https://www.udemy.com/the-ultimate-mysql-bootcamp-go-from-sql-beginner-to-expert
docker-compose up -dSHOW DATABASES;CREATE DATABASE hello_world_db;DROP DATABASE hello_world_db;USE hello_world_dbSELECT database();This amongst other things truncates data that are two long for a VARCHAR field with a warning instead of giving an error. It requires the user to login again.
SET @@global.sql_mode=''enable it again, like so;
SET @@sql_mode='STRICT_TRANS_TABLES';A database is a a collection of tables this is true for relational databases.
VARCHAR(<maximum characters>)CREATE TABLE cats
(
name VARCHAR(50),
age INT
);DESCRIBE cats;SHOW COLUMNS FROM cats;DROP TABLE cats;INSERT INTO cats(name, age)
VALUES('Blue', 1);and verify it, like so:
SELECT * from cats;INSERT INTO cats(name, age)
VALUES ('Peanut', 2),
('Butter', 4),
('Jelly', 7);If a warning is shown when a command is executed you can show it, like so:
SHOW WARNINGS;Examples of warnings are inserting strings that are above the maximum limit for a VARCHAR
If not specified otherwise a column in a table is allays nullable meaning that the column value
doesn't have to be specified on insertion. To override this behaviour add NOT NULL to the column
declaration in the CREATE TABLE statement.
CREATE TABLE cats
(
name VARCHAR(50) NOT NULL,
age INT NOT NULL
);This will result in a warning about the column not having a default value and MySQL will actually pick one anyway. The default for an INT is 0.
CREATE TABLE cats3( name VARCHAR(100) DEFAULT 'unnamed', age INT DEFAULT 99);
INSERT INTO cats3(age) VALUES(13);This results in a table like:
+---------+-----+
| name | age |
+---------+-----+
| unnamed | 13 |
+---------+-----+
You still might want to specify NOT NULL to prevent the operator to explicitly provide a NULL
value for a column, like so:
CREATE TABLE cats4( name VARCHAR(100) NOT NULL DEFAULT 'unnamed', age INT NOT NULL DEFAULT 99);
INSERT INTO cats4(age) VALUES(13);Leading to this:
ERROR: 1048 (23000): Column 'age' cannot be null
https://www.udemy.com/the-ultimate-mysql-bootcamp-go-from-sql-beginner-to-expert/learn/v4/t/lecture/7019750?start=0 A unique identifier for a row and thereby a unique entity of a collection.
CREATE TABLE unique_cats(
cat_id INT NOT NULL,
name VARCHAR(100),
age INT,
PRIMARY KEY (cat_id)
);Results in this:
+--------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+--------------+------+-----+---------+-------+
| cat_id | int(11) | NO | PRI | NULL | |
| name | varchar(100) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
+--------+--------------+------+-----+---------+-------+
A Primary Key must be Unique so the this will lead to an error:
INSERT INTO unique_cats(cat_id, name, age) VALUES(1, 'Fred', 23);
INSERT INTO unique_cats(cat_id, name, age) VALUES(1, 'James', 3);ERROR: 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
It is common to use the username as the Primary Key
It is redundant to have to specify the id of the row manually for each insertion. The
AUTO_INCREMENT column property solves this, like so:
CREATE TABLE unique_cats2(
cat_id INT NOT NULL AUTO_INCREMENT,
name VARCHAR(100),
age INT,
PRIMARY KEY (cat_id)
);It is now possible to insert multiple entities, like so:
INSERT INTO unique_cats2 (name, age) VALUES('Skippy', 4) ;
INSERT INTO unique_cats2 (name, age) VALUES('Jiff', 3) ;
INSERT INTO unique_cats2 (name, age) VALUES('Jiff', 3) ;And each of the inserts will be treated as a separate entity.
-- Let's drop the existing cats table:
DROP TABLE cats;
-- Recreate a new cats table
CREATE TABLE cats
(
cat_id INT NOT NULL AUTO_INCREMENT,
name VARCHAR(100),
breed VARCHAR(100),
age INT,
PRIMARY KEY (cat_id)
);
-- Show the created cats table
DESCRIBE cats;
-- Insert data
INSERT INTO cats(name, breed, age)
VALUES ('Ringo', 'Tabby', 4),
('Cindy', 'Maine Coon', 10),
('Dumbledore', 'Maine Coon', 11),
('Egg', 'Persian', 4),
('Misty', 'Tabby', 13),
('George Michael', 'Ragdoll', 9),
('Jackson', 'Sphynx', 7);SELECT * FROM cats;The star means retrieve all columns from table cats.
SELECT name, age FROM cats;SELECT * FROM cats WHERE age=4;Will result in:
+--------+-------+---------+-----+
| cat_id | name | breed | age |
+--------+-------+---------+-----+
| 1 | Ringo | Tabby | 4 |
| 4 | Egg | Persian | 4 |
+--------+-------+---------+-----+
WHERE statements matching strings are case insensitive by default.
WHERE statements can match either user input or values of other columns, like so:
SELECT * FROM cats WHERE cat_id=age;This will retrieve a list of all cats that has a cat_id that is equal to their age
It is possible to rename the output of some fields using an alias, like so:
SELECT cat_id as id, name FROM cats;How do we alter existing database
Change all cats that are of breed 'Tabby' to breed 'Shorthair'
UPDATE cats SET breed='Shorthair' WHERE breed='Tabby';Always do a SELECT statement to test the UPDATE statement to make sure that
the correct data was targeted
SELECT * FROM cats WHERE name='Jackson';
UPDATE cats SET name='Jack' WHERE name='Jackson';
SELECT * FROM cats WHERE name='Jackson';
SELECT * FROM cats WHERE name='Jack';
SELECT * FROM cats WHERE name='Ringo';
UPDATE cats SET breed='British Shorthair' WHERE name='Ringo';
SELECT * FROM cats WHERE name='Ringo';
SELECT * FROM cats;
SELECT * FROM cats WHERE breed='Maine Coon';
UPDATE cats SET age=12 WHERE breed='Maine Coon';
SELECT * FROM cats WHERE breed='Maine Coon';-- Delete all cats named 'Egg' from the table `cats`
DELETE FROM cats WHERE name='Egg';
-- Delete all cats from the table `cats`
DELETE FROM cats;Always do a SELECT statement to test the DELETE statement to make sure that
the correct data was targeted
- Start the CLI by
mysqlsh --sql -P3306 -h127.0.0.1 -uroot -ppassword
- Source the file within the CLI, like so:
\source ./cats.sql
- Start the CLI by
mysqlsh --sql -P3306 -h127.0.0.1 -uroot -ppassword
- Create book_shop database
CREATE DATABASE book_shop; USE book_shop;
- Source the file within the CLI, like so:
\source ./books/book-data.sql
MySQL Reference Manual - String Functions
MySQL Reference Manual - CONCAT Combine data for cleaner output
SELECT CONCAT(author_fname,' ', author_lname) as 'Name' from books;Results in:
+----------------------+
| Name |
+----------------------+
| Jhumpa Lahiri |
| Neil Gaiman |
| Neil Gaiman |
| Jhumpa Lahiri |
| Dave Eggers |
| Dave Eggers |
| Michael Chabon |
| Patti Smith |
| Dave Eggers |
| Neil Gaiman |
| Raymond Carver |
| Raymond Carver |
| Don DeLillo |
| John Steinbeck |
| David Foster Wallace |
| David Foster Wallace |
+----------------------+
SELECT CONCAT_WS(' - ', title, author_fname, author_lname) FROM books;Results in:
+------------------------------------------------------------------------+
| CONCAT_WS(' - ', title, author_fname, author_lname) |
+------------------------------------------------------------------------+
| The Namesake - Jhumpa - Lahiri |
| Norse Mythology - Neil - Gaiman |
| American Gods - Neil - Gaiman |
| Interpreter of Maladies - Jhumpa - Lahiri |
| A Hologram for the King: A Novel - Dave - Eggers |
| The Circle - Dave - Eggers |
| The Amazing Adventures of Kavalier & Clay - Michael - Chabon |
| Just Kids - Patti - Smith |
| A Heartbreaking Work of Staggering Genius - Dave - Eggers |
| Coraline - Neil - Gaiman |
| What We Talk About When We Talk About Love: Stories - Raymond - Carver |
| Where I'm Calling From: Selected Stories - Raymond - Carver |
| White Noise - Don - DeLillo |
| Cannery Row - John - Steinbeck |
| Oblivion: Stories - David - Foster Wallace |
| Consider the Lobster - David - Foster Wallace |
+------------------------------------------------------------------------+
SELECT SUBSTRING('Hello World', 1, 4);Results in:
+-------------------------------+
| SUBSTRING('Hello World', 1, 4) |
+-------------------------------+
| Hell |
+-------------------------------+
Get position 7 to end of string
SELECT SUBSTRING('Hello World', 7);Results in:
+-----------------------------+
| SUBSTRING('Hello World', 7) |
+-----------------------------+
| World |
+-----------------------------+
Get five characters starting from end of string
SELECT SUBSTRING('Hello World', -5);Select the first ten characters of the title of each book in table books with column name
'Short Title'
SELECT SUBSTRING(title, 1,10) as 'Short Title' from books;To add an extra '...' at the end of the 'Short Title' the SUBSTRING command can be used in
conjunction with CONCAT, like so:
SELECT CONCAT(SUBSTRING(title, 1,10), '...') as 'Short Title' from books;Results in:
+---------------+
| Short Title |
+---------------+
| The Namesa... |
| Norse Myth... |
| American G... |
| Interprete... |
| A Hologram... |
| The Circle... |
| The Amazin... |
| Just Kids... |
| A Heartbre... |
| Coraline... |
| What We Ta... |
| Where I'm ... |
| White Nois... |
| Cannery Ro... |
| Oblivion: ... |
| Consider t... |
+---------------+
Looks nice doesn't it
Replace all e's in all book titles with the number 3
SELECT REPLACE(title, 'e', '3') from books;Nest multiple string functions
SELECT SUBSTRING(REPLACE(title, 'e', '3'), 1, 20) as 'Weird String' from books;SELECT REVERSE('Hello World');Results in:
+------------------------+
| REVERSE('Hello World') |
+------------------------+
| dlroW olleH |
+------------------------+
SELECT CHAR_LENGTH('Hello World');Results in:
+----------------------------+
| CHAR_LENGTH('Hello World') |
+----------------------------+
| 11 |
+----------------------------+
Show a human readable string like 'Eggers is 6 characters long' for each author in table books
SELECT CONCAT(author_lname, ' is ', CHAR_LENGTH(author_lname), ' characters long.') FROM books;SELECT UPPER('Hello World');SELECT LOWER('Hello World');INSERT INTO books (title, author_fname, author_lname, released_year, stock_quantity, pages)
VALUES ('10% Happier', 'Dan', 'Harris', 2014, 29, 256),
('fake_book', 'Freida', 'Harris', 2001, 287, 428),
('Lincoln In The Bardo', 'George', 'Saunders', 2017, 1000, 367);The DISTINCT keyword comes straight after SELECT and only retrieves the unique entries of values
in a column.
Get distinct author last names
SELECT DISTINCT author_lname FROM books;Results in:
+----------------+
| author_lname |
+----------------+
| Lahiri |
| Gaiman |
| Eggers |
| Chabon |
| Smith |
| Carver |
| DeLillo |
| Steinbeck |
| Foster Wallace |
| Harris |
| Saunders |
+----------------+
Get distinct pairs of author first and last names
SELECT DISTINCT author_fname, author_lname FROM books;Orders results by the values in a column.
SELECT DISTINCT author_fname, author_lname FROM books ORDER BY author_lname;- The default order is ASCENDING
- The default order can be reversed by adding the keyword
DESCafterORDER BY
Get all columns of the first record from the table books
SELECT * FROM books LIMIT 1;Get all columns of the second to fifth record from the table books
SELECT * FROM books LIMIT 2,5;Matches parts of column values
-
The % sign are wildcards and works like a asterisk
-
The _ sign are wildcards matches exactly one character
-
The
%and_can be escaped by\ -
The patterns are case insensitive by default.
Get title and author_fname column from table books where author_fname contains the string
'da'.
SELECT title, author_fname from books WHERE author_fname LIKE '%da%';Get title and author_fname column from table books where author_fname starts with the
string 'da'.
SELECT title, author_fname from books WHERE author_fname LIKE 'da%';Get title and stock_quantity from table books where stock_quantity us exactly to
characters or numbers long.
SELECT title, stock_quantity FROM books WHERE stock_quantity LIKE '__';Get title from table books where title has a percentage sign in it.
SELECT title FROM books WHERE title LIKE '%\%%' ;Get title from table books where title has a underscore sign in it.
SELECT title FROM books WHERE title LIKE '%\_%' ;How many books are in our database?
SELECT COUNT(*) as 'Amount of Books' FROM books;Results in:
+-----------------------------+
| Amount of Author First Name |
+-----------------------------+
| 19 |
+-----------------------------+
How many distinct author first name are there in our books table
SELECT COUNT(DISTINCT author_fname) as 'Amount of Distinct Author First Name' FROM books;Results in:
+--------------------------------------+
| Amount of Distinct Author First Name |
+--------------------------------------+
| 12 |
+--------------------------------------+
How many distinct authors are there in our books table
SELECT COUNT(DISTINCT author_fname, author_lname) as 'Amount of Distinct Author First Name' FROM books;How many titles contains the string 'the'
SELECT COUNT(*) FROM books WHERE title LIKE '%the%';GROUP BY shows records based on groups in a column.
The output of this query is a bit confusing since it only shows the first book of each group.
SELECT title, author_lname FROM books GROUP BY author_lname;By combining the GROUP BY with a COUNT it is easier to understand what is going on.
SELECT author_lname, COUNT(*) FROM books GROUP BY author_lname;Results in:
+----------------+----------+
| author_lname | COUNT(*) |
+----------------+----------+
| Carver | 2 |
| Chabon | 1 |
| DeLillo | 1 |
| Eggers | 3 |
| Foster Wallace | 2 |
| Gaiman | 3 |
| Harris | 2 |
| Lahiri | 2 |
| Saunders | 1 |
| Smith | 1 |
| Steinbeck | 1 |
+----------------+----------+
This shows the count the number of books each author has written.
It is possible to group by multiple fields, like so:
SELECT author_fname, author_lname, COUNT(*) FROM books GROUP BY author_lname, author_fname;Which will avoid duplicated author_lname
Get the earliest released book from table books
SELECT MIN(released_year) FROM books;Get the latest released book from table books
SELECT MAX(released_year) FROM books;- The Ultimate MySQL bootcamp - A Problem with MIN and MAX
Get the book with the least pages in the
bookstable
SELECT * FROM books WHERE pages = (SELECT Min(pages) FROM books);MySQL executes the subquery first and then uses the result as input for the outer query.
This is not effective since two queries needs to be executed. As an alternative ORDER BY can
be used to achieve the same thing, like so:
SELECT title, pages FROM books ORDER BY pages LIMIT 1;Get the first released book for all authors
SELECT author_fname, author_lname, MIN(released_year)
FROM books
GROUP BY author_lname, author_fname;Calculate the total amount of pages from all books in the books tables
SELECT SUM(pages) from books;Calculate the total amount an author has produced in total in the books tables
SELECT author_fname, author_lname, SUM(pages) from books GROUP BY author_fname, author_lname;Calculate the average release year of all books in the books tables
SELECT AVG(released_year) from books;Calculate the average stock quantity per released_year the books tables
SELECT released_year, AVG(stock_quantity) from books GROUP BY released_year;Calculate the average pages per author in table books
SELECT author_fname, author_lname, AVG(PAGES) FROM books GROUP BY author_lname, author_fname;
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