SQL join: selecting the last records in a one-to-many relationship

Question

Suppose I have a table of customers and a table of purchases. Each purchase belongs to one customer. I want to get a list of all customers along with their last purchase in

Answer 1

Without getting into the code first, the logic/algorithm goes below:

1. Go to the transaction table with multiple records for the same client.

2. Select records of clientID and the latestDate of client's activity using group by clientID and max(transactionDate)

      select clientID, max(transactionDate) as latestDate 
      from transaction 
      group by clientID
   

3. inner join the transaction table with the outcome from Step 2, then you will have the full records of the transaction table with only each client's latest record.

      select * from 
      transaction t 
      inner join (
        select clientID, max(transactionDate) as latestDate
        from transaction 
        group by clientID) d 
      on t.clientID = d.clientID and t.transactionDate = d.latestDate) 
   

4. You can use the result from step 3 to join any table you want to get different results.