[V4MD] WHERE clause in JOIN request

[Caliste]

SELECT 
tblPerson.idPerson, tblActivityCateg.idMasterCateg
FROM 
tblPerson
LEFT JOIN 
tblPersActivity ON 
tblPerson.idPerson=tblPersActivity.idPerson
LEFT JOIN 
tblActivity ON tblPersActivity.idActivity=tblActivity 
.idActivity
LEFT JOIN 
tblActivityCateg ON tblActivityCateg.idActivityCateg=tblActivity 
.idActivityCateg
 
This request returns 
:
tblPerson.idPerson    
tblActivityCateg.idMasterCateg
    
126                                12
    
127                                12
    
128                                13
    
129                                15
    
130                                14
    
131                            <null>
    
132                            <null>
 
- if I add the WHERE 
clause :
WHERE 
tblActivityCateg.idMasterCateg=12
> It returns 
always the same result as without WHERE clause
 
- if I add the WHERE 
clause :
WHERE 
tblActivityCateg.idMasterCateg IS NOT NULL
>It returns always the same result as without WHERE 
clause
 
Do you know why . is there an issue 
?
 
Thanks