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Database: MySQL

siriuskoan

Outline

  • Relational and Non-relational Database
  • SQL
    • Basic Statements
    • Primary Key and Foreign Key
  • MySQL
    • Installation
    • mysqldump

Relational and Non-relational Database

There are two types of database, one is relational and the other one is non-relational.

  • Relational Database (also called Relational Database Management System, RDBMS, or SQL)
    • It stores data in tables. The data in tables and rows is called as records.
    • It is structured and confined in format.
    • The data is easy to navigate.
    • Some popular examples: MySQL, MariaDB, PostgreSQL

layout: two-cols

Relational and Non-relational Database

::left::

  • Non-relational Database (also called NoSQL)
    • No tables, no rows, less structured.
    • More flexibility and adaptability.
    • Some popular examples: MongoDB, Redis

::right::

SQL

SQL, standing for Structured Query Language, is a language for managing data in relational database management system.

The statement consists of clause, expression and predicate (bool), and ends with ;.

SQL keywords are case insensitive.

SQL

For example, UPDATE users SET level = level * 2 WHERE name = 'siriuskoan' is a simple SQL statement.

  • UPDATE users is UPDATE clause.
  • SET level = level * 2 is SET clause.
  • level * 2 is an expression.
  • WHERE name = 'siriuskoan' is WHERE clause.
  • name = 'siriuskoan' is predicate.
  • 'siriuskoan' is expression.

SQL - Basic Statements

CRUD means Create, Read, Update and Delete, they are the operations can be done to data.

We will talk about CRUD commands in SQL.

We will use the following table called users to do operations.

SQL - Basic Statements

WHERE statement yields boolean value, and it cannot be used alone.

It is a filter, so it can yields more than one records.

If there are many conditions, the predicate can be concatenated with logical operators AND, OR, NOT.

For example,

  • WHERE username = 'siriuskoan' will return true when the username column of this record is siriuskoan.
  • WHERE age < 18 will return true when the age column of a record is less than 18;
  • WHERE age < 18 AND age > 65 will return true when the age column of a record is less than 18 OR greater than 65.

SQL - Basic Statements

SELECT statement is "R" in CRUD. It can select data from a table.

The syntax is

SELECT column1, column2, ...
FROM table_name
WHERE condition;

WHERE clause can be omitted.

For example,

  • SELECT * FROM users will yield the original table.
  • SELECT username FROM users will yield the three usernames.
  • SELECT id FROM users WHERE username = 'siriuskoan' will yield 1 since the id column of the record whose username column is siriuskoan.

SQL - Basic Statements

INSERT INTO statement is "C" in CRUD, it can insert new record to a table.

The syntax is

INSERT INTO table_name (column1, column2, ...)
VALUES (value1, value2, ...);

(column1, column2, ...) can be omitted if all columns are assigned.

For example,

  • INSERT INTO users (username, level) VALUES ('cat', 100000) inserts a record whose id is 4 (by AUTOINCREMENT), username is cat and level is 100000.

SQL - Basic Statements

UPDATE statement is "U" in CRUD, it can modify the existing records in a table.

The syntax is

UPDATE table_name
SET column1 = value1, column2 = value2, ...
WHERE condition;

WHERE clause can be omitted, but all records will be updated.

For example,

  • UPDATE users SET level = 10 WHERE username = 'siriuskoan' updates the level column to 10 of the record whose username column is siriuskoan.
  • UPDATE users SET level = level + 100 WHERE id > 1 updates the level column to the original value plus 10 of the record whose id column is greater than 1.

SQL - Basic Statements

DELETE statement is "D" in CRUD, it can remove existing records in a table.

The syntax is

DELETE FROM table_name WHERE condition;

WHERE clause can be omitted, but all records will be deleted.

For example,

  • DELETE FROM users WHERE username = 'cat' removes the fourth row since its username column is cat.

SQL - Basic Statements

CREATE DATABASE statement can create a new database.

The syntax is

CREATE DATABASE new_database_name;

DROP DATABASE statement can drop (delete) an existing database. It means all records, all tables and the database will be removed.

The syntax is

DROP DATABASE database_name;

SQL - Basic Statements

CREATE TABLE statement is used to create a new table in a database. We have to specify the structure of this table.

The syntax is

CREATE TABLE table_name (
    column1 data_type,
    column2 data_type,
    ...
);

SQL - Basic Statements

For example,

CREATE TABLE users (
    id INT,
    username VARCHAR(255),
    level INT
);

The statement can create a table with the same columns as the original table.

SQL - Basic Statements

DROP TABLE statement can remove an existing table, including all records in it.

The syntax is

DROP TABLE table_name;

For example,

  • DROP TABLE users will remove the original table.

SQL - Basic Statements

ALTER TABLE statement is used to "alter" an existing table by adding, removing or modifying columns.

To add a column, the syntax is

ALTER TABLE table_name
ADD column_name data_type [DEFAULT default_value];

For example,

  • ALTER TABLE users ADD age int DEFAULT 0; will add a new column age whose data type is INT with default value 0 to users table.

SQL - Basic Statements

To drop a column, the syntax is

ALTER TABLE table_name
DROP COLUMN column_name;

For example,

  • ALTER TABLE users DROP COLUMN age will deletes the age column.

SQL - Basic Statements

To modify a column, the syntax is

ALTER TABLE table_name
MODIFY COLUMN column_name data_type;

For example,

  • ALTER TABLE users MODIFY COLUMN age varchar(255) changes the data type of age column from INT to VARCHAR(255);

SQL - Primary Key and Foreign Key

When creating database, we can set some constraints such as UNIQUE, NOT NULL and DEFAULT which we have mentioned above.

In addition to them, PRIMARY KEY and FOREIGN KEY are also important constraints, and we will discuss them.

For example (from w3school),

CREATE TABLE Orders (
    OrderID INT NOT NULL PRIMARY KEY,
    OrderNumber INT NOT NULL,
    PersonID INT FOREIGN KEY REFERENCES Persons(PersonID)
);

SQL - Primary Key and Foreign Key

  • Primary Key (PK)

    • PRIMARY KEY constraint uniquely identifies each record in a table.
    • For example, id can be the primary key since it is unique.
    • A table can have only one primary key, but it can consists of many columns.

  • Foreign Key (FK)

    • FOREIGN KEY constraint is a field in one table, and the field refers to the primary key in another table.
    • For example, id is the PK in table users, then there can be a FK called owner which refers to id column in users in table cars.

MySQL - Primary Key and Foreign Key

Finally, let's see the schema of the original table.

CREATE TABLE users (
    id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
    username VARCHAR(255) NOT NULL UNIQUE,
    level INT NOT NULL
);

MySQL

MySQL is a relational database management system. It is also the "M" in LAMP.

Though MySQL is an open source software originally, it is a commercial software now.

MariaDB is one of its fork, and it is open source.

MySQL - Installation

  1. Install MySQL.

    $ sudo apt install mysql-server

  2. Check whether MySQL is running.

    $ service mysql status

  3. Do installation.

    $ sudo mysql_secure_installation

  4. It will ask you some questions such as whether you want to remove test database, and let you set new root password.

  5. Login MySQL and see whether it works correctly.

    $ mysql -u root -p

MySQL - Installation

After installation, you can try to modify its config file /etc/mysql/my.cnf. There are some config you may want to check such as port, log location, etc.

After you login the server, you can try to enter some statement and see whether it works.

Let's create a database and a table now.

CREATE DATABASE users;
CREATE TABLE users (
    id INT NOT NULL AUTO_INCREMENT,
    username VARCHAR(255) NOT NULL UNIQUE,
    level INT NOT NULL
);

MySQL - mysqldump

mysqldump is a tool for database backup.

  • Backup

    • mysqldump -h localhost -u root -p users > users_db_backup.sql - Backup users database on localhost.
    • mysqldump -h localhost -u root -p users users > users_table_backup.sql - Backup table users of database users on localhost.
    • mysqldump -u root -p --all-databases > backup.sql - Backup all databases.

  • Restoration

    • mysql -u root -p < users_db_backup.sql - Restore database users.
    • mysql -u root -p users < users_table_backup.sql - Restore table users to database users.
    • mysql -u root -p < backup.sql - Restore all databases.