decode and where clause

here is my question about decode. thanks in advance for your help and patience.
i want to reach details of an accused person|firm details according to the type column in accused table related with a specific file. if type in accused table is 'CON' (consultant) or 'STA' (staff), details are taken from accused_person_details table by type and no. if type in accused table is 'FIR' (firm) details are taken from accused_firm_details table by no. if type is 'OTH' (other), details (name, surname) are in accused table.

 SELECT accused_table.file_type,accused_table.file_number, DECODE( accused_table.type,'FIR',accused_firm.firm_name,'CON', accused_person_details.no||' '||accused_person_details.name||'
'||accused_person_details.surname,'STA', accused_person_details.no||'
'||accused_person_details.name||' '||accused_person_details.surname,'OTH',
accused_table.firstname||' '||accused_table.surname) FROM accused, accused_person_details, accused_firm_details WHERE (accused_table.file_type='IDA') and (accused.file_number=6) and ( ( accused.type.no=hfirma.tax_number) or ( ( accused.type=accused_person_details.type) and (accused.no=accused_person_details.no) ) );

accused table ;

 file_type            VARCHAR2(3)
 file_number       NUMBER(6)
....
 name           VARCHAR2(20)
 surname            VARCHAR2(20)
 type                  VARCHAR2(3)
 no                     VARCHAR2(12)

accused_person_details table;
no                       VARCHAR2(12)
type                    VARCHAR2(3)
name             VARCHAR2(20)
surname              VARCHAR2(20)

accused_firm_details table;
tax_number        VARCHAR2(12)
firm_name          VARCHAR2(50)