Example of getting UTF-8 working with next.jdbc with MySQL, to enable Unicode and emojis, along with using using "REPLACE" instead of "INSERT" to enable upserts.

db.clj

(ns db.db2
  (:require
    [next.jdbc :as j]
    [next.jdbc.sql :as jsql]
    [next.jdbc.sql.builder :as jb]
    [clojure.spec.alpha :as s]))

(def mysql-db {:dbtype   "mysql"
               :dbname   "twitter"
               :host     "127.0.0.1"
               :user     "xxx"
               :password "xxx"
               :useUnicode "yes"
               :characterEncoding "UTF-8"})

(def ds (j/get-datasource mysql-db))

(defn insert-tweets-with-conn!
  [conn tweets]
  (let [ks [:id :tweet :hash256]
        rows (map #((apply juxt ks) %) tweets)
        ;_ (println rows)
        q (jb/for-insert-multi :tweets ks rows {})
        ;_ (println q)
        qr (clojure.string/replace (first q) #"^INSERT" "REPLACE")
        ;_ (println qr)
        qnew (assoc-in q [0] qr)
        _ (println qnew)]
    (j/execute! conn qnew)))

(def insert-tweets! (partial insert-tweets-with-conn! ds))


(commment
  (insert-tweets! [{:id 1235 :tweet "abc🎉" :hash256 "abcxxx"}])
  )

@seancorfield also wrote — I think this is what every dev (and DBA) should know about modern realities of strings and their storage in databases

(we have a couple of columns still on utf8 in our biggest tables because running the ALTER TABLE will lock the table long enough to cause disruption to our users -- at some point we'll probably tackle that during a scheduled maintenance window... but we very rarely have scheduled maintenance any more now that our processes are nearly all Clojure apps running in a (small) cluster!)