如何只获取一组数据中的最新记录？

Question

I need an help to solve this SQL problem: I've a dataset like this

Customer     Field A    Date

A            y        2015-01-21

A            z        2015-02-24

B            y        2015-02-01

B            g        2015-02-25

C            z        2015-02-25

C            x        2015-03-27

I would like to get ONLY one row per customer and only the most recent record. So, result must be:

A           z         2015-02-24

B           g         2015-02-25

C           x         2015-03-27

I think I need to get the DISTINCT of all customers and then JOIN them to the same table with a MAX clause on Date field. Something like this:

SELECT DISTINCT customer FROM mytable a
INNER JOIN mytable b ON a.customer = b.customer

but I don't know how to continue...

Answer 1

Or use a correlated sub-query to find a customer's max date:

SELECT *
FROM mytable a
WHERE Date = (select max(Date) from mytable b
              where a.customer = b.customer)

Answer 2

You can self-join on the max date of each customer

SELECT a.*
FROM mytable a 
INNER JOIN 
(
  select customer, max(date_column) as mdate
  from mytable 
  group by customer 
) b ON a.customer = b.customer
   AND a.date_column = b.mdate