SQL Databases

What is SQL?

  • SQL, Structured Query Language is a language used to organize and manipulate data in a database
  • Originally developed by IBM in the 70's
    • Quickly became the most popular database language
    SELECT id, email
FROM users
WHERE first_name = 'Teppo';

Relational Database Management Systems

  • In relational databases, values are stored in tables
    • Each table has rows and columns
    • Data is displayed in a two-dimensional matrix
  • Values in a table are related to each other
    • Values can also be related to values in other tables
  • A relational database management system (RDBMS) is a program that executes queries to relational databases

https://db-engines.com/en/ranking

PostgreSQL

  • Free and open-source, cross-platform relational database management system
  • Emphasizes extensibility and SQL compliance
  • Fully ACID-compliant (atomicity, consistency, isolation and durability)
  • Used in conjunction with pgAdmin
    • Administration and development platform for PostgreSQL
    • Cross-platform, features a web interface
    • Basically a control panel application for your PostgreSQL database

PostgreSQL: Creating a Database

With pgAdmin:

  1. Right-click Servers > my-postgres > Databases
  2. Select Create > Database...
  3. Insert a name for the database and hit Save

With psql:

  1. Enter command
    CREATE DATABASE <database-name>;

PostgreSQL: Querying

With pgAdmin:

  1. Right-click sqlpractice > Query tool...
  2. Insert a query into the Query Editor and hit Execute (F5)

With psql:

  1. List all available databases with \l
  2. Connect to the created database with \c <database-name>
  3. List all tables in the database with \dt
  4. Type a query and press enter

Editing Data with pgAdmin

  • Inspect and edit data in pgAdmin by right-clicking a table and selecting View/Edit Data

  • Individual values in the table can be directly modified by double clicking the value and then editing the value in the visual user interface
  • Save the changes by pressing the Save Data Changes button


Exercise 1: Preparing the database

Using either PgAdmin or PSQL,

  1. Create a new database called sqlpractice
  2. Insert the provided example query to the new database
  3. Verify that the query has created new tables to your database

Basic SQL queries

  • SELECT
  • INSERT
  • DELETE
  • UPDATE
  • CREATE & DROP

Querying data with SELECT

  • Syntax:
    SELECT column1, column2, column3 FROM table_name;
  • Examples:
    SELECT full_name, email FROM users;
SELECT full_name AS name, email FROM users;
SELECT * FROM users;

Filtering data with WHERE

  • Syntax:
    SELECT column1, column2 FROM table_name WHERE condition;
  • Text is captured in single quotes.
  • LIKE condition uses the % sign as a wildcard.
  • IS and IS NOT are also valid operators.
  • Example:
    SELECT full_name FROM users
WHERE full_name = 'Teppo Testaaja';

SELECT * FROM books WHERE name LIKE '%rr%';
SELECT * FROM books WHERE author IS NOT null;

Ordering data with ORDER BY

  • Syntax:
    SELECT column1 FROM table_name ORDER BY column1 ASC;
  • Examples:
    SELECT full_name FROM users
ORDER BY full_name ASC;

SELECT full_name FROM users
ORDER BY full_name DESC;

Combining data with JOIN

  • Also known as INNER JOIN
  • Corresponds to intersection from set theory

JOIN examples

SELECT
users.id, users.full_name, borrows.id,
borrows.user_id, borrows.due_date, borrows.returned_at
FROM users
JOIN borrows ON
users.id = borrows.user_id;

SELECT
U.full_name AS name,
B.due_date AS due_date,
B.returned_at AS returned_at
FROM users AS U
JOIN borrows AS B ON
U.id = B.user_id;

Combining with LEFT JOIN

  • Also known as LEFT OUTER JOIN
  • Example:
    SELECT
U.full_name AS name,
B.due_date AS due_date,
B.returned_at AS returned_at
FROM users AS U
LEFT JOIN borrows AS B ON
U.id = B.user_id;

Exercise 2: Querying the library

Using SQL queries, get

  1. all columns of borrows that are borrowed before 2020-10-27
  2. all columns of borrows that are returned
  3. columns user.full_name and borrows.borrowed_at of the user with an id of 1
  4. columns book.name, book.release_year and language.name of all books that are released after 1960

Inserting data with INSERT

  • Syntax
    INSERT INTO table_name (column1, column2, column3) VALUES (value1, value2, value3);
  • Example
    INSERT INTO users (full_name, email, created_at)
VALUES ('Pekka Poistuja', 'pekka.poistuja@buutti.com', NOW());
  • Since id is not provided, it will be automatically generated.

Updating data with UPDATE

  • Syntax
    UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition;
  • Notice: if a condition is not provided, all rows will be updated!
    • If updating only one row, it is usually best to use id.
  • Example
    UPDATE users SET email = 'taija.testaaja@gmail.com' WHERE id = 2;

Removing data with REMOVE

  • Syntax
    DELETE FROM table_name WHERE condition;
  • Again, if the condition is not provided, DELETE affects all rows
  • Before deleting, it is a good practice to execute an equivalent SELECT query to make sure that only the proper rows will be affected.
  • Example:
    SELECT * FROM users WHERE id = 5;
DELETE FROM users WHERE id = 5;

Exercise 3: Editing data

  1. Postpone the due date of the borrow with an id of 1 by two days in the borrows table
  2. Add a couple of new books to the books table
  3. Delete one of the borrows.

Initializing data with CREATE TABLE

  • Before data can be manipulated, a database and its tables need to be initialized.
  • Syntax
    CREATE TABLE table_name (
  column1 datatype,
  column2 datatype,
  ...
);
  • Example:
    CREATE TABLE "users" (
  "id" SERIAL PRIMARY KEY,
  "full_name" varchar NOT NULL,
  "email" varchar UNIQUE NOT NULL,
  "created_at" timestamp NOT NULL
);

Removing data with DROP

  • In order to remove tables or databases, we use a DROP statement
    DROP TABLE table_name;
DROP DATABASE database_name;
  • These statements do not ask for confirmation and there is no undo feature. Take care when using a drop statement.
  • I'm legally obliged to include this XKCD comic here.

NoSQL

  • In addition to SQL databases, there's NoSQL
  • There are many differing definitions, but...
    • most agree that NoSQL databases store data in a format other than tables
    • They can still store relational data - just differently
  • Four different database types:
    • Document databases
    • Key-value databases
    • Wide-column stores
    • Graph databases
  • Example database engines include MongoDB, Redis and Cassandra
  • For more info, see MongoDB: What is NoSQL?

Object-Relational Mappers

  • Object-Relational Mappers, or ORMs allow the developer to write code instead of SQL to perform CRUD operations on their database
  • An abstraction layer between code and database
    • Can make database logic easier to read and write
    • Prevents SQL injection by sanitizing input (see the comic before...)
    • ...but sometimes you'd rather just write the SQL queries themselves
  • Some popular ORMs:
    • Hibernate (Java)
    • EFCore (.NET)
    • Sequelize (Node.js)
    • TypeORM (TypeScript)