SQL group by on nested select statement - sql

Question

My data looks like this and here's google spread sheet
I want all the rows selected, along with an added extra row which is life of item (per item_code) in terms of weeks i.e. DATEDIFF(day, min(txn_date), txn_date)/7
I tried something like this:
SELECT txn_date, txn_qty, item_code,
( SELECT DATEDIFF(day, min(txn_date), txn_date)/7
FROM my_table
WHERE item_code like 'X6%'
GROUP BY item_code
) weeks_life
FROM my_table
WHERE item_code like 'X6%'
as for output, it should look like this:

Answer 1

I would use DATEDIFF and a MAX OVER clause for this.
declare #TEMP table (transaction_id int, txn_date date, txn_qty int, code char(5))
INSERT INTO #TEMP VALUES ( 1,'2016-09-20',1,'X6113')
INSERT INTO #TEMP VALUES ( 2,'2016-09-22',4,'X6113')
INSERT INTO #TEMP VALUES ( 3,'2016-11-08',7,'X6117')
INSERT INTO #TEMP VALUES ( 4,'2016-12-10',3,'X6117')
INSERT INTO #TEMP VALUES ( 5,'2016-12-22',1,'X6112')
INSERT INTO #TEMP VALUES ( 6,'2017-01-19',2,'X6118')
INSERT INTO #TEMP VALUES ( 7,'2017-02-11',4,'X6119')
INSERT INTO #TEMP VALUES ( 8,'2016-06-30',1,'X6117')
INSERT INTO #TEMP VALUES ( 9,'2016-08-03',5,'X6110')
INSERT INTO #TEMP VALUES (10,'2016-09-11',7,'X6118')
INSERT INTO #TEMP VALUES (11,'2016-10-29',1,'X6110')
INSERT INTO #TEMP VALUES (12,'2016-11-12',335,'X6113')
INSERT INTO #TEMP VALUES (13,'2017-01-06',1,'X6110')
INSERT INTO #TEMP VALUES (14,'2017-02-06',12,'X6113')
select transaction_id
,txn_date
,DATEDIFF(WEEK,MIN(txn_date) OVER(PARTITION BY code order by code),txn_date) as life_weeks
,txn_qty
,code
from #TEMP
References from MSDN
MAX (Transact-SQL)
OVER Clause (Transact-SQL)
DATEDIFF (Transact-SQL)

Answer 2

You can use window functions. I think this is what you want:
SELECT txn_date, txn_qty, item_code,
DATEDIFF(day,
MIN(txn_date) OVER (PARTITION BY item_code),
txn_date
)/7 as weeks_life
FROM my_table
WHERE item_code like 'X6%';

Answer 3

If you want to use an inline subquery like that, you could do this:
select
txn_date
, txn_qty
, item_code
, ceiling((
datediff(day, (
select min(txn_date)
from t i
where i.item_code = t.item_code
), txn_date)
) / 7.0) as weeks_life
from t
where item_code like 'X6%'
order by
item_code
, txn_date
cross apply() version:
select
txn_date
, txn_qty
, item_code
, ceiling((datediff(day, x.min_txn_date, txn_date)) / 7.0) as weeks_life
from t
cross apply (
select min_txn_date = min(txn_date)
from t i
where i.item_code = t.item_code
) as x
where item_code like 'X6%'
order by
item_code
, txn_date
inner join version:
select
t.txn_date
, t.txn_qty
, t.item_code
, ceiling((datediff(day, x.min_txn_date, t.txn_date)) / 7.0) as weeks_life
from t
inner join (
select min_txn_date = min(txn_date), item_code
from t i
group by item_code
) as x
on x.item_code = t.item_code
where t.item_code like 'X6%'
order by
t.item_code
, t.txn_date
rextester demo: http://rextester.com/BYTL99094
returns:
+------------+---------+-----------+------------+
| txn_date | txn_qty | item_code | weeks_life |
+------------+---------+-----------+------------+
| 2016-08-03 | 5 | X6110 | 0 |
| 2016-10-29 | 1 | X6110 | 13 |
| 2017-01-06 | 1 | X6110 | 23 |
| 2016-12-22 | 1 | X6112 | 0 |
| 2016-09-20 | 1 | X6113 | 0 |
| 2016-09-22 | 4 | X6113 | 1 |
| 2016-11-12 | 335 | X6113 | 8 |
| 2017-02-06 | 12 | X6113 | 20 |
| 2016-06-30 | 1 | X6117 | 0 |
| 2016-11-08 | 7 | X6117 | 19 |
| 2016-12-10 | 3 | X6117 | 24 |
| 2016-09-11 | 7 | X6118 | 0 |
| 2017-01-19 | 2 | X6118 | 19 |
| 2017-02-11 | 4 | X6119 | 0 |
+------------+---------+-----------+------------+