oracle - SQL -Grab unique column combination from table -

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in oracle, have table called "mytable". table has columns 'a' , 'b'. want find every unique combination of 'a' , 'b'. how this? i'd prefer in sql rather pl/sql.

example:

column | column b

dog           cat cat           dog horse         cat dog           cat

a unique combination above should return 3 rows.

thank you

select distinct columna, columnb table

or 

select columna, columnb table group columna, columnb

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