SQL query to produce the output shown below

Question

I have to visualize Projected Out of Stock Items and the date it goes out of stock in the inventory.

I have a table which shows when the item may go out of stock. This table shows the Projected Inventory for every Monday of the week.

Item    Location    Projected     Inventory Date
A1         L1           0          Aug 20, 2018
A1         L1           0        August 27, 2018
A1         L1           54        Sep 03,20-18
A1         L1           49        Sep 10, 2018
A1         L1           44          Sep 17
A1         L1           39          Sep 24
A1         L1           32          Oct 1
A1         L1           25          Oct 8
A1         L1           18          Oct 15
A1         L1           12          Oct 22
A1         L1           5           Oct 29
A1         L1           55          Nov 5
A1         L1           45          Nov 12

An item is considered to be Out of Stock if the : Projected Invenotry<20 . Since the data is for every Monday of the week, it may take few weeks for the item to come back on stock(ie PI > 20 ).

As can be seen in the table, Item A1 goes out of stock on Aug 20, 2018 and comes back on stock on Sep 03, 2018. The output table desired is :

Item    Location    PI    Date Out of Stock
A1       L1         0       Aug 20, 2018
A1       L1         18      Oct 15, 2018

This is quite complex for me. Any help would be greatly appreciated.

Regards, Prajwal

Answer 1

You could use (MySQL 8.0+):

WITH cte AS (
  SELECT *, SUM(CASE WHEN Projected < 20 THEN 0 ELSE 1 END)
        OVER(PARTITION BY Item, Location ORDER BY Inventory_Date) AS subgrp
  FROM tab
), cte2 AS (
  SELECT *, ROW_NUMBER() 
          OVER(PARTITION BY Item, Location, subgrp ORDER BY Inventory_Date) AS rn
  FROM cte
)
SELECT *
FROM cte2
WHERE (Item, Location, subgrp,rn) IN (SELECT Item, Location, subgrp, MIN(rn) 
                                      FROM cte2 c 
                                      WHERE Projected < 20
                                      GROUP BY Item, Location, subgrp)
ORDER BY Inventory_Date;

DBFiddle Demo

It will select first value below 20 per group that consists of Item/Location/Streak_of_values_below_20.

Answer 2

Try this:

select @lag := 0;
select item, location, projected, `date` from (
    select @lag projectedLag, @lag:=projected,
           item, location, projected, `date`
    from tab
    order by `date`
) a where projectedLag >= 20 and projected < 20