SQL queries in Clojure with next.jdbc library

Using next.jdbc to connect to a database and run queries only a few steps

  • add seancorfield/next.jdbc as a project dependcy
  • require the seancorfield/next.jdbc in the relevant project namespace definitions
  • define a database specification (hash-map of database details or JDBC string)
  • create a connection (optionally using a connection pool)
  • execute SQL statements (individual, batch, transaction)

Clojure connect to database

next.jdbc supersedes clojure.java.jdbc

seancorfield/next.jdbc supersedes clojure.java.jdbc which used to be the defacto library for database backed projects. next.jdbc is faster and exposes a more modern API design (according to the author of clojure.java.jdbc). Migration from clojure.java.jdbc is documented on the next.jdbc repository

Live Coding example

Summary of using next.jdbc

Include ... as a dependency in the project

{:deps
 {org.clojure/clojure    {:mvn/version "1.10.1"}
 seancorfield/next.jdbc {:mvn/version "1.1.569"}}}

Require next.jdbc into the project namespace

(ns practicalli.database-access
  (:require [next.jdbc :as jdbc]))

Specify the database connection

Define a data source connection using a next.jdbc hash map or a JDBC URL

An example next.jdbc specification for H2 database

{:dbtype "h2" :dbname "banking-on-clojure"}

An example JDBC connection string for postgres database

"jdbc:postgresql://<hostname>:port/<database-name>?user=<username>&password=<password>&sslmode=require"

Running SQL queries

execute! runs an SQL statement and returns the results as a vector of hash maps. The hash maps use table and column name to create qualified keywords in the results.

A Clojure string contains the SQL statement.

(jdbc/execute!
      connection
      [(str "insert into account_holders(
               account_holder_id,first_name,last_name,email_address,residential_address,social_security_number)
             values(
               '" account-holder-id "', 'Jenny', 'Jetpack', 'jen@jetpack.org', '42 Meaning Lane, Altar IV', 'AB101112C' )")])

Hint::Datafy results

Hash maps returned by execute! use Datafy and are therefore navigable using Clojure data browsers

Using connections and queries effectively

Define a name for the database connection using the form (jdbc/get-datasource {:dbtype "..." :dbname "..." ...})

(def db-spec (jdbc/get-datasource {:dbtype "h2" :dbname "banking-on-clojure"}))

Use the with-open Clojure core function to automatically close connections after running SQL expressions

(with-open [connection (jdbc/get-connection db-spec)]
      (jdbc/execute! connection [...]))

Defining a generic function provides a simple way to run any SQL query for a specified data base connection.

(defn query-database
  [db-spec sql-statement]
  (with-open [connection (jdbc/get-connection db-spec)]
    (jdbc/execute! connection sql-statement)))

Using next.jdbc friendly functions

next.jdbc provides higher level abstractions over execute! function. These friendly functions take a database connection, a table name as a Clojure keyword and a hash map that contains the values in the query. As the query is a hash map it can also be represented by a Clojure specification (clojure.spec or Malli).

Function names ending with a bang, !, change the contents of the database

  • insert! - insert new rows
  • query - read data
  • update! - update existing rows
  • delete! - remove rows

Generic insert function with next.jdbc.sql

To save repetition, define a generic function that uses insert and takes a database table name, data to insert and the database connection.

(defn insert-data
  [db-spec table record-data]
  (with-open [connection (jdbc/get-connection db-spec)]
    (jdbc-sql/insert! connection table record-data)))

Call the generic insert function with the database connection, table name and query specification

(insert-data
  db-spec
  :public.account_holders
  {:account_holder_id      (java.util.UUID/randomUUID)
   :first_name             "Rachel"
   :last_name              "Rocketpack"
   :email_address          "rach@rocketpack.org"
   :residential_address    "1 Ultimate Question Lane, Altar IV"
   :social_security_number "BB104312D"} )

Transactions for multiple queries

TODO: add example of using transactions for multiple queries

HINT::next.jdbc getting started guide

next.jdbc getting started guide is very detailed.

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