Union and Joins

I have 3 tables:
Tabel 1- tblLogin (only relevant fields)
UserName,  BUM (Business Unit Manager), ASM (Area Sales Manager)
Tabel 2 - tblWork1
strSeenBy, strData
Tabel 3 - tblWork2
strSeenBy, strData
Tabel 4 -tblWork3
strSeenBy, strData
strSeenBy = UserName

Table 1 is holding the informations about all users. Table 2-4 holds information about users work. In one day, a user might have records in 1, 2 or all 3 tblWork. What I need is to print a report on how many working days (strData) has each users.
In order to find out how many working days has a single user, I use UNION SELECT DISTINCT strData with all table from 2-4. I don't know how to make is loop through table1 to count this for all the user belonging to an BUM(ASM).
To distinct between ASM and BUM I have a an If-then:
If UserISBUM then
sql for BUM
Else
sql for ASM
End if.