You are trying to use T3 before its existence (unfortunately VFP doesn't support such SQL):
Select T1.mdag1, T2.mdag2, T3.mdag3, T4.mdag4, T5.mdag5, T6.mdag6, T7.mdag7 ,;
Nvl(T1.mdagdeel, nvl(T2.mdagdeel, nvl(T3.mdagdeel, nvl(T4.mdagdeel, nvl(T5.mdagdeel, nvl(T6.mdagdeel, T7.mdagdeel)))))) As mdagdeel, ;
Sys(2015) As newId ;
from (Select mdag1, mdagdeel ;
from crsReport0 ;
where !Empty(mdag1)) T1 ;
full Join;
(Select mdag2, mdagdeel ;
from crsReport0 ;
where !Empty(mdag2)) T2 On ;
T1.mdagdeel = T2.mdagdeel ;
full Join ;
(Select mdag3, mdagdeel ;
from crsReport0 ;
where !Empty(mdag3)) T3 On ;
Nvl(T1.mdagdeel, T2.mdagdeel) = T3.mdagdeel ;
full Join ;
(Select mdag4, mdagdeel ;
from crsReport0 ;
where !Empty(mdag4)) T4 On ;
Nvl(T1.mdagdeel, nvl(T2.mdagdeel, T3.mdagdeel)) = T4.mdagdeel ;
full Join ;
(Select mdag5, mdagdeel ;
from crsReport0 ;
where !Empty(mdag5)) T5 On ;
Nvl(T1.mdagdeel, nvl(T2.mdagdeel, nvl(T3.mdagdeel, T4.mdagdeel))) = T5.mdagdeel ;
full Join ;
(Select mdag6, mdagdeel ;
from crsReport0 ;
where !Empty(mdag6)) T6 On ;
Nvl(T1.mdagdeel, nvl(T2.mdagdeel, nvl(T3.mdagdeel, nvl(T4.mdagdeel, T5.mdagdeel)))) = T6.mdagdeel ;
full Join ;
(Select mdag7, mdagdeel ;
from crsReport0 ;
where !Empty(mdag7)) T7 On ;
Nvl(T1.mdagdeel, nvl(T1.mdagdeel, nvl(T2.mdagdeel, nvl(T3.mdagdeel, nvl(T4.mdagdeel, nvl(T5.mdagdeel, T6.mdagdeel)))))) = T7.mdagdeel ;
Into Cursor crsReport ;
order by 8 ;
nofilter
