What is MySQL?

MySQL is a popular open-source relational database management system (RDBMS) that uses Structured Query Language (SQL).

SQL Injection

SQL Injection is a code injection technique that exploits vulnerabilities in the database layer of an application. It involves inserting malicious SQL statements into an entry field for execution.

Practical Application of MySQL

Retrieve, update, and delete data
Create and manipulate tables and databases
Manage users and permissions

Example: Connecting to MySQL Database

mysql -u root -p

Enter your password to proceed.

Command Line Interaction

The `mysql` Utility

The mysql utility is a command-line tool for interacting with MySQL/MariaDB databases.

Authentication

Use -u for the username and -p for the password

Note: Passing the password directly in the command line is discouraged due to security concerns

Connecting to a Database

To connect to a specific database:

mysql -u username -p -h host_address -P port

Example: Logging In

mysql -u root -p

For remote connection:

mysql -u root -h your_host -P 3306 -p

Creating and Managing Databases and Tables

Creating a Database

Log into the MySQL terminal
Execute CREATE DATABASE users; to create a new database named users

CREATE DATABASE users;

Defining Tables

Tables store data in rows and columns. Each column has a specific data type, like INT for integers or VARCHAR for variable-length strings.

Select a database with USE users;
Create a table with CREATE TABLE

CREATE TABLE logins (
    id INT,
    username VARCHAR(100),
    password VARCHAR(100),
    date_of_joining DATETIME
);

SQL Statements

Let's dive deep into SQL Statements INSERT, SELECT, DROP, ALTER, and UPDATE.

`INSERT` Statement

Objective

Learn how to use the INSERT statement to add new records to a database table.

Explanation and Steps

Syntax:

INSERT INTO table_name VALUES (column1_value, column2_value, column3_value, ...);

Selective Insertion: You can specify columns to insert values selectively, useful for skipping columns with default values.

INSERT INTO table_name(column2, column3, ...) VALUES (column2_value, column3_value, ...);

Step-by-Step Application

Basic Insertion: Insert values into all columns by specifying them in order. Example: INSERT INTO logins VALUES(1, 'admin', 'p@ssw0rd', '2020-07-02');
Selective Insertion: Specify column names to insert values into specific columns, skipping those with default or auto-increment values. Example: INSERT INTO logins(username, password) VALUES('administrator', 'adm1n_p@ss');
Insert Multiple Records: Insert multiple records in one statement by separating each record with a comma. Example: INSERT INTO logins(username, password) VALUES ('john', 'john123!'), ('tom', 'tom123!');

Common Errors and Best Practices

Not Null Constraint: Skipping columns with NOT NULL constraint without providing a default value results in an error.
Security Best Practice: Never insert passwords in cleartext. Use hashing/encryption.

`SELECT` Statement

Objective

Retrieve data from tables using the SELECT statement.

Explanation and Steps

Basic Syntax: SELECT * FROM table_name; retrieves all columns.

Selective Retrieval: SELECT column1, column2 FROM table_name; fetches specified columns only.

Application Examples

Retrieve All Records: SELECT * FROM logins; shows all columns and records.
Selective Columns: SELECT username,password FROM logins; displays only usernames and passwords.

Tips and Tricks

Using Wildcards: The asterisk `` is a wildcard that selects all columns. Be specific in column selection for efficiency.

`DROP` Statement

Objective

Understand how to remove tables or databases using the DROP statement.

Explanation and Steps

Syntax: DROP TABLE table_name;

Usage: Removes a table entirely without confirmation.

Example

To remove the logins table: DROP TABLE logins;

Warnings and Best Practices

Irreversible Action: DROP permanently deletes the table. Use with caution.

`ALTER` Statement

Objective

Modify table structures using the ALTER statement.

Key Operations

Add Column: ALTER TABLE table_name ADD column_name column_type;
Rename Column: ALTER TABLE table_name RENAME COLUMN old_name TO new_name;
Modify Column Type: ALTER TABLE table_name MODIFY column_name new_type;
Drop Column: ALTER TABLE table_name DROP column_name;

Examples

Add a new column: ALTER TABLE logins ADD newColumn INT;
Rename a column: ALTER TABLE logins RENAME COLUMN newColumn TO oldColumn;
Modify column type: ALTER TABLE logins MODIFY oldColumn DATE;
Drop a column: ALTER TABLE logins DROP oldColumn;

Tips

Privileges: Ensure you have the necessary permissions to alter tables.

`UPDATE` Statement

Objective

Update existing records in a table based on specified conditions.

Explanation and Steps

Syntax: UPDATE table_name SET column1=newvalue1, column2=newvalue2, ... WHERE condition;

Application: Change record values that meet the condition.

Example

To change passwords for records with id greater than 1: UPDATE logins SET password = 'change_password' WHERE id > 1;

Common Pitfalls and Best Practices

Conditional Updates: Always use a WHERE clause to avoid unintended mass updates.
Security Consideration: Like with INSERT, ensure password values are hashed/encrypted.

Best Practices and Tips

SQL Statements: While not case-sensitive, writing SQL keywords in uppercase (e.g., SELECT, CREATE TABLE) and database entities in lowercase is recommended for readability.
Security Practices: Always be wary of SQL injection vulnerabilities. Validate and sanitize user inputs.

What is the difference between `ALTER` and `UPDATE`?

`ALTER` Statement

Purpose: Used to modify the structure of an existing table or database objects. This includes operations like adding, deleting, or modifying columns in a table, changing data types of columns, renaming tables, and more.

Operation Level: The ALTER statement operates at the schema or structure level of the database. It changes how the table or its elements are defined but does not directly modify the data within the rows.

Common Uses:

Adding a new column to a table: ALTER TABLE table_name ADD column_name datatype;
Removing a column from a table: ALTER TABLE table_name DROP COLUMN column_name;
Changing the data type of a column: ALTER TABLE table_name MODIFY COLUMN column_name new_datatype;
Renaming a table: ALTER TABLE old_table_name RENAME TO new_table_name;

`UPDATE` Statement

Purpose: Used to modify existing records in a table. It updates the values of specified columns in rows that meet the condition defined in the WHERE clause.

Operation Level: The UPDATE statement operates at the data level. It directly changes the contents of one or more rows in a table but does not alter the table's structure.

Common Uses:

Updating the value of a column in specific rows: UPDATE table_name SET column1 = value1 WHERE condition;
Updating multiple columns at once: UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition;

Key Differences

Scope: ALTER changes the table design or structure, while UPDATE modifies the data within the table.
Operation: ALTER is used for modifying the schema of the database, such as adding or deleting columns. UPDATE is used for changing the data stored in the table rows.
Impact: Changes made by ALTER can affect the overall layout and functionality of the database and its relations. UPDATE affects the actual data stored, which can impact the results of queries but not the structure of the database.

In summary, ALTER is used for structural changes to the database, whereas UPDATE is used for modifying the data within the tables. Each serves a distinct purpose in database management and manipulation in MySQL.

Checkpoint

Exercise: Try creating a new table named test_table with columns test_id (INT) and test_name (VARCHAR(50)). Use the CREATE TABLE statement.

Feedback: Verify the table creation with SHOW TABLES; and describe its structure using DESCRIBE test_table;.

Query Results

In this section, we will learn how to control the results output of any query.

Sorting Results

Explanation

Sorting results allows you to order the output of a query based on one or more columns in ascending or descending order.

Steps

Use the ORDER BY clause followed by the column name to sort the results. Example: SELECT * FROM logins ORDER BY password;
By default, the sort is done in ascending order. To sort in descending order, use DESC after the column name. Example: SELECT * FROM logins ORDER BY password DESC;
To sort by multiple columns, separate the column names with commas. The secondary sort is applied for duplicate values in the first column. Example: SELECT * FROM logins ORDER BY password DESC, id ASC;

Output Examples

mysql> SELECT * FROM logins ORDER BY password;
+----+---------------+------------+---------------------+
| id | username      | password   | date_of_joining     |
+----+---------------+------------+---------------------+
|  2 | administrator | adm1n_p@ss | 2020-07-02 11:30:50 |
|  3 | john          | john123!   | 2020-07-02 11:47:16 |
|  1 | admin         | p@ssw0rd   | 2020-07-02 00:00:00 |
|  4 | tom           | tom123!    | 2020-07-02 11:47:16 |
+----+---------------+------------+---------------------+

mysql> SELECT * FROM logins ORDER BY password DESC;
+----+---------------+------------+---------------------+
| id | username      | password   | date_of_joining     |
+----+---------------+------------+---------------------+
|  4 | tom           | tom123!    | 2020-07-02 11:47:16 |
|  1 | admin         | p@ssw0rd   | 2020-07-02 00:00:00 |
|  3 | john          | john123!   | 2020-07-02 11:47:16 |
|  2 | administrator | adm1n_p@ss | 2020-07-02 11:30:50 |
+----+---------------+------------+---------------------+

mysql> SELECT * FROM logins ORDER BY password DESC, id ASC;
+----+---------------+-----------------+---------------------+
| id | username      | password        | date_of_joining     |
+----+---------------+-----------------+---------------------+
|  1 | admin         | p@ssw0rd        | 2020-07-02 00:00:00 |
|  2 | administrator | change_password | 2020-07-02 11:30:50 |
|  3 | john          | change_password | 2020-07-02 11:47:16 |
|  4 | tom           | change_password | 2020-07-02 11:50:20 |
+----+---------------+-----------------+---------------------+

`LIMIT` Results

Explanation

The LIMIT clause allows you to restrict the number of records returned by a query. This is useful when dealing with large result sets.

Steps

Use the LIMIT keyword followed by the number of records you want to retrieve. Example: SELECT * FROM logins LIMIT 2;
To specify an offset (starting point) for the records, include the offset before the limit count, separated by a comma. Example: SELECT * FROM logins LIMIT 1, 2; Note: The offset starts from 0, so LIMIT 1, 2 will start from the 2nd record and return 2 records.

Output Examples

mysql> SELECT * FROM logins LIMIT 2;
+----+---------------+------------+---------------------+
| id | username      | password   | date_of_joining     |
+----+---------------+------------+---------------------+
|  1 | admin         | p@ssw0rd   | 2020-07-02 00:00:00 |
|  2 | administrator | adm1n_p@ss | 2020-07-02 11:30:50 |
+----+---------------+------------+---------------------+

mysql> SELECT * FROM logins LIMIT 1, 2;
+----+---------------+------------+---------------------+
| id | username      | password   | date_of_joining     |
+----+---------------+------------+---------------------+
|  2 | administrator | adm1n_p@ss | 2020-07-02 11:30:50 |
|  3 | john          | john123!   | 2020-07-02 11:47:16 |
+----+---------------+------------+---------------------+

`WHERE` Clause

Explanation

The WHERE clause is used to filter results based on specified conditions. It allows you to retrieve specific records that meet the given criteria.

Steps

Use the WHERE keyword followed by the condition to filter the results. Example: SELECT * FROM logins WHERE id > 1;

Output Example

mysql> SELECT * FROM logins WHERE id > 1;
+----+---------------+------------+---------------------+
| id | username      | password   | date_of_joining     |
+----+---------------+------------+---------------------+
|  2 | administrator | adm1n_p@ss | 2020-07-02 11:30:50 |
|  3 | john          | john123!   | 2020-07-02 11:47:16 |
|  4 | tom           | tom123!    | 2020-07-02 11:47:16 |
+----+---------------+------------+---------------------+

In the example above, only records where the value of id is greater than 1 are returned.

Checkpoint

Write a query to sort the logins table by date_of_joining in descending order.
Retrieve the first 3 records from the logins table.
Find all records in the logins table where the username starts with the letter 'a'.

SQL Operators

SQL supports logical operators to combine multiple conditions in a query. The most common logical operators are AND, OR, and NOT.

`AND` Operator

Explanation

The AND operator takes two conditions and returns true if both conditions evaluate to true.

Syntax

condition1 AND condition2

Examples

mysql> SELECT 1 = 1 AND 'test' = 'test';
+---------------------------+
| 1 = 1 AND 'test' = 'test' |
+---------------------------+
|                         1 |
+---------------------------+

mysql> SELECT 1 = 1 AND 'test' = 'abc';
+--------------------------+
| 1 = 1 AND 'test' = 'abc' |
+--------------------------+
|                        0 |
+--------------------------+

In MySQL, any non-zero value is considered true (usually represented as 1), and 0 is considered false.

`OR` Operator

Explanation

The OR operator takes two conditions and returns true if at least one of the conditions evaluates to true.

Syntax

condition1 OR condition2

Examples

mysql> SELECT 1 = 1 OR 'test' = 'abc';
+-------------------------+
| 1 = 1 OR 'test' = 'abc' |
+-------------------------+
|                       1 |
+-------------------------+

mysql> SELECT 1 = 2 OR 'test' = 'abc';
+-------------------------+
| 1 = 2 OR 'test' = '

`NOT` Operator

Explanation

The NOT operator toggles a boolean value, converting true to false and vice versa.

Syntax

NOT condition

Examples

mysql> SELECT NOT 1 = 1;
+-----------+
| NOT 1 = 1 |
+-----------+
|         0 |
+-----------+

mysql> SELECT NOT 1 = 2;
+-----------+
| NOT 1 = 2 |
+-----------+
|         1 |
+-----------+

Symbol Operators

Explanation

The AND, OR, and NOT operators can also be represented using symbols:

AND: &&
OR: ||
NOT: !

Examples

mysql> SELECT 1 = 1 && 'test' = 'abc';
+-------------------------+
| 1 = 1 && 'test' = 'abc' |
+-------------------------+
|                       0 |
+-------------------------+

mysql> SELECT 1 = 1 || 'test' = 'abc';
+-------------------------+
| 1 = 1 || 'test' = 'abc' |
+-------------------------+
|                       1 |
+-------------------------+

mysql> SELECT 1 != 1;
+--------+
| 1 != 1 |
+--------+
|      0 |
+--------+

Using Operators in Queries

Examples

List all records where the username is NOT 'john':

SELECT * FROM logins WHERE username != 'john';

Select users who have an id greater than 1 AND a username NOT equal to 'john':

SELECT * FROM logins WHERE username != 'john' AND id > 1;

Operator Precedence

Explanation

SQL supports various operations such as arithmetic, comparison, and logical operations. The order of evaluation is determined by operator precedence.

Precedence Order (from highest to lowest)

Division (/), Multiplication (``), and Modulus (%)
Addition (+) and Subtraction (``)
Comparison (=, >, <, <=, >=, !=, LIKE)
NOT (!)
AND (&&)
OR (||)

Example

SELECT * FROM logins WHERE username != 'tom' AND id > 3 - 2;

Evaluation steps:

Subtraction: 3 - 2 = 1
Comparison: username != 'tom' AND id > 1
AND: Return records where both conditions are true

Checkpoint

Write a query to find all records where the username is 'admin' OR 'administrator'.
Retrieve records where the id is less than 5 AND the username is NOT 'john'.

That concludes the MySQL guide covering SQL statements, query results, operators, and best practices.

SQLi Series - Intro to MySQL and SQL Injection - 01

Table of contents

What is MySQL?

SQL Injection

Practical Application of MySQL

Example: Connecting to MySQL Database

Command Line Interaction

The mysql Utility

Authentication

Connecting to a Database

Example: Logging In

Creating and Managing Databases and Tables

Creating a Database

Defining Tables

SQL Statements

INSERT Statement

Objective

Explanation and Steps

Step-by-Step Application

Common Errors and Best Practices

SELECT Statement

Objective

Explanation and Steps

Application Examples

Tips and Tricks

DROP Statement

Objective

Explanation and Steps

Example

Warnings and Best Practices

ALTER Statement

Objective

Key Operations

Examples

Tips

UPDATE Statement

Objective

Explanation and Steps

Example

Common Pitfalls and Best Practices

Best Practices and Tips

What is the difference between ALTER and UPDATE?

ALTER Statement

UPDATE Statement

Key Differences

Checkpoint

Query Results

Sorting Results

Explanation

Steps

Output Examples

LIMIT Results

Explanation

Steps

Output Examples

WHERE Clause

Explanation

Steps

Output Example

Checkpoint

SQL Operators

AND Operator

Explanation

Syntax

Examples

OR Operator

Explanation

Syntax

Examples

NOT Operator

Explanation

Syntax

Examples

Symbol Operators

Explanation

Examples

Using Operators in Queries

Examples

Operator Precedence

Explanation

The `mysql` Utility

`INSERT` Statement

`SELECT` Statement

`DROP` Statement

`ALTER` Statement

`UPDATE` Statement

What is the difference between `ALTER` and `UPDATE`?

`ALTER` Statement

`UPDATE` Statement

`LIMIT` Results

`WHERE` Clause

`AND` Operator

`OR` Operator

`NOT` Operator