Brief History of SQL
SQL (Structured Query Language) was developed in the 1970s by IBM researchers Donald D. Chamberlin and Raymond F. Boyce. It was initially called SEQUEL (Structured English Query Language) and was designed to manipulate and retrieve data stored in IBM's relational database management system (RDBMS). Later, SQL became a standard for database management, with ANSI and ISO establishing official versions. Today, SQL is widely used in various database systems like MySQL, PostgreSQL, SQL Server, and Oracle.
Basic Concepts of SQL
Database
A database is a collection of structured data stored electronically. It organizes information systematically, making it easy to access, manage, and update.
Tables
A table is a fundamental component of an SQL database. It consists of rows and columns, where each row represents a record, and each column represents an attribute of that record.
SQL Statements
SQL is a query language used to interact with databases. The main types of SQL statements include:
- DDL (Data Definition Language): Used to define and modify the database structure (e.g.,
CREATE,ALTER,DROP). - DML (Data Manipulation Language): Used to manipulate data within tables (e.g.,
INSERT,UPDATE,DELETE). - DQL (Data Query Language): Used to retrieve data from tables (e.g.,
SELECT). - DCL (Data Control Language): Used to control access to data (e.g.,
GRANT,REVOKE). - TCL (Transaction Control Language): Used to manage transactions (e.g.,
COMMIT,ROLLBACK).
Primary Key
A primary key is a unique identifier for each record in a table. It ensures that no two rows have the same value in the primary key column.
Foreign Key
A foreign key is a column that establishes a relationship between two tables. It references a primary key in another table to maintain referential integrity.
Normalization
Normalization is the process of organizing a database to reduce redundancy and improve data integrity. It involves dividing large tables into smaller ones and defining relationships between them.
Joins
Joins are used to combine data from multiple tables based on a related column. Common types of joins include:
- INNER JOIN: Returns only matching records from both tables.
- LEFT JOIN: Returns all records from the left table and matching records from the right table.
- RIGHT JOIN: Returns all records from the right table and matching records from the left table.
- FULL JOIN: Returns all records from both tables, including unmatched ones.
Indexes
Indexes improve the speed of data retrieval operations by creating a data structure that allows quick lookups. They are commonly used on frequently searched columns.
Stored Procedures
A stored procedure is a set of SQL statements that can be executed as a single unit. It helps in reducing redundancy and improving performance.
Triggers
Triggers are automated actions executed when specific database events occur, such as inserting, updating, or deleting a record.
Views
A view is a virtual table based on an SQL query. It allows users to retrieve specific data without modifying the original table.
Transactions
Transactions ensure that a series of SQL operations are executed as a single unit. If any part of the transaction fails, the entire process can be rolled back to maintain data integrity.