SQL: An Idiot's Guide

Right, so you want to learn SQL? Good on ya. Look, it’s not as scary as it sounds — think of it like talking to a really pedantic librarian who needs you to be very specific about what book you want and where it lives. I’ve pulled together the basics here in plain English, no fluff, no jargon overload. Grab a cuppa (or something stronger) and let’s crack on.

SQL, or Structured Query Language, is a programming language designed for managing and manipulating relational databases. It is widely used in the field of database management and is considered a standard for interacting with relational database systems.

As of now, SQL is extremely common and widely used across various industries and applications. Most relational database management systems (RDBMS), such as MySQL, PostgreSQL, Oracle Database, Microsoft SQL Server, and SQLite, use SQL as their standard query language.

Many web applications, enterprise systems, data analytics platforms, and other software solutions rely on SQL for tasks like data retrieval, insertion, updating, and deletion.

Learning SQL is considered essential for individuals working in fields related to database administration, data analysis, software development, and more.

I will be using this page to distil information for my own usage. Here I will list a brass tacks guide on how to use the language and where I have found use in seeking help.

Two of the most useful sources I have found and have built this post around are:

Codeacademy cheatsheets

W3Schools

Query Syntax

AND Operator

The AND operator allows multiple conditions to be combined. Records must match both conditions that are joined by AND to be included in the result set. The given query will match any car that is blue and made after 2014.

SELECT model
FROM cars
WHERE color = 'blue'
  AND year > 2014;

AS Clause

Columns or tables can be aliased using the AS clause. This allows columns or tables to be specifically renamed in the returned result set. The given query will return a result set with the column for name renamed to movie_title.

SELECT name AS 'movie_title'
FROM movies;

OR Operator

The OR operator allows multiple conditions to be combined. Records matching either condition joined by the OR are included in the result set. The given query will match customers whose state is either 'CA' or 'NY'.

SELECT name
FROM customers
WHERE state = 'CA'
   OR state = 'NY';

% Wildcard

The % wildcard can be used in a LIKE operator pattern to match zero or more unspecified character(s). The given query will match any movie that begins with The, followed by zero or more of any characters.

SELECT name
FROM movies
WHERE name LIKE 'The%';

SELECT Statement

The SELECT * statement returns all columns from the provided table in the result set. The given query will fetch all columns and records (rows) from the movies table.

SELECT *
FROM movies;

_ Wildcard

The _ wildcard can be used in a LIKE operator pattern to match any single unspecified character. The given query will match any movie which begins with a single character, followed by ove.

SELECT name
FROM movies
WHERE name LIKE '_ove';

ORDER BY Clause

The ORDER BY clause can be used to sort the result set by a particular column either alphabetically or numerically. It can be ordered in two ways:

• DESC is a keyword used to sort the results in descending order.
• ASC is a keyword used to sort the results in ascending order (default).

SELECT *
FROM contacts
ORDER BY birth_date DESC;

LIKE Operator

The LIKE operator can be used inside of a WHERE clause to match a specified pattern. The given query will match any movie that begins with Star in its title.

SELECT name
FROM movies
WHERE name LIKE 'Star%';

DISTINCT Clause

Unique values of a column can be selected using a DISTINCT query. For a table contact_details having five rows in which the city column contains Chicago, Madison, Boston, Madison, and Denver, the given query would return:

• Chicago
• Madison
• Boston
• Denver

SELECT DISTINCT city
FROM contact_details;

BETWEEN Operator

The BETWEEN operator can be used to filter by a range of values. The range of values can be text, numbers, or date data. The given query will match any movie made between the years 1980 and 1990, inclusive.

SELECT *
FROM movies
WHERE year BETWEEN 1980 AND 1990;

LIMIT Clause

The LIMIT clause is used to narrow, or limit, a result set to the specified number of rows. The given query will limit the result set to 5 rows.

SELECT *
FROM movies
LIMIT 5;

NULL Values

Column values can be NULL, or have no value. These records can be matched (or not matched) using the IS NULL and IS NOT NULL operators in combination with the WHERE clause. The given query will match all addresses where the address has a value or is not NULL.

SELECT address
FROM records
WHERE address IS NOT NULL;

WHERE Clause

The WHERE clause is used to filter records (rows) that match a certain condition. The given query will select all records where the pub_year equals 2017.

SELECT title
FROM library
WHERE pub_year = 2017;

Joining Tables

Outer Join

An outer join will combine rows from different tables even if the join condition is not met. In a LEFT JOIN, every row in the left table is returned in the result set, and if the join condition is not met, then NULL values are used to fill in the columns from the right table.

SELECT column_name(s)
FROM table1
LEFT JOIN table2
  ON table1.column_name = table2.column_name;

WITH Clause

The WITH clause stores the result of a query in a temporary table (temporary_movies) using an alias.

Multiple temporary tables can be defined with one instance of the WITH keyword.

WITH temporary_movies AS (
   SELECT *
   FROM movies
)
SELECT *
FROM temporary_movies
WHERE year BETWEEN 2000 AND 2020;

UNION Clause

The UNION clause is used to combine results that appear from multiple SELECT statements and filter duplicates.

For example, given a first_names table with a column name containing rows of data “James” and “Hermione”, and a last_names table with a column name containing rows of data “James”, “Hermione” and “Cassidy”, the result of this query would contain three names: “Cassidy”, “James”, and “Hermione”.

SELECT name
FROM first_names
UNION
SELECT name
FROM last_names

CROSS JOIN Clause

The CROSS JOIN clause is used to combine each row from one table with each row from another in the result set. This JOIN is helpful for creating all possible combinations for the records (rows) in two tables.

The given query will select the shirt_color and pants_color columns from the result set, which will contain all combinations of combining the rows in the shirts and pants tables. If there are 3 different shirt colors in the shirts table and 5 different pants colors in the pants table then the result set will contain 3 x 5 = 15 rows.

SELECT shirts.shirt_color,
   pants.pants_color
FROM shirts
CROSS JOIN pants;

Foreign Key

A foreign key is a reference in one table’s records to the primary key of another table. To maintain multiple records for a specific row, the use of foreign key plays a vital role. For instance, to track all the orders of a specific customer, the table order (illustrated at the bottom of the image) can contain a foreign key.

Primary Key

A primary key column in a SQL table is used to uniquely identify each record in that table. A primary key cannot be NULL. In the example, customer_id is the primary key. The same value cannot re-occur in a primary key column. Primary keys are often used in JOIN operations.

Inner Join

The JOIN clause allows for the return of results from more than one table by joining them together with other results based on common column values specified using an ON clause. INNER JOIN is the default JOIN and it will only return results matching the condition specified by ON.

SELECT *
FROM books
JOIN authors
  ON books.author_id = authors.id;

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Wrapping Up

And that’s the gist of it — the core SQL commands you’ll actually use day-to-day. Obviously there’s loads more depth to dive into (subqueries, window functions, indexing, the whole lot), but if you’ve got the bits above nailed down, you’re in a solid spot. Bookmark this page, come back when you forget something (you will, we all do), and just keep writing queries. The best way to learn is by doing.

Cheers 🍻