Racunanje u sql upitu

Pozdrav,

Ako imam recimo:

SELECT tabla.a * tabla.b AS ab, tabla.a * tabla.c AS ac FROM tabla

Kako bi mogao, ako se uopce moze napravit ovo:

SELECT tabla.a * tabla.b AS ab, tabla.a * tabla.c AS ac, ab + ac AS ukupno FROM tabla

da ne moram radit

SELECT tabla.a * tabla.b AS ab, tabla.a * tabla.c AS ac, (tabla.a * tabla.b) + (tabla.a * tabla.c) AS ukupno FROM tabla