Postgres

Postgres is a relational database management system.

Getting Started

This tutorial will be for getting started with Postgres using node on a Mac.

Local Machine

Installation

Install Postgres using homebrew: brew install postgres

In a new terminal window, start up the local database: postgres -U username -d database_name, filling in the blanks as needed. If this does not work, ensure you have the local database installed at your username. The DB can be also started using brew services start postgres (Be sure to stop the server when completed with your work using brew services stop postgres).

To enter into interactive mode, in your terminal, enter psql. From here, you can enter in any queries you'd like. Ensure they all end with a semicolon, or else you will not get any return back and it will throw.

Initialization

Create a new table in your database: CREATE TABLE numbers( age integer );

Insert a value into the new table: INSERT INTO numbers VALUES (732);

Javascript

Initialize a new npm project, and then install pg, pg-format, and express.

Create a new file in your project called server.js and copy this into it's contents:

const express = require('express'); // Server
var pg = require('pg');             // Postgres 
var format = require('pg-format');  // For dynamic SQL queries (a la MySQL ?)

const app = express();              // Start your server

var PGUSER = '<your username>';     // Definte the username and database
var PGDATABASE = '<your username>'; // previously set up in Postgres.

var age = 732;                      // Used so we can reference our earlier insertion.

var config = {
  user: PGUSER,
  database: PGDATABASE,
  max: 10,                          // max number of clients in the pool
  idleTimeoutMillis: 30000          // how long a client is allowed to remain idle before being closed
};

var pool = new pg.Pool(config);     // Create a new connection instance

pool.connect(function (err, client, done) { // Connect to the DB
  if (err) {
    console.log(err);
  }
  app.listen(3000, function () {     // Connect to the Express server
    console.log('listening on 3000')
  });
  // Build a dynamic query using `age`
  var ageQuery = format('SELECT * from numbers WHERE age = %L', age);
  // Query the DB for all rows from the numbers table where age is = 732
  client.query(ageQuery, function (err, result) {
    if (err) {
      console.log(err);
    }
    console.log(result.rows[0]);
  });
});

Now in your terminal, run node server.js and assuming the database is still running, if should return:

listening on 3000
{ age: 732 }

Troubleshooting

If you use the types bigserial or bigint, you'll notice they return as strings. This is because Javascript can't natively handle 64-bit integers, so it will stringify these types instead. Javascript can do 2^53 before losing precision, so as long as you don't have numbers over 9 quadrillion, you can use the native Number type in Javascript (there is the Javascript BigInt type, but it has it's drawbacks, including the inability to interact with other numbers directly). To do this, you can set a custom parser for that type[9,10]:

var types = require('pg').types
types.setTypeParser(20, Number);

Schemas

Schemas are within a database but are ways of organizing tables, objects, functions, etc. within the database. In a database with table1, table2, and table3, a schema could hold table1 and table2, with another schema holding table2 and table3.

To create a schema, use CREATE SCHEMA name, followed by the other components.

CREATE SCHEMA schema_example
  CREATE TABLE people(
    id INT PRIMARY KEY,
    name TEXT NOT NULL,
    age INT,
    birthday DATE
  )
  CREATE VIEW john_club AS
    SELECT 
      *
    FROM
      people
    WHERE
      name = 'John';

Querying JSONB Columns

The way to retrieve the values found at given columns in a JSONB column is by using the single or double arrow, -> and ->> respectively. The former will retrieve the value in its original type, while the latter will return the value as a string.

SELECT
  column_name -> 'property_name' AS display_name
FROM
  table_name;

Commands

Command	Description
`\dt`	Display all tables in the database
`TABLE x`	Show the contents of table x.
`SELECT schema_name FROM information_schema.schemata;`	Show all schema in a given database
`COALESCE(arg1, arg2, ...);`	Return the first non-null argument found, or null if none[11]

Inserting a constant among multiple rows

I found this quirky syntax when working on a project recently and thought it would be helpful to document for the future. In a subquery, one can specify a default value that will be used for all returned rows. In this case, the return for the final SELECT query will be ('Bob', 1337). The value used in the subquery will be added to all returned rows.

CREATE TABLE agents (
  id INT PRIMARY KEY,
  name VARCHAR(64)
);

INSERT INTO 
  agents (name)
VALUES
  ('Bob');

CREATE TABLE bullshit (
  id INT primary key,
  name VARCHAR(64) NOT NULL,
  age INT NOT NULL
);

INSERT INTO 
  bullshit (name, age)
  SELECT 
    name, 1337
  FROM
    agents;

SELECT * FROM bullshit;

DROP TABLE agents;
DROP TABLE bullshit;

References

Last modified: 202408240222