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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