Intro to SQL

Goals

  • Understand what SQL is and what it’s used for
  • Learn basic postgres commands for navigating databases in the terminal

SQL

SQL (pronounced sequel) stands for Structured Query Language. Its the most popular language for interacting with databases. SQL allows us to create databases and tables, change the data that exists within them, retrieve very specific data from a large set that we’re interested in, etc.

Most frequently, when you’re writing SQL, you’d likely be doing it from the terminal or writing a script that can be run to generate your queries for you. There are some exceptions though – here are some instances of SQL in PHP codebases. Pretty gnarly when you see it in this context, right?

Lucky for us, people have built many abstractions over SQL so that we don’t have to write code that looks like this.

Writing SQL in the Terminal

  • SELECT * FROM <tablename>;
  • SELECT * FROM <tablename> WHERE column='value';
  • INSERT INTO <tablename> (title) VALUES ('Baz');
  • DELETE FROM <tablename> WHERE column='value';

Gotchas

  • Don’t forget your semi-colons after each SQL query!
  • Postgres requires single quotes in scenarios like WHERE column='value'

Common Postgres Commands

  • \l - list all databases
  • CREATE DATABASE <databasename>; - create a new database
  • \c <databasename> - connect to a particular database
  • \dt - after you’re connected to a database, show the tables it contains
  • \d <tablename> - list all columns in a table
  • \q - quit

Sample Workflow Using Raw SQL

Let’s go through an exercise in using raw SQL in the terminal. You will almost never use raw SQL in the APIs you build, but it’s helpful to see this in case you need to debug at the database level.

# Open PostgreSQL client
psql

# List all databases
\l

# Create a new database
CREATE DATABASE name;

# Connect to a database
\c tablename

# Create table
CREATE TABLE students (
  id serial PRIMARY KEY,
  name varchar (50) NOT NULL,
  program varchar (50) NOT NULL,
);

# View records in a table
SELECT * FROM tablename;

# Insert record
INSERT INTO students (name, program) VALUES ('Robbie', 'FE');

# View again and only certain columns
SELECT name FROM students;

# Delete record
DELETE FROM students WHERE name='Robbie';

# Quit the PostgreSQL client
\q 

Practice (20 minutes)

Practice writing SQL commands by going through the lessons here, that will teach you how to make selections and filter data.

Checks for Understanding

  • What is postgres? How is it related to SQL?
  • How do you open and close the terminal PostgreSQL client?
  • How would you select all columns and all records from a table called “invoices?”
  • From a table called “customers”, how would you select the email column from records that have a billing address in Arizona?

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