Select unique pair of string regarless of the order

Question

I have an sql table composed of unique pair of strings:

Table: 
("a","b")
("a","c")
("a","d")
("b","a")
("d","a")

How can I select the unique element regardless of the order and produce th following output:

Output: 
("a","b")
("a","c")
("a","d")

i.e. unique pair