Question

Users

+--------+--------------+----------+--------------+
| userID | userUsername | userName | userLastName |
+--------+--------------+----------+--------------+
|      6 | richard      | Ricardo  | Vega         |
|     10 | jason        | Jason    | Bourne       |
+--------+--------------+----------+--------------+

Restocks

+-----------+-------------+--------+--------+-----------------+
| restockID | restockDate | itemID | userID | restockQuantity |
+-----------+-------------+--------+--------+-----------------+
|         1 | 2012-02-29  |      1 |      6 |              48 |
|         2 | 2012-02-29  |      1 |     10 |             100 |
|         3 | 2012-02-29  |      2 |     10 |              50 |
|         4 | 2012-02-29  |      2 |      6 |             100 |
|         5 | 2012-02-29  |      2 |      6 |             200 |
|         6 | 2012-02-29  |      2 |     10 |            2000 |
|         7 | 2012-02-29  |      1 |     10 |            2000 |
+-----------+-------------+--------+--------+-----------------+

Items

+--------+--------------------+
| itemID | itemName           |
+--------+--------------------+
|      1 | Coca Cola (lata)   |
|      2 | Cerveza Sol (lata) |
+--------+--------------------+

Ok guys, i have supplied some sample data as requested. I need to get this table:

+--------+--------------------+---------------+-------------+----------+--------------+--------------+
| itemID | itemName           | itemExistence | restockDate | userName | userLastName | userUsername |
+--------+--------------------+---------------+-------------+----------+--------------+--------------+
|      2 | Cerveza Sol (lata) |          2350 | 2012-02-29  | Jason    | Bourne       | jason        |
|      1 | Coca Cola (lata)   |          2148 | 2012-02-29  | Ricardo  | Vega         | richard      |
+--------+--------------------+---------------+-------------+----------+--------------+--------------+

But, i need restockDate to be THE LATEST ONE for each itemName. In the example, it shows the first restock and not the latest one. I just need to show what’s the existence for the item and when was restocked for last time, not first time.

If my tables are not good or so, please suggest a new schema.

I know maybe this is a lot so i will tip 5 USD (Paypal) to the one how can help me with this. Promise.

Answer 1

As discussed in comments, many restocks can be performed on the same day so it is not possible to compare dates in this case. Two options are presented here: Use the incremental PK from restocks table or restructure the table. For the first case, this is the solution:

select i.itemID, i.itemName, i.itemExistence, r.restockDate, u.userName,
  u.userLastName, u.userUsername
from items i
left join (
    select r1.restockDate, r1.itemID, r1.userID from restocks r1
    left join restocks r2
    on r1.itemId = r2.itemId and r1.restockId < r2.restockId
    where r2.restockDate is null
) as r on i.itemID = r.itemID
inner join users u on r.userID = u.userID

For the second case, the restructre would imply changing the date field to a unique datetime that would uniquely identify a record. That is the best solution, however, it does require to also update any previous data present in the table. That means, to update all the records that have the same date for a single product restock and set different date times to them.

The lazy one (like me), would go for the first option 🙂 Let me know if you have any doubt about this.