Relational Algebra to SQL Subqueries

-- NAME         PYTHON-NOTATION                             HASKELL-NOTATION
-- selektion:   S[a.name != b.name]( ... )                  S String Rel
-- projektion:  P[a.name, b.budget]( ... )                  P String Rel
-- join:        J[a.budget = b.budget, proj]( RELA, RELB )  J String String Rel Rel
-- rename:      R[budget->geld, name->zeug]( ... )          R [(String,String)] Rel
-- from:        F[a]( ... )                                 F String Rel
--              T[""]()                                     T String
--
-- Beispiel: (P "a.name,a.einnahmen,b.alter,CONCAT(b.vname, b.nname)" (J "a.angestellter = b.id" "a.name, a.einnahmen, b.alter, b.vname, b.nname" (T "a") (T "b")))

data Rel = T String | F String Rel | R [(String,String)] Rel | J String String Rel Rel | P String Rel | S String Rel deriving Show

render rel                  = "SELECT * FROM " ++ (render' rel)

render' (T a)               = a
render' (F new rel)         = (render' rel) ++ " AS " ++ new
render' (R a rel)           = "(SELECT " ++ (rename a) ++ " FROM " ++ (render' rel) ++ ")"
render' (J cond sel a b)    = "(SELECT " ++ sel ++ " FROM " ++ (render' a) ++ ", " ++ (render' b) ++ " WHERE " ++ cond ++ ")"
render' (P sel rel)         = "(SELECT " ++ sel ++ " FROM " ++ (render' rel) ++ ")"
render' (S cond rel)        = "(SELECT * FROM " ++ (render' rel) ++ " WHERE " ++ cond ++ ")"

rename []           = ""
rename ((o,n):[])   = o ++ " AS " ++ n
rename ((o,n):rs)   = o ++ " AS " ++ n ++ ", " ++ (rename rs)