What is SQL?
SQL (Structured Query Language) is a standardized programming language designed for managing and manipulating relational databases. It is used to query, insert, update, and delete data, as well as to create and modify database structures.
Quick Facts
| Full Name | Structured Query Language |
|---|---|
| Created | 1974 by IBM (Donald Chamberlin and Raymond Boyce) |
| Specification | Official Specification |
How It Works
SQL was developed at IBM in the early 1970s and became an ANSI standard in 1986. It provides a declarative approach to database operations, where users specify what data they want rather than how to retrieve it. SQL consists of several sub-languages: DDL (Data Definition Language) for schema, DML (Data Manipulation Language) for data, DCL (Data Control Language) for permissions, and TCL (Transaction Control Language) for transactions. While the core syntax is standardized, different database systems (MySQL, PostgreSQL, SQL Server, Oracle) have their own extensions and variations.
Key Characteristics
- Declarative language - specify what, not how
- ANSI/ISO standardized with vendor extensions
- Supports CRUD operations (Create, Read, Update, Delete)
- Includes DDL, DML, DCL, and TCL sub-languages
- Relational model based on set theory
- Supports transactions with ACID properties
Common Use Cases
- Database querying and reporting
- Data manipulation and transformation
- Database schema design and management
- Backend application data access
- Data analysis and business intelligence
Example
Loading code...Frequently Asked Questions
What is the difference between SQL and NoSQL databases?
SQL databases are relational, using structured tables with predefined schemas and supporting ACID transactions. NoSQL databases include document stores, key-value stores, and graph databases with flexible schemas. SQL is best for complex queries and transactions; NoSQL excels at horizontal scaling and unstructured data.
What are JOINs in SQL and what types are there?
JOINs combine rows from multiple tables based on related columns. INNER JOIN returns matching rows from both tables. LEFT JOIN returns all rows from the left table plus matches. RIGHT JOIN returns all rows from the right table plus matches. FULL OUTER JOIN returns all rows when there's a match in either table.
How can I prevent SQL injection attacks?
Use parameterized queries (prepared statements) instead of string concatenation to build SQL queries. Never insert user input directly into SQL strings. Use ORM frameworks that handle escaping automatically. Implement input validation and apply the principle of least privilege for database accounts.
What is the difference between WHERE and HAVING clauses?
WHERE filters rows before grouping and cannot use aggregate functions. HAVING filters groups after GROUP BY and can use aggregate functions. For example, use WHERE to filter individual records, and HAVING to filter groups based on aggregate calculations like COUNT(*) > 5.
What are indexes in SQL and when should I use them?
Indexes are data structures that speed up data retrieval by creating pointers to rows based on column values. Use indexes on columns frequently used in WHERE clauses, JOINs, and ORDER BY. Avoid over-indexing as indexes slow down INSERT/UPDATE operations and consume storage space.