SQL (Structured Query Language): Web Development Explained

SQL, or Structured Query Language, is a standard programming language specifically designed for managing data held in a relational database management system (RDBMS), or for stream processing in a relational data stream management system (RDSMS). It is particularly useful in handling structured data, i.e., data incorporating relations among entities and variables.

SQL offers two main advantages: first, it introduced the concept of accessing many records with one single command; and second, it eliminates the need to specify how to reach a record, e.g., with or without an index.

History of SQL

SQL was initially developed at IBM by Donald D. Chamberlin and Raymond F. Boyce in the early 1970s. This version, initially called SEQUEL (Structured English Query Language), was designed to manipulate and retrieve data stored in IBM's original quasi-relational database management system, System R, which a group at IBM San Jose Research Laboratory had developed.

The acronym SEQUEL was later changed to SQL because "SEQUEL" was a trademark of the UK-based Hawker Siddeley aircraft company. In the late 1970s, SQL was used by Relational Software, now Oracle.

Standardization

After the system had been sold to the UK, it was standardized in ANSI in 1986 and ISO in 1987. Since then, the standard has been revised to include a larger set of features. Despite the existence of such standards, most SQL code requires at least some changes before being ported to different database systems.

The SQL standard is divided into nine parts. SQL has been standardized by both the International Organization for Standardization (ISO) and the American National Standards Institute (ANSI).

Language Elements

SQL is based on the relational algebra, and for certain operations, the two are equivalent. SQL includes operators and functions for calculating values on stored values. SQL includes a mechanism for defining data structures, as well as operators and functions for updating and querying those data structures.

SQL is a declarative language, which means the logic of the computation is hidden from the user. Many procedural extensions for SQL have been added to the standard, including control-of-flow constructs, transaction controls, variables, and exception handling.

Queries

The most common operation in SQL, the query, makes use of the declarative SELECT statement. SELECT retrieves data from one or more tables, or expressions. Standard SQL statements have no persistent effects on the database. Some non-standard implementations of SELECT can have persistent effects, such as the SELECT INTO syntax that exists in some databases.

Queries allow the user to describe desired data, leaving the database management system (DBMS) to carry out planning, optimizing, and performing the physical operations necessary to produce that result as it chooses.

Data Manipulation

After the database is created, manipulated and queried, SQL provides commands for a variety of purposes including adding new rows, changing existing rows, deleting existing rows, etc. These commands are collectively known as Data Manipulation Language (DML).

DML is a subset of SQL commands that modify the database instance by inserting, modifying, and deleting its data. A DML is often a content data language for a corresponding SQL data schema.

SQL in Web Development

In web development, SQL is used to serve and manipulate data stored in RDBMS. SQL is used in combination with a scripting language like PHP or a framework like Django, to create dynamic web content. It can be used to insert new data, update existing data, retrieve data and delete data.

Web developers use SQL to write applications that require database connectivity. They use SQL to create views, stored procedures, and functions that application software can use to enforce business rules consistently.

Server-Side Scripting and SQL

Server-side scripting languages like PHP, Python, Ruby, and .NET often incorporate SQL to handle data storage and manipulation. These languages use SQL to communicate with the database, send commands, and retrieve results.

For example, a PHP script running on a web server might use SQL commands to read data from a database, format it as HTML, and send it to a client's web browser. The client sees only the HTML, not the SQL that was used to generate it.

Frameworks and SQL

Many modern web development frameworks, such as Django (Python), Rails (Ruby), and Laravel (PHP), incorporate SQL through an Object-Relational Mapping (ORM) layer. This layer allows developers to interact with the database using the syntax and concepts of their programming language, rather than writing SQL commands.

However, even when using an ORM, understanding SQL and its concepts can be critical. Complex queries or performance optimizations often require writing raw SQL, and understanding how the ORM translates code into SQL can help prevent bugs and inefficiencies.

SQL Injection

SQL injection is a code injection technique that attackers can use to exploit vulnerabilities in a web application's database layer. This form of attack involves injecting malicious SQL statements into an entry field for execution to dump the database contents to the attacker.

SQL injection attacks can be prevented by careful programming practices, such as using parameterized queries or prepared statements, and by validating and sanitizing all user input. Understanding SQL and its potential vulnerabilities is crucial for web developers to secure their applications.

Prevention

Preventing SQL injection attacks involves securing the database layer of a web application. This can be achieved by using parameterized queries or prepared statements, which separate SQL code from data. This separation prevents an attacker from changing the intent of a query.

Another important prevention measure is input validation and sanitization. This involves checking all user input to ensure it is safe before it is included in a SQL query. This can prevent an attacker from inserting malicious SQL code into user input fields.

Conclusion

SQL is a powerful tool in web development, allowing developers to interact with relational databases, create dynamic content, and enforce business rules. Understanding SQL and its concepts is crucial for any web developer, as it underpins many of the technologies used in modern web development.

However, as with any powerful tool, SQL can be misused, and understanding its potential vulnerabilities, such as SQL injection attacks, is also important. By using best practices, such as parameterized queries and input validation, developers can secure their applications and protect their data.