if i have a table containing this data:

a b c
_|_|_
1|b|c
2|e|g
3|e|g
4|d|z
5|r|m
6|d|t

and now i want to select all values but only once for the b column

like: SELECT * from table  But same values in d only once;
a b c
_|_|_
1|b|c select
2|e|g select
3|e|g don't because there is already selected a value for b containing e
4|d|z select
5|r|m select
6|d|t dont't because there is already selected a value for b containing d