a-certain-transaction-is-done