Select example

(ns bits.sql-syntax)

(def test-db [{:id 1, :name "A"},
              {:id 0, :name "B"},
              {:id 3, :name "C"},
              {:id 2, :name "D"},
              {:id 4, :name "E"}])

(defn where [arg-vec]
  (let [[k f comp-arg] arg-vec]
    (fn [elem] (f (k elem) comp-arg))))

(defn select [selection & args]
  (let [arg-map (into {} (map vec (partition 2 args)))
        {from :from, where-vec :where, order-key :order-by} arg-map]
    (if from
      (let [filt-ordered
            (cond->> from
              where-vec (filter (where where-vec))
              order-key (sort-by order-key))]
        (mapcat #(map % filt-ordered) selection)))))

(select [:name] :from test-db :where [:id < 2] :order-by :id)
 ; Prints ("B" "A")

(defmacro select ; define the macro that acts as a select command
[vara _ coll _ wherearg _ orderarg]
`(filter ;call filter command to select only those entries that match the criteria in where arg. Unquote filter block so it is not executed here
((fn ; The first argument to filter is this internally created function
[[word func arg]] ;which takes three arguments, an identifier(:id), a function(>) and a value(2)
(~func (~word ~coll) ~arg) ;call the passed in function(func) on each id in coll(persons)
) (first ~wherearg) (second ~wherearg) (third ~wherearg) ); the wherearg parameter (:id < 2) is passed into this internally created function
) ~coll ; the second argument to filter is coll(persons)
)

(defmacro select
[vara _ coll _ wherearg _ orderarg]
`(filter
((fn
[[word func arg]]
(~func (~word ~coll) ~arg)
) ~wherearg )
) ~coll
)

You shouldn't be unquoting func. This runs without error, although I haven't tested if it actually works:

(defmacro select
  [vara _ coll _ wherearg _ orderarg]
  `(filter
     (fn [[word# func# arg#]]
       (func# (word# ~coll) arg#))
     ~wherearg)
  ~coll)