How to pull data based on current and last update? - sql

Question

Our data table looks like this:
Machine Name
Lot Number
Qty
Load TxnDate
Unload TxnDate
M123
ABC
500
10/1/2020
10/2/2020
M741
DEF
325
10/1/2020
M123
ZZZ
100
10/5/2020
10/7/2020
M951
AAA
550
10/5/2020
10/9/2020
M123
BBB
550
10/7/2020
I need to create an SQL query that shows the currently loaded Lot number - Machines with no Unload TxnDate - and the last loaded Lot number based on the unload TxnDate.
So in the example, when I run a query for M123, the result will show:
Machine Name
Lot Number
Qty
Load TxnDate
Unload TxnDate
M123
ZZZ
100
10/5/2020
10/7/2020
M123
BBB
550
10/7/2020
As you can see although Machine Name has 3 records, the results only show the currently loaded and the last loaded. Is there anyway to replicate this? The Machine Name is dynamic, so my user can enter the Machine Name and see the results the machine based on the missing Unload TxnDate and the last Unload Txn Date

Answer 1

You seem to want the last two rows. That would be something like this:
select t.*
from t
where machine_name = 'M123'
order by load_txn_date desc
fetch first 2 rows only;
Note: not all databases support the first first clause. Some spell it limit, or select top, or even something else.

Answer 2

If you want two rows per machine, one option uses window functions:
select *
from (
select t.*,
row_number() over(
partition by machine_name, (case when unload_txn_date is null then 0 else 1 end)
order by coalesce(unload_txn_date, load_txn_date) desc
) rn
from mytable t
) t
where rn = 1
The idea is to separate rows between those that have an unload date, and those that do not. We can then bring the top record per group.
For your sample data, this returns:
Machine_Name | Lot_Number | Qty | Load_Txn_Date | Unload_Txn_Date | rn
:----------- | :--------- | --: | :------------ | :-------------- | -:
M123 | BBB | 550 | 2020-10-07 | null | 1
M123 | ZZZ | 100 | 2020-10-05 | 2020-10-07 | 1
M741 | DEF | 325 | 2020-10-01 | null | 1
M951 | AAA | 550 | 2020-10-05 | 2020-10-09 | 1

Answer 3

You might use the following query, presuming that you're on a database having Window(or Analytic) Function
WITH t AS
(
SELECT COALESCE(Unload_Txn_Date -
LAG(Load_Txn_Date) OVER
(PARTITION BY Machine_Name ORDER BY Load_Txn_Date DESC),0) AS lg,
MAX(CASE WHEN Unload_Txn_Date IS NULL THEN Load_Txn_Date END) OVER
(PARTITION BY Machine_Name) AS mx,
t.*
FROM tab t
), t2 AS
(
SELECT DENSE_RANK() OVER (ORDER BY mx DESC NULLS LAST) AS dr, t.*
FROM t
WHERE mx IS NOT NULL
)
SELECT Machine_Name,Lot_Number,Qty,Load_Txn_Date,Unload_Txn_Date
FROM t2
WHERE dr = 1 AND lg = 0
ORDER BY Load_Txn_Date
where if previous row's Unload_Txn_Date is equal to the current Load_Txn_Date, then it's accepted that there's no interruption will occur for the job, while determining the last Unload Txn Dates with no unload date values per each machine. And then, the result set returns through filtering by the values derived from the window functions within the penultimate query.
Demo