MySQL

MySQL is an open-source RDBMS created in 1995

• MySQL InnoDB Storage Engine

Setup: Install the MySQL Community Server from this link

Features:

• Web App
• Ease of use
• Speed
• Scalability

Data Types

• CHAR: It can only store strings of fixed length CHAR(10) can only store 10 characters

Type	Description
INT	Whole numbers
DECIMAL(M,N)	Decimal numbers (exact value) (number of digits M, precision N)
FLOAT
VARCHAR(L)	String of length L (for small strings)
TEXT	for longer strings
BLOB	Binary Large Object, Stores large data
DATE	'YYYY-MM-DD'
TIMESTAMP	'YYYY-MM-DD HH:MM:SS'

Database Queries

Collation and Character Set:

• Default latin1_swedish_ci and latin1

SHOW CHARACTER SET;
SHOW COLLATION;

SHOW VARIABLES LIKE 'c%';

CREATE DATABASE rose DEFAULT CHARACTER SET latin1 COLLATE latin1_swedish_cs;

• count(*) is fast and good to be used
  • Smallest secondary non-null index

EXPLAIN SELECT COUNT(*) FROM todos;

DB Info

-- list databases
SHOW DATABASES;

-- list tables
SHOW TABLES;
SHOW TABLES FROM music;

-- list columns
SHOW COLUMNS FROM track;

-- list triggers
SHOW TRIGGERS;

-- show statement used to create database, table
SHOW CREATE DATABASE music;
SHOW CREATE TABLE track;

-- show table schema
DESCRIBE student;

Database

CREATE DATABASE lucy;

-- avoid errors
CREATE DATABASE IF NOT EXISTS lucy;

USE lucy;

Table

SELECT *
FROM customers
WHERE last_name REGEXP '^field'

Column Constraints:

• UNIQUE

• NOT NULL

• PRIMARY KEY: entry for this column should be unique and cannot be NULL

• DEFAULT value: set default value

• ENUM

• AUTO_INCREMENT: Add unique number to each row

• CHECK

Example:

-- Users table
CREATE TABLE Users (
  id INT PRIMARY KEY AUTO_INCREMENT,
  email VARCHAR(255) NOT NULL UNIQUE,
  bio TEXT,
  country VARCHAR(2)
);

-- Rooms table
CREATE TABLE Rooms (
  id SMALLINT(5) NOT NULL DEFAULT 0,
  street CHAR(128) DEFAULT NULL,
  owner_id INT NOT NULL,
  PRIMARY KEY (id),
  FOREIGN KEY (owner_id) REFERENCES Users(id) ON DELETE SET NULL,
);

• ON DELETE SET NULL: set value as NULL when the foreign key entry is delete from the other table

• ON DELETE SET CASCADE: delete entry when the foreign key entry is delete from the other table

Create temporary tables which will be deleted once the monitor connection is closed:

CREATE TEMPORARY TABLE

• Show table:

SHOW CREATE TABLE todos;

Column Types

1. INT[(width)] [UNSIGNED] [ZEROFILL]:

   • UNSIGNED can be used

   • width: Not the max number of digits but the min number of digits, like INT(4) will save 33 as 0033.

   • width and ZEROFILL arguments to left-pad the values with 0's.

Index

CREATE INDEX email_index on Users(email);

SELECT Statement

The SELECT statement is used to query the database and retrieve data from one or more tables

The arithmetic operators in order of precedence:

Operator	Name	Order of precedence
*	Multiplication	1
/	Division	1
DIV	Integer division	1
% (MOD)	Modulo (remainder)	1
+	Addition	2
-	Subtraction	2

What determines the sequence of operations?

• Order of precedence
• Parentheses

Functions

• CONCAT(string1[, string2]...): Concatenate strings
• LEFT(str, number_of_characters): Extract leftmost characters
• RIGHT(str, number_of_characters): Extract rightmost characters
• DATE_FORMAT(date, format_string): Format date
• ROUND(number[, number_of_decimal_places]): Round number to specified decimal places

Date and Time

Handle date and time values

Triggers

A trigger is a stored program invoked automatically in response to an event such as INSERT, UPDATE, or DELETE that occurs in the associated table

• You can define a trigger that is invoked automatically before a new row is inserted into a table

CREATE TRIGGER before_employee_update BEFORE
UPDATE
  ON employees FOR EACH ROW
INSERT INTO
  employees_audit
SET
  action = 'update',
  employeeNumber = OLD.employeeNumber,
  lastName = OLD.lastName,
  changeDate = NOW();

• Default delimiter is ;

mysql> delimiter //
mysql> CREATE TRIGGER upd_check BEFORE UPDATE ON account
       FOR EACH ROW
       BEGIN
           IF NEW.amount < 0 THEN
               SET NEW.amount = 0;
           ELSEIF NEW.amount > 100 THEN
               SET NEW.amount = 100;
           END IF;
       END;//
mysql> delimiter ;

Locks

show session variables like '%isolation%';

Add Table lock:

lock table PRODUCTS write;

Settings

• set autocommit=0;: Disable auto-commit (default: ON)
• set session transaction isolation level read commited;: Good for OLTP Databases (default: repeatable-read)