SQL commands are grouped into five main categories based on their purpose: Data Definition Language (DDL), Data Manipulation Language (DML), Data Control Language (DCL), Transaction Control Language (TCL), and Data Query Language (DQL). Each type serves a distinct role in managing databases. DDL focuses on structuring the database, DML handles data modification, DCL manages access permissions, TCL controls transactions, and DQL retrieves data. These categories provide a framework for developers to interact with databases systematically.
Data Definition Language (DDL) commands define or modify the database schema. Examples include CREATE, ALTER, and DROP. CREATE TABLE builds a new table with specified columns and data types, while ALTER TABLE adds or removes columns. DROP TABLE deletes a table entirely. DDL commands are critical during initial database design or structural updates. For instance, adding an email column to a users table with ALTER TABLE users ADD email VARCHAR(255); modifies the schema without affecting existing data. These commands are typically executed sparingly and require careful planning, as they directly impact the database’s architecture.
Data Manipulation Language (DML) and Data Query Language (DQL) handle data interaction. DML includes INSERT, UPDATE, and DELETE, which add, modify, or remove records. For example, INSERT INTO users (name, age) VALUES ('Alice', 30); adds a new row. DQL, often considered a subset of DML, revolves around SELECT for querying data. A query like SELECT name FROM users WHERE age > 25; retrieves specific records. While DML focuses on changing data, DQL extracts it for analysis or display. These commands are used frequently in applications, such as updating a user’s profile or fetching product details from a catalog.
Data Control Language (DCL) and Transaction Control Language (TCL) manage permissions and transactional integrity. DCL includes GRANT and REVOKE, which control user access. For example, GRANT SELECT ON orders TO analyst; allows a user to read order data. TCL commands like COMMIT and ROLLBACK ensure transactions (a series of operations) either fully complete or revert on failure. If a banking app transfers funds between accounts, COMMIT finalizes the changes after both updates succeed, while ROLLBACK undoes them if an error occurs. These commands maintain security and data consistency, especially in multi-user environments.