T_SQL Rank function not working as expected - sql-server

Question

once again I really need your expertise.It looks like I am not ranking well here...
I have two records returned and they are both ranked one even though their date time is slightly different. by some minutes.Thanks guys , I really appreciate your help as always.
SELECT CD.MEMACT,
CD.DATETIME,--DATETIME
CD.AG_ID,
RANK() OVER (PARTITION BY
CD.MEMACT,
CD.DATETIME,
CD.AG_ID
ORDER BY CD.DATETIME)RANKED
FROM MEM_ACT_TBL
WHERE CD.MEMACT='1024518'

Answer 1

You provided this sample data as violating your expectations:
MEMACT DATETIME AG_ID RANK
------- --------------- ------- ----
1024518 12/26/2013 7:43 Ag_1541 1
1024518 12/26/2013 7:53 Ag_2488 1
But in your example code, you PARTITION BY CD.MEMACT, CD.DATETIME, CD.AG_ID. This explicitly means to treat all rows that do not share these values as completely separate ranking series. Since the two rows you presented above do not have the same MEMACT, DATETIME, and AG_ID values, then they each start their ranking from 1.
If you were expecting the two rows above to ascend in rank, then you must not partition by anything but MEMACT, since only that column is the same between the rows. You would then most likely order by DATETIME and then AG_ID (to get determinism if you have two rows with the same date) like so:
PARTITION BY CD.MEMACT ORDER BY CD.DATETIME, CD.AG_ID
It never makes sense in any of the ranking functions to both PARTITION BY and ORDER BY the same column as this will ensure that the ORDER BY has no differing data to work with as you've already segregated all the separate series by the same column.

Answer 2

Without seeing your data I could say it is due to the partitioning method. Your 'partitioning' statement is essentially WHAT you group by to determine number position resetting. The 'order by' determines the sequence of HOW you are ordering that data. See this simple example where in my first windowed function I use one too many partition by's and thus just show a whole bunch of redundant ones.
Simple self extracting example:
declare #Person Table ( personID int identity, person varchar(8));
insert into #Person values ('Brett'),('Sean'),('Chad'),('Michael'),('Ray'),('Erik'),('Queyn');
declare #Orders table ( OrderID int identity, PersonID int, Desciption varchar(32), Amount int);
insert into #Orders values (1, 'Shirt', 20),(1, 'Shoes', 50),(2, 'Shirt', 22),(2, 'Shoes', 52),(3, 'Shirt', 20),(3, 'Shoes', 50),(3, 'Hat', 20),(4, 'Shirt', 20),(5, 'Shirt', 20),(5, 'Pants', 30),
(6, 'Shirt', 20),(6, 'RunningShoes', 70),(7, 'Shirt', 22),(7, 'Shoes', 40),(7, 'Coat', 80)
Select
p.person
, o.Desciption
, o.Amount
, row_number() over(partition by person, Desciption order by Desciption) as [Wrong I Used Too Many Partitions]
, row_number() over(partition by person order by Desciption) as [Correct By Alpha of Description]
, row_number() over(partition by person order by Amount desc) as [Correct By Amount of Highest First]
from #Person p
join #Orders o on p.personID = o.PersonID
I would guess you could do this instead:
SELECT
CD.MEMACT,
CD.DATETIME,--DATETIME
CD.AG_ID,
RANK() OVER (PARTITION BY CD.MEMACT ORDER BY CD.DATETIME, CD.AG_ID) RANKED
FROM MEM_ACT_TBL
WHERE CD.MEMACT='1024518'