dawn-intro-SQL

###An introduction to SQL
###-or-
###In which we suffer through databases to appreciate Active Records

---

##What is a Relational Database (RDB)?

Relational databases were invented in the 1970's as a way to structure data so that it can be queried by a "relational algebra." The basic idea of relational model, though, was to use collections of data, Tables, where each database manages Relations among the data in various tables. Each table is organized like a spreadsheet with a Row (also known as "record") for each data item and with attributes of those items arranged in Columns*.

Authors Table

id	first_name	last_name	year_of_birth	year_of_death
1	Rudyard	Kipling	1865	1936
2	Lewis	Carroll	1832	1892
3	H.G.	Wells	1866	1946

Books Table

id	title	publication_year	isbn	author_id
1	The Jungle Book	1894	9788497896696	1
2	Alice's Adventures in Wonderland	1865	9781552465707	2
3	Rikki-Tikki-Tavi	1894	1484123689	1
4	Through the Looking-Glass	1871	9781489500182	2
5	The Time Machine	1895	9781423794417	3

Primary Key: The primary key of a relational table uniquely identifies each record in the table. This column is automatically assigned a btree index in postgres.

##What is SQL?

SQL, Structured Query Language, is a specialized language used to create, manipulate, and query tables in relational databases.

• Data Definition Language

  • Define and update database's structure
  • CREATE, ALTER, RENAME, DROP, TRUNCATE
  • Data Types
  • Constraints

• Data Manipulation Language

  • CRUD data within the database
  • SELECT, INSERT, UPDATE, DELETE, ORDER BY
  • UPSERT (attempts an UPDATE, or on failure, INSERT) is part of SQL 3 but not yet in Postgres
  • Queries
  • Aggregation: GROUP BY, SUM, AVG, MIN

• Data Control Language (beyond our scope)

  • GRANT access to parts of the table

##Creating and Modifying RDB Structure

#Database Setup

Let's create our first relational database (RDB) using the Terminal.

createdb practice

Then let's connect to it by name so we can practice our SQL.

psql practice

In your Terminal, you should see a prompt like the following:

practice=#

To quit/exit the database console, type:

\q

Console Tips

• Don't forget to close your SQL Commands with a semi-colon (";")!
• If you see practice-# you're stuck in the middle of inputting a sql command (and likely forgot the trailing semi-colon). Just type ctrl+c to start fresh.

#Workflow Setup

To save your progress on the in-class examples and the challenges, I suggest creating files that store your SQL commands. To run a .sql file, use the following command in your terminal:

psql -f <file_name>

To run a sql file against a specific database, use:

psql -f <file_name> -d <database_name>

You can also create (and destroy) tables from within a SQL file. At the top of your SQL file, I suggest you write the following:

DROP DATABASE IF EXISTS database_name;
CREATE DATABASE database_name;

Feel free to use the pqsl console to try out the following. Once you're comfortable with it, try using a .sql file.

If you would like to load, execute, and save .sql files in a safe, nurturing sandbox environment, head on over to this online SQL interpreter. It's handy!

#Our First Table

Now let's try to create our first Table within the new database. Note: please feel free to shorten attribute names so they're easier to type.

CREATE TABLE author (
  id SERIAL primary key,
  firstName VARCHAR(255),
  year_of_birth INTEGER, /* also known as yob */
  year_of_death NUMERIC DEFAULT 'NaN',
  description TEXT,
  created_at TIMESTAMP NOT NULL DEFAULT now()
);

• SERIAL TYPE
• Primary Key
• MORE DATA TYPES

#Altering Tables and Columns

We can ALTER this table after is created.

ALTER TABLE author ADD COLUMN last_name varchar(255);

An author doesn't need a description column, so let's remove it.

ALTER TABLE author DROP COLUMN description;

Oops, Table names should always be plural. We'll fix the author table name.

ALTER TABLE author RENAME TO authors;

Oops, it looks like our firstName column is camelCased. All column names should be snake_case. We can also rename columns.

ALTER TABLE authors RENAME COLUMN firstName TO first_name;

Let's DROP our table!

DROP TABLE authors;

##Creating, Reading, Updating, and Deleting data in our RDB

The library's having a fundraiser! Here's another table we might have in the database:

CREATE TABLE products (
  id SERIAL primary key,
  name VARCHAR(255),
  price numeric NOT NULL DEFAULT 'NaN',
  quantity integer NOT NULL DEFAULT 0
);

#Inserting Data into RDB

How do we get data into a table? With INSERT!

INSERT INTO products
  (name, price, quantity)
  VALUES
  ('bookmark', 0.50, 200);

Let's add a few more items to our products table

'book cover', 2.00, 75
'book bag', 60.00, 15
'reading light', 25.00, 10

#Reading Data from RDB

To retrieve data from inside our database, we use the command SELECT.

SELECT * FROM products;

Let's look at only some attributes of each product.

SELECT name, price FROM products;

We can use ORDER BY to sort the selected items.

SELECT name FROM products ORDER BY price;

The WHERE keyword allows us to narrow down our query results. We can grab just the bookmark record.

SELECT * FROM products
  WHERE name = 'bookmark';

We can grab the more expensive items only.

SELECT * FROM products
  WHERE price > 20.00
  ORDER BY price;

We can even use regular expressions to find products with "book" at the start of their names.

SELECT * FROM products
  WHERE name LIKE 'book%';

#Updating Simple Data in the RDB

So far we've had a great time using SELECT to read data from our TABLE. We can also change data. Here comes our first sale, a bookmark!

UPDATE products
  SET quantity = quantity - 1
  WHERE name = 'bookmark';

Let's also correct the spelling of book bag to bookbag.

UPDATE products
  SET name = 'bookbag'
  WHERE name = 'book bag';

You might wonder why you don't see anything change after you update an entry. If you'd like, you can tell Postgres to return the modified record. It just isn't the standard behavior.

UPDATE products
  SET quantity = quantity - 1
  WHERE name = 'bookmark'
  RETURNING *;

#Deleting Simple Data from an RDB

Let's remove a row in our products table. Book covers don't sell that well.

DELETE FROM products
  WHERE name = 'book cover'
  RETURNING *;

We could also DELETE everything but the bookmarks with the <> (not equal) operator.

DELETE FROM products
  WHERE name <> 'bookmark';

You can DELETE everything from a table using

DELETE FROM products;

Challenge: Insert four items into the products table.

#ALIAS

You can make your queries easier to read using an alias. Aliases in SQL use the keyword AS.

SELECT * FROM products
  AS prod  -- alias for the products table
  WHERE prod.name = 'bookmark';

SELECT name, price AS cost, quantity  -- alias for the price column only
  FROM products
  WHERE name = 'bookmark';

Note also that -- starts a SQL comment.

#DISTINCT

We can use selection to filter out rows that aren't distinct. First, let's add duplicate bookbag records.

INSERT INTO products
  (name, price, quantity)
  VALUES
  ('bookbag', 50.00, 20),
  ('bookbag', 65.00, 10);

Then we'll select, looking for records with distinct names. Which of the bookbag records do you think will be selected?

SELECT DISTINCT ON (name) *
  FROM products;

#Aggregation

SELECT SUM(quantity) AS total_inventory_count from products;
SELECT name, MIN(price) AS lowest_avaialable_price
FROM products
GROUP BY name
ORDER BY lowest_avaialable_price;