SQL Commands | DDL, DQL, DML, DCL and TCL Commands

SQL commands are crucial for managing databases effectively. These commands are divided into categories such as Data Definition Language (DDL), Data Manipulation Language (DML), Data Control Language (DCL), Data Query Language (DQL), and Transaction Control Language (TCL).

In this article, we will explain the different types of SQL commands, including DDL, DML, DCL, DQL, and TCL. These SQL sublanguages serve specific purposes and are important for effective database management.

What are SQL Commands?

SQL commands are the fundamental building blocks for communicating with a database management system (DBMS). It is used to interact with the database with some operations. It is also used to perform specific tasks, functions, and queries of data. SQL can perform various tasks like creating a table, adding data to tables, dropping the table, modifying the table, set permission for users.

SQL Commands are mainly categorized into five categories: 

• DDL – Data Definition Language
• DQL – Data Query Language
• DML – Data Manipulation Language
• DCL – Data Control Language
• TCL - Transaction Control Language

1. DDL - Data Definition Language

DDL or Data Definition Language actually consists of the SQL commands that can be used for defining, altering, and deleting database structures such as tables, indexes, and schemas. It simply deals with descriptions of the database schema and is used to create and modify the structure of database objects in the database

Common DDL Commands

Example:

CREATE TABLE employees (
    employee_id INT PRIMARY KEY,
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    hire_date DATE
);

In this example, a new table called employees is created with columns for employee ID, first name, last name, and hire date.

2. DQL - Data Query Language

DQL statements are used for performing queries on the data within schema objects. The purpose of the DQL Command is to get some schema relation based on the query passed to it. This command allows getting the data out of the database to perform operations with it. When a SELECT is fired against a table or tables the result is compiled into a further temporary table, which is displayed or perhaps received by the program.

DQL Command

Example:

SELECT first_name, last_name, hire_date
FROM employees
WHERE department = 'Sales'
ORDER BY hire_date DESC;

This query retrieves employees' first and last names, along with their hire dates, from the employees table, specifically for those in the 'Sales' department, sorted by hire date.

3. DML - Data Manipulation Language

The SQL commands that deal with the manipulation of data present in the database belong to DML or Data Manipulation Language and this includes most of the SQL statements. It is the component of the SQL statement that controls access to data and to the database. Basically, DCL statements are grouped with DML statements.

Common DML Commands

Example:

INSERT INTO employees (first_name, last_name, department) 
VALUES ('Jane', 'Smith', 'HR');

This query inserts a new record into the employees table with the first name 'Jane', last name 'Smith', and department 'HR'.

4. DCL - Data Control Language

DCL (Data Control Language) includes commands such as GRANT and REVOKE which mainly deal with the rights, permissions, and other controls of the database system. These commands are used to control access to data in the database by granting or revoking permissions.

Common DCL Commands

Example of DCL

GRANT SELECT, UPDATE ON employees TO user_name;

This command grants the user user_name the permissions to select and update records in the employees table.

5. TCL - Transaction Control Language

Transactions group a set of tasks into a single execution unit. Each transaction begins with a specific task and ends when all the tasks in the group are successfully completed. If any of the tasks fail, the transaction fails. Therefore, a transaction has only two results: success or failure. We can explore more about transactions here.

Common TCL Commands

Example:

BEGIN TRANSACTION;
UPDATE employees SET department = 'Marketing' WHERE department = 'Sales';
SAVEPOINT before_update;
UPDATE employees SET department = 'IT' WHERE department = 'HR';
ROLLBACK TO SAVEPOINT before_update;
COMMIT;

In this example, a transaction is started, changes are made, and a savepoint is set. If needed, the transaction can be rolled back to the savepoint before being committed.

Most Important SQL Commands

There are also a few other SQL Commands we often rely on when writing powerful queries. While they don’t fit neatly into the five main categories, they’re absolutely essential for working with data effectively.

Conclusion

SQL commands such as DDL, DML, DCL, DQL, and TCL are foundational for effective database management. From creating and modifying tables with DDL commands to managing transactions with TCL commands in SQL, understanding each type of command enhances our database skills. Whether we are manipulating data, or managing data, SQL provides all sets of tools. Now, with this detailed guide, we hope you have gained a deep understanding of SQL commands, their categories, and syntax with examples.