aspnet-basics/sql-databases.md

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title: SQL Databases
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# SQL Databases

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## 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
  ```sql
  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

---

![](imgs/7-databases-with-entity-framework_0.png)

[https://db-engines.com/en/ranking](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

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With pgAdmin:
1) Right-click *Servers > my-postgres > Databases*
2) Select  _Create > Database..._
3) Insert a name for the database and hit _Save_

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With psql:
1) Enter command
    ```sql
    CREATE DATABASE <database-name>;
    ```

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</div>

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![](imgs/7-databases-with-entity-framework_1.png)

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### 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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![w:345](imgs/7-databases-with-entity-framework_2.png)

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![](imgs/7-databases-with-entity-framework_3.png)

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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](code-examples/example-query.sql)

## Editing Data with pgAdmin

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* Inspect and edit data in pgAdmin by right-clicking a table and selecting _View/Edit Data_

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![](imgs/7-databases-with-entity-framework_4.png)

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---

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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

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![](imgs/7-databases-with-entity-framework_5.png)
![](imgs/7-databases-with-entity-framework_6.png)

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## Exercise 1: Preparing the database
<!--_class: "exercise invert" -->

Using either PgAdmin or PSQL,

1) Create a new database called `sqlpractice`
2) Insert the provided [example query](code-examples/example-query.sql)  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:
  ```sql
  SELECT column1, column2, column3 FROM table_name;
  ```
* Examples:
  ```sql
  SELECT full_name, email FROM users;
  SELECT full_name AS name, email FROM users;
  SELECT * FROM users;
  ```

### Filtering data with `WHERE`

* Syntax:
  ```sql
  SELECT column1, column2 FROM table_name WHERE condition;
  ```
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* Text is captured in **_single quotes_**.
* `LIKE` condition uses the `%` sign as a wildcard.
* `IS` and `IS NOT` are also valid operators.

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* Example:
  ```sql
  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;
  ```

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</div>

### Ordering data with `ORDER BY`

* Syntax:
  ```sql
  SELECT column1 FROM table_name ORDER BY column1 ASC;
  ```
* Examples:
  ```sql
  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](https://en.wikipedia.org/wiki/Set_theory)

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![w:500px](imgs/7-databases-with-entity-framework_7.png)

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#### JOIN examples

```sql
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;
```

```sql
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`

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* Also known as `LEFT OUTER JOIN`
* Example:
  ```sql
  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;
  ```

</div>
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![](imgs/7-databases-with-entity-framework_8.png)

</div>
</div>

### Exercise 2: Querying the library
<!--_class: "exercise invert" -->

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
  ```sql
  INSERT INTO table_name (column1, column2, column3) VALUES (value1, value2, value3);
  ```
* Example
  ```sql
  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
  ```sql
  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
  ```sql
  UPDATE users SET email = 'taija.testaaja@gmail.com' WHERE id = 2;
  ```

## Removing data with `REMOVE`

* Syntax
  ```sql
  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:
  ```sql
  SELECT * FROM users WHERE id = 5;
  DELETE FROM users WHERE id = 5;
  ```

### Exercise 3: Editing data
<!--_class: "exercise invert" -->

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.

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* Syntax
  ```sql
  CREATE TABLE table_name (
    column1 datatype,
    column2 datatype,
    ...
  );
  ```

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* Example:
  ```sql
  CREATE TABLE "users" (
    "id" SERIAL PRIMARY KEY,
    "full_name" varchar NOT NULL,
    "email" varchar UNIQUE NOT NULL,
    "created_at" timestamp NOT NULL
  );
  ```

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</div>

### Removing data with `DROP`

* In order to remove tables or databases, we use a `DROP` statement
  ```sql
  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.

![](https://imgs.xkcd.com/comics/exploits_of_a_mom_2x.png)

## 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](https://www.mongodb.com/), [Redis](https://redis.io/) and [Cassandra](https://cassandra.apache.org/_/index.html)
* For more info, see [MongoDB: What is NoSQL?](https://www.mongodb.com/resources/basics/databases/nosql-explained)


## 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)