Questions about Exists-Not exists clause

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I've got, imagine, the next two tables:

table A:

field1 char(3)
field2 varchar(50)
field3 numeric(6)

table B:

field4 date
field5 time
field6 numeric(6)

I need to know which rows in table A are not in table B, and the join
fields are the two numeric ones (field3 and field6).

I've been taking a look at the EXISTS statement, but I can't see how I
should do it.

Some advice??
Thanks in advance.

Josep Sànchez
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