SQL and Databases

Follow along with code examples here!

We've learned how to build a server application using Express. It can serve the static assets for a frontend and can handle requests through an API. But the data is not persistent!

This week, we'll learn about the tools needed to build a truly "fullstack" web application with a proper database.

Let's dive in!

Table of Contents

Terms

  • Fullstack - refers to the combination of frontend (client-side) and backend (server-side) technologies.

  • PERN - an acronym for a specific set of technologies used to build a fullstack web application: Postgres, Express, React, and Node. This acronym is useful when asked "what stack do you use?"

  • Database - a structured collection of data that is organized in a manner for easy retrieval.

  • Database Management System (DBMS) - a piece of software used to create and maintain a database.

  • Postgres - a popular "relational" database management system that stores data in a table-like manner

  • Table - a collection of related data organized in rows and columns.

    • A row represents a single object/instance/record in the table

    • A column represents a property/attribute/field of that object. Columns have data types such as integer, string, date, boolean, etc...

    • A primary key serves as the unique identifier for a row in a table

  • SQL (Structured Query Language) - a language used by relational database management systems to create, read, update, or delete data from a database.

What is a database?

A database is a structured collection of data. The data could be stored in a file or in RAM. In the old days, it was stored on a physical "punch card".

a punch card

Nowadays, the data is often stored on a dedicated "database server" managed by a piece of software called a database management system (DBMS) such as Postgres, MongoDB, SQLite, MySQL, Firebird, Apache Cassandra, and many more.

client server database diagram

A server application can send queries to the DBMS using the Structured Query Language (SQL) and the DBMS responds with the requested data.

By separating the database from the server application, we achieve greater separation of concerns:

  • The database layer is focused solely on managing and securing the data.

  • The application layer is focused solely on receiving client requests, retrieving the appropriate data from the database, and sending it back to the client.

  • If we need to update or restart the application server, the database server can continue running without losing the data.

What is Postgres?

PostgreSQL is a popular "relational" DBMS that organizes its data in tables. Below is an example of a table called films with data about various films:

A film table showing data about films

Each table is made up of rows and columns.

  • Each row represents a single object/instance/record in the table

  • Each column represents a property/attribute/field of that object. Columns have data types such as integer, string, date, or boolean.

  • Every table has a column called id (or film_id in this case) that serves as the primary key - a unique identifier for each row in the table.

Q: In JavaScript, how would this data be organized?

What is SQL?

SQL (Structured Querying Language) is the language used by many relational DBMSs to create, modify, and retrieve data from a database.

SQL is written in query statements that are composed of keywords and clauses. Keywords determine the operation to be performed. Clauses modify the operation.

  • Common keywords: SELECT, FROM, INSERT INTO, UPDATE, DELETE

  • Common clauses: WHERE, GROUP BY, ORDER BY

For example:

SELECT title, release_year FROM films WHERE length <= 90;

Q: What does the query above look for?

Answer

The title and release year of the movies that are 90 minutes or less in length from the film table.

SELECT * FROM films;

Q: What about this one?

Answer

This query will get all of the data from the films table.

Tips to avoid SQL errors

  • SQL is NOT case sensitive. However, it is a standard practice to write keywords in all-caps to distinguish them from values.

  • SQL strings MUST be written with single quotes '' to avoid unwanted errors.

  • SQL queries MUST end in a semicolon ; to avoid unwanted errors.

Summary

  • Database - a structured collection of data that is organized in a manner for easy retrieval.

  • Database Management System (DBMS) - a piece of software used to create and maintain a database.

client server database diagram

  • Postgres - a popular "relational" database management system that stores data in a table-like manner

  • Table - a collection of related data organized in rows and columns.

    • A row represents a single object/instance/record in the table

    • A column represents a property/attribute/field of that object. Columns have data types such as integer, string, date, boolean, etc...

    • A primary key serves as the unique identifier for a row in a table

A film table showing data about films

  • SQL (Structured Query Language) - a language used by relational database management systems to create, read, update, or delete data from a database.

SELECT title, release_year FROM films WHERE length <= 90;
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