SQL查询为每个产品选择最后一个条目，最后一个选择最后一个

Question

Good Afternoon,

I'm trying to build a simple inventory system and i wish i could have a query able to select the last entry for each product (component ; ref) and also the one just before the last one for each product (to compare).

This is what i have so far :

My query so far is :

SELECT u1.*
FROM $usertable u1
JOIN ( 
SELECT component, ref, MAX(date) date
FROM $usertable
GROUP BY component, ref 
) u2
USING(component, ref, date) ORDER BY component ASC, ref ASC

Can you please help me to figure out the solution ?

Answer 1

You can do so by using a subquery in where clause

SELECT u1.*
FROM $usertable u1
WHERE (
        SELECT  COUNT(*) 
        FROM $usertable u2
        WHERE u2.component= u1.component
         AND u2.ref= u1.ref
         AND  u2.date>= u1.date
        ) <= 2
ORDER BY component ASC, ref ASC

Answer 2

It should work. http://www.sqlfiddle.com/#!2/288980/1

You could try specifying the Join condition.

SELECT u1.*
FROM $usertable u1
JOIN ( 
    SELECT component, ref, MAX(date) date
    FROM $usertable
    GROUP BY component, ref 
      ) u2
  on u2.component = u1.component
  and u2.ref = u1.ref
  and u2.date = u1.date
ORDER BY component ASC, ref ASC

http://www.sqlfiddle.com/#!2/288980/1