StartDate and EndDate values are multiplied, Can't get the right values

Question

Building: Im trying to build a report where I can calculate the difference between two dates (Example between 1-1-2001 and 31-12-2001) but also showing (per name per identifier) the current balance of a customer per date(As shown in the Report attachment).
Problem: The problem is that the totals are not correct, they get multiplied on random numbers (Checking it with Count and CountDistinct I learned that I probably made a mistake in my SQL, so I copied SQL in this question as well).
I have tried plenty myself and tried to find solutions on the interwebs, but none of them seem to work.  So I am desperate enough to ask for help :).
SELECT
CASE WHEN t1.identifier IS NULL THEN t2.identifier ELSE t1.identifier
END as identifier
, CASE WHEN t1.firstName IS NULL THEN t2.firstName ELSE t1.firstName
END as firstName
, CASE WHEN t1.lastName IS NULL THEN t2.lastName ELSE t1.lastName
END as lastName
, CASE WHEN t1.initials IS NULL THEN t2.initials ELSE t1.initials
END as initials
, CASE WHEN t1.name IS NULL THEN t2.name ELSE t1.name
END as name
, CASE WHEN t1.employmentNumber IS NULL THEN t2.employmentNumber ELSE t1.employmentNumber
END as employmentNumber
, CASE WHEN t1.externalIdentifier IS NULL THEN t2.externalIdentifier ELSE t1.externalIdentifier
END as externalIdentifier
, t1.balance AS S_balance
, t1.unvested AS S_unvested
, t2.balance AS V_unvested
, t2.unvested AS V_balance
, SUM(t1.TotalBalance + t1.TotalUnvested) Total1
FROM
SELECT
par.identifier
,par.firstName
,par.lastName
,par.initials
,pos.name
,pos.balance
,pos.unvested
,par.employmentNumber
,SUM(pos.balance) TotalBalance
,sum(pos.unvested) TotalUnvested
,Right(par.externalIdentifier,(Len(par.externalIdentifier)-4)) as externalIdentifier
FROM
participant par
INNER JOIN position pos
ON par.identifier = pos.participantIdentifier
WHERE
(pos.balance != 0 OR pos.unvested != 0)
AND pos.name NOT LIKE (@Euro)
AND par.Metadata_start_validity <=@Datum
AND (par.Metadata_end_validity >=@Datum
OR par.Metadata_end_validity IS NULL)
AND pos.Metadata_start_validity <=@Datum
AND (pos.Metadata_end_validity >=@Datum
OR pos.Metadata_end_validity IS NULL)
GROUP BY
par.identifier
,par.firstName
,par.lastName
,par.initials
,pos.name
,pos.balance
,pos.unvested
,par.employmentNumber
,par.externalIdentifier
) as t1
INNER JOIN
SELECT
par.identifier
,par.firstName
,par.lastName
,par.initials
,pos.name
,pos.balance
,pos.unvested
,par.employmentNumber
,Right(par.externalIdentifier,(Len(par.externalIdentifier)-4)) as externalIdentifier
FROM
participant par
INNER JOIN position pos
ON par.identifier = pos.participantIdentifier
WHERE (pos.balance != 0 OR pos.unvested != 0)
AND pos.name NOT LIKE (@Euro)
AND par.Metadata_start_validity <=@V_Datum
AND (par.Metadata_end_validity >=@V_Datum
OR par.Metadata_end_validity IS NULL)
AND pos.Metadata_start_validity <=@V_Datum
AND (pos.Metadata_end_validity >=@V_Datum
OR pos.Metadata_end_validity IS NULL)
) as t2
ON
t1.identifier = t2.identifier
GROUP BY
CASE WHEN t1.identifier IS NULL THEN t2.identifier ELSE t1.identifier
END
, CASE WHEN t1.firstName IS NULL THEN t2.firstName ELSE t1.firstName
END
, CASE WHEN t1.lastName IS NULL THEN t2.lastName ELSE t1.lastName
END
, CASE WHEN t1.initials IS NULL THEN t2.initials ELSE t1.initials
END
, CASE WHEN t1.name IS NULL THEN t2.name ELSE t1.name
END
, CASE WHEN t1.employmentNumber IS NULL THEN t2.employmentNumber ELSE t1.employmentNumber
END
, CASE WHEN t1.externalIdentifier IS NULL THEN t2.externalIdentifier ELSE t1.externalIdentifier
END
, t1.balance
, t1.unvested
, t2.balance
, t2.unvested
, t1.TotalBalance
, t1.TotalUnvested

Answer 1

Please post DDL, so that people do not have to guess what the keys, constraints, Declarative Referential Integrity, data types, etc. in your schema are. Learn how to follow ISO-11179 data element naming conventions and formatting rules (you have no idea,
do you?). Temporal data should use ISO-8601 formats. Code should be in Standard SQL as much as possible and not local dialect. 
This is minimal polite behavior on SQL forums. Did you know that camelCase does not work? Google the research. There are no generic “<nothing in particular>_name”, <magic universal>_identifier”, etc in a valid data model. 
This codes tells us you have attribute splitting in the schema:
 T1.balance AS S_balance, T1.unvested AS S_unvested, 
 T2.balance AS V_unvested, T2.unvested AS V_balance, 
I am sorry you have only one “Participant” and one “Position”, which is what your singular table name tell the world. 
Your “vested” and “unvested” are a status value, but you put them into separate tables. This is as silly as having “Male_Personnel” and “Female_Personnel” tables instead of the correct “Personnel” table. This why you are re-creating the name-address columns
every time! 
What kind of entity is an “external”? You have an “external_identifier”, so there has to be such an entity. Even worse you have:
RIGHT(PAR.external_identifier, (LEN(PAR.external_identifier)-4))
This tells us that this vague column is a concatenation of two or more data elements. This is not a valid schema design. And when you write:
AR.identifier = POS.participant_identifier
Your magical generic “identifier” changes to a participant. In RDBMS, a data element has one and only one name, and doer not change from table to table.  Can it also become a squid? An automobile? 
I see you are an old punch card programmer. Back in the old days, we put one expression per card (line) so we could re-arrange the deck. This also meant that the comma went at the front line. And we even parentheses and keywords on single cards! WOW! 
You do not know about COALESCE() and use CASE the way that you would use IF-THEN in COBOL. Look at this code. Why did you think that “@datum” was a clear, precise data element name? It is the most generic name for a single data element value (not even the element
itself). 
 AND PAR.metadata_start_validity <= @vague_generic_something
 AND (PAR.metadata_end_validity >= @vague_generic_something
 OR PAR.metadata_end_validity IS NULL)
COBOL, C and the other procedural languages that you are trying to write in SQL do no have the SQL shorthand, so you use Boolean operators. We would write:
@vague_generic_something
 BETWEEN PAR.metadata_start_validity 
   AND COALESCE(PAR.metadata_end_validity, @vague_generic_something)
But we never put metadata into a schema. NEVER. “Validity” makes no sense. Is it a date? A physical location? What? It has to be on a scale on which theta operators apply. Your validity hs no ordering! 
You do not know that SQL uses <> and not != from the C family. 
We do not use bit flags; that was assembly language, not SQL. The term “Euro” is a monetary unit of measurement, not value. 
You are getting garbage data because you have a garbage schema. Post the DDL and someone might be able to help you replace it. 
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