Custom Column Validation or DataGrid Cell Validation

Hi to all ,
How can i implement some custom constraint or to make make some
validation of cell in DataGrid.
for example i shave 2 tables and i must check in detail table if one
column is unique only for some related record no all records to be unique.
and second thing is after getting value of one DataGrid cell to run a
query and based on the result to fill other grid cell in the same row.
What is the best way to do this .

Thanks in advance.