Get Max date by another column - sql

Question

I am trying to write a simple query to get the MAX DEMAND_DATE for each INV_CART_ID. Here is my existing query:
SELECT BUSINESS_UNIT, INV_CART_ID, INV_ITEM_ID, CART_COUNT_QTY, DEMAND_DATE
FROM PS_CART_CT_INF_INV A
WHERE A.INV_ITEM_ID = 1
AND A.BUSINESS_UNIT = '11MMS'
AND A.CART_COUNT_QTY <> 0
ORDER BY DEMAND_DATE DESC
Current Output:
Desired Output:
BUSINESS_UNIT INV_CART_ID INV_ITEM_ID CART_COUNT_QTY DEMAND_DATE
11MMS 405 1 5.0000 2018-05-29
11MMS OUTPT_INFUSION 1 4.0000 2018-05-29
11MMS 938 1 15.0000 2018-05-31
11MMS 286 1 1.0000 2018-05-07
11MMS 708 1 4.0000 2018-04-05
This is what I have tried doing so far:
SELECT MAX(DEMAND_DATE) AS DEMAND_DATE, INV_CART_ID, BUSINESS_UNIT,
INV_ITEM_ID, CART_COUNT_QTY
FROM PS_CART_CT_INF_INV A
WHERE A.INV_ITEM_ID = 1
AND A.BUSINESS_UNIT = '11MMS'
AND A.CART_COUNT_QTY <> 0
AND A.DEMAND_DATE IN (SELECT MAX (DEMAND_DATE) FROM PS_CART_CT_INF_INV B
WHERE A.INV_ITEM_ID = B.INV_ITEM_ID GROUP BY INV_CART_ID)
GROUP BY INV_CART_ID, BUSINESS_UNIT, INV_ITEM_ID, CART_COUNT_QTY
However it doesn't return all INV_CART_ID #'s and is not retrieving the correct row (wrong DEMAND_DATE):

Answer 1

Use ROW_NUMBER:
WITH cte AS (
SELECT BUSINESS_UNIT, INV_CART_ID, INV_ITEM_ID, CART_COUNT_QTY, DEMAND_DATE,
ROW_NUMBER() OVER (PARTITION BY INV_CART_ID ORDER BY DEMAND_DATE DESC) rn
FROM PS_CART_CT_INF_INV
WHERE
INV_ITEM_ID = 1 AND
BUSINESS_UNIT = '11MMS' AND
CART_COUNT_QTY <> 0
)
SELECT
BUSINESS_UNIT, INV_CART_ID, INV_ITEM_ID, CART_COUNT_QTY, DEMAND_DATE
FROM cte
WHERE rn = 1
ORDER BY DEMAND_DATE DESC;
If you don't want to use analytic functions, then I still would not use your current approach. Instead, I would join to a subquery, like this:
SELECT
t1.BUSINESS_UNIT,
t1.INV_CART_ID,
t1.INV_ITEM_ID,
t1.CART_COUNT_QTY,
t1.DEMAND_DATE
FROM PS_CART_CT_INF_INV t1
INNER JOIN
(
SELECT INV_CART_ID, MAX(DEMAND_DATE) AS MAX_DEMAND_DATE
FROM PS_CART_CT_INF_INV
WHERE INV_ITEM_ID = 1 AND BUSINESS_UNIT = '11MMS' AND CART_COUNT_QTY <> 0
GROUP BY INV_CART_ID
) t2
ON t1.INV_CART_ID = t2.INV_CART_ID AND t1.DEMAND_DATE = t2.MAX_DEMAND_DATE
WHERE
t1.INV_ITEM_ID = 1 AND
t1.BUSINESS_UNIT = '11MMS' AND
t1.CART_COUNT_QTY <> 0;
The issue with your current query, even once corrected, is that it is using a correlated subquery in the WHERE clause. These are known to be potential performance killers, and so should be avoided if possible.

Answer 2

I think you want :
SELECT BUSINESS_UNIT, INV_CART_ID, INV_ITEM_ID, CART_COUNT_QTY, DEMAND_DATE
FROM PS_CART_CT_INF_INV AS a
WHERE INV_ITEM_ID = 1 AND BUSINESS_UNIT = '11MMS' AND
CART_COUNT_QTY <> 0 AND
DEMAND_DATE = (SELECT MAX(b.DEMAND_DATE)
FROM PS_CART_CT_INF_INV as b
WHERE a.INV_CART_ID = b.INV_CART_ID
);
However, this would gives you duplicate records, if you want to avoid duplicates then you can use identity column or pk instead in WHERE clause :
. . .
WHERE pk = (SELECT TOP (1) b.pk
FROM PS_CART_CT_INF_INV as b
WHERE a.INV_CART_ID = b.INV_CART_ID
ORDER BY b.DEMAND_DATE DESC
);