SQL query , group by only one column - sql

Question

i want to group this query by project only because there are two records of same project but i only want one.
But when i add group by clause it asks me to add other columns as well by which grouping does not work.
*
DECLARE #Year varchar(75) = '2018'
DECLARE #von DateTime = '1.09.2018'
DECLARE #bis DateTime = '30.09.2018'
select new_projekt ,new_geschftsartname, new_mitarbeitername, new_stundensatz
from Filterednew_projektkondition ps
left join Filterednew_fakturierungsplan fp on ps.new_projekt = fp.new_hauptprojekt1
where ps.statecodename = 'Aktiv'
and fp.new_startdatum >= #von +'00:00:00'
and fp.new_enddatum <= #bis +'23:59:59'
--and new_projekt= Filterednew_projekt.new_
--group by new_projekt
*
look at the column new_projekt . row 2 and 3 has same project, but i want it to appear only once. Due to different other columns this is not possible.
if its of interested , there is another coluim projectcondition id which is unique for both

Answer 1

You can't ask a database to decide arbitrarily for you, which records should be thrown away when doing a group. You have to be precise and specific
Example, here is some data about a person:
Name, AddressZipCode
John Doe, 90210
John Doe, 12345
SELECT name, addresszipcode FROM person INNER JOIN address on address.personid = person.id
There are two addresses stored for this one guy, the person data is repeated in the output!
"I don't want that. I only want to see one line for this guy, together with his address"
Which address?
That's what you have to tell the database
"Well, obviously his current address"
And how do you denote that an address is current?
"It's the one with the null enddate"
SELECT name, addresszipcode FROM person INNER JOIN address on address.personid = person.id WHERE address.enddate = null
If you still get two addresses out, there are two address records that are null - you have data that is in violation of your business data modelling principles ("a person's address history shall have at most one adddress that is current, denoted by a null end date") - fix the data
"Why can't i just group by name?"
You can, but if you do, you still have to tell the database how to accumulate the non-name data that it shows you. You want an address data out of it, it has 2 it wants to show you, you have to tell it which to discard. You could do this:
SELECT name, MAX(addresszipcode) FROM person INNER JOIN address on address.personid = person.id GROUP BY name
"But I don't want the max zipcode? That doesn't make sense"
OK, use the MIN, the SUM, the AVG, anything that makes sense. If none of these make sense, then use something that does, like the address line that has the highest end date, or the lowest end date that is a future end date. If you only want one address on show you must decide how to boil that data down to just one record - you have to write the rule for the database to follow and no question about it you have to create a rule so make it a rule that describes what you actually want
Ok, so you created a rule - you want only the rows with the minimum new_stundenstatz
DECLARE #Year varchar(75) = '2018'
DECLARE #von DateTime = '1.09.2018'
DECLARE #bis DateTime = '30.09.2018'
select new_projekt ,new_geschftsartname, new_mitarbeitername, new_stundensatz
from
(SELECT *, ROW_NUMBER() OVER(PARTITON BY new_projekt ORDER BY new_stundensatz) rown FROM Filterednew_projektkondition) ps
left join
Filterednew_fakturierungsplan fp on ps.new_projekt = fp.new_hauptprojekt1
where ps.statecodename = 'Aktiv'
and fp.new_startdatum >= #von +'00:00:00'
and fp.new_enddatum <= #bis +'23:59:59'
and ps.rown = 1
Here I've used an analytic operation to number the rows in your PS table. They're numbered in order of ascending new_stundensatz, starting with 1. The numbering restarts when the new_projekt changes, so each new_projekt will have a number 1 row.. and then we make that a condition of the where
(Helpful side note for applying this technique in future.. Ff it were the FP table we were adding a row number to, we would need to put AND fp.rown= 1 in the ON clause, not the WHERE clause, because putting it in the where would make the LEFT join behave like an INNER, hiding rows that don't have any FP matching record)