Same table Alias in Union parts

[PAscal]

Hello,
here is a problem met with an union query

    select * from a,b, c
    where a.l=b.l
    and b.m=c.m
    union
    select * from a,b,(select * from x) c
    where a.l=b.l
    and b.m=c.m

it can be solved simply by surrounding the whole query by (…)

Regards
PAscal