Takeing unique values from one column in table

Question

I am a beginner in Hashkell ​​and Yesod, and I am interested in how to take all the unique values ​​
from one column in the mysql table.
A concrete example is that I have an Address entity and I want to take all the values ​​for the city attribute :

Address json
    state Text sqltype=varchar(255)
    city Text sqltype=varchar(255)
    street Text Maybe sqltype=varchar(255)
    number Text sqltype=varchar(255)
    deriving Show Typeable

I want to pick up all the values ​​of the city to use them in the form.
I would like to have these values ​​in the dropdown list:

filters <- runInputPost $ ManFilter
        <$> iopt (selectFieldList city) "City"
    defaultLayout [whamlet | <h1> # {show $ filters} |]
 where
  city ​​:: [(Text, Text)]
  city ​​= [("NY", "NY"), ("London", "London"), ("Madrid", "Madrid")]

And another question how to automatically create a tuple city :: [(Text, Text)] from the
values ​​I would get from the database?

If anyone can help I would be grateful, because I'm pretty stuck with the forms.
Also write if it is a good procedure, what is wrong and of course any advice is welcome. Thank you!