Ultimate Guide to SQL: DDL, DML, DCL, TCL and DQL
Explore the essential SQL commands for creating and managing databases!
Structured Query Language (SQL) is the standard language for managing and manipulating relational databases. Whether you are creating a new database, managing existing data, or controlling user access, SQL provides the tools needed to perform these tasks efficiently.
SQL commands are generally classified into five types:
- DDL (data definition language): To define the structure of the database.
- DML (data manipulation language): To manage data in tables.
- DCL (data control language): To control access to the database.
- TCL (transaction control language): To manage database transactions.
- DQL (data query language): To recover data.
Hello, my name is CyCoderX and today in this article we will explore each category, its role and examples of common commands to help you understand their importance in database management.
Let’s go!
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What is DDL (Data Definition Language)?
DDL commands define the structure of the database. They are used to create, modify and drop database objects such as tables, indexes and schemas.
Common DDL commands:
- CREATE: Creates new database objects like tables or indexes.
CREATE TABLE employees (
id INT PRIMARY KEY,
name VARCHAR(100),
salary DECIMAL(10, 2)
);
- TO MODIFY: Modifies an existing database object.
ALTER TABLE employees ADD department VARCHAR(50);
- DROP: Permanently deletes a database object.
DROP TABLE employees;
- TRUNCATE: Deletes all data from a table without recording individual row deletions.
TRUNCATE TABLE employees;
DDL commands directly affect the structure of the database and changes are often irreversible without a backup.
What is DML (data manipulation language)?
DML commands are used to manipulate data in database tables. These commands allow you to insert, update, and delete data.
Common DML commands:
- INSERT: Adds new records to a table.
INSERT INTO employees (id, name, salary, department)
VALUES (1, 'Alice', 75000, 'IT');
- UPDATE: Modifies existing records in a table.
UPDATE employees
SET salary = 80000
WHERE id = 1;
- DELETE: Deletes records from a table based on a condition.
DELETE FROM employees
WHERE id = 1;
DML commands work on table data and allow you to manage the information stored in the database. Unlike DDL, DML operations can often be rolled back if necessary.
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What is DCL (Data Control Language)?
DCL commands are used to manage permissions and access to the database. These commands ensure that only authorized users can perform specific actions.
Common DCL commands:
- TO AGREE: Gives specific privileges to a user.
GRANT SELECT, INSERT ON employees TO user1;
- REVOKE: Removes privileges previously granted to a user.
REVOKE INSERT ON employees FROM user1;
DCL commands play a vital role in database security, ensuring that sensitive data is only accessible to people with the appropriate permissions.
What is TCL (Transaction Control Language)?
TCL commands manage transactions within a database. A transaction is a sequence of one or more SQL operations executed as a single unit.
Common TCL commands:
- COMMIT: Saves all changes made during the current transaction.
COMMIT;
- GO BACK: Cancels the changes made during the current transaction.
ROLLBACK;
- SAVE POINT: Defines a point in a transaction that you can return to later.
SAVEPOINT sp1;
TCL commands ensure data integrity and enable recovery from errors during complex operations.
What is DQL (Data Query Language)?
DQL commands are used to retrieve data from a database. Unlike DDL, DML, DCL or TCL, DQL mainly focuses on data querying.
Common DQL command:
- SELECT: Retrieves data from one or more tables.
SELECT name, salary
FROM employees
WHERE department = 'IT';
The SELECT statement supports various clauses such as WHERE, GROUP BY, and ORDER BY to filter and organize the retrieved data.
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Comparison table of SQL command types
Here’s a quick comparison of the five categories of SQL commands:
Conclusion
SQL commands are divided into five categories, each serving a specific purpose in database management:
- DDL: Defines the structure of the database.
- DML: Manipulates data in tables.
- DCL: Controls access and permissions.
- TCL: Manages transactions for data integrity.
- DQL: Retrieves data from the database.
Understanding these categories is essential to mastering SQL and effectively managing relational databases. Whether you’re building a database, analyzing data, or ensuring security, knowing when and how to use each type of command is key to success.
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Final words:
Thank you for taking the time to read my article. Article first published on Medium by CyCoderX.
Hi, my name is CyCoderX! Engineer passionate about sharing knowledge, I write articles on Python, SQL, Data Science, Data Engineering and much more!
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