conversi ms-sql7 vs postgresql 7.3

Hi..

I have table xx:
id debet credit balance
1 1000 0 0
2 2000 0 0
3 0 2500 0
4 0 100 0

command in ms-sql 7 can use calculate field (column) balance from id=1 to
id=4:
"update xx set bal=balance=bal+debet-credit"
result:
id debet credit balance
1 1000 0 1000
2 2000 0 3000
3 0 2500 500
4 0 100 400

How command sql can use in psotgresql 7.3?

thank's a lot
betty.