Hi,

Sorry by my mistake, as a matte of fact the fields
ending with "_matr" are from table "matricula", as
the "_aluno" are from table "aluno" and "_paramper"
are from the table "param_per"

Completing my previous message: if I change the positions
of "matricula" and "aluno" (between "from" and "join")
I get the erro on the fourth line fetched.

The IBExpert shows the following plans:

Plan
PLAN JOIN (JOIN (MATRICULA NATURAL, ALUNO INDEX (RDB$PRIMARY3)),
PARAM_PER INDEX (RDB$PRIMARY14))

Adapted Plan
PLAN JOIN (JOIN (MATRICULA NATURAL, ALUNO INDEX (INTEG_8)),
PARAM_PER INDEX (INTEG_38))

But the Pk of PARAM_PER is periodoano_paramper, which is
a Char(6)!

Regards.

Luciano RM