aspnet-basics/sql-databases.md

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true	true	mathjax	buutti	SQL Databases

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

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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)

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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
   • Here's an example query

Editing Data with pgAdmin

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

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• 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.

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• 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)