An Overview of SQL Commands: DDL, DML, DCL, TCL, DQL – Types, Syntax, and Examples
Structured Query Language (SQL) is a programming language used for managing and manipulating relational databases. It provides a set of commands that allow users to create, modify, and retrieve data from databases. SQL commands can be categorized into different types based on their functionality. In this article, we will provide an overview of the main types of SQL commands: Data Definition Language (DDL), Data Manipulation Language (DML), Data Control Language (DCL), Transaction Control Language (TCL), and Data Query Language (DQL). We will discuss the purpose, syntax, and provide examples for each type.
1. Data Definition Language (DDL):
DDL commands are used to define and manage the structure of database objects such as tables, views, indexes, and constraints. The main DDL commands are:
– CREATE: Used to create a new database object.
– ALTER: Used to modify the structure of an existing database object.
– DROP: Used to delete a database object.
– TRUNCATE: Used to remove all data from a table.
Syntax example:
CREATE TABLE employees (
id INT PRIMARY KEY,
name VARCHAR(50),
age INT
);
2. Data Manipulation Language (DML):
DML commands are used to manipulate data within the database objects. The main DML commands are:
– INSERT: Used to insert new records into a table.
– UPDATE: Used to modify existing records in a table.
– DELETE: Used to delete records from a table.
– SELECT: Used to retrieve data from one or more tables.
Syntax example:
INSERT INTO employees (id, name, age) VALUES (1, ‘John Doe’, 30);
3. Data Control Language (DCL):
DCL commands are used to control access and permissions within the database. The main DCL commands are:
– GRANT: Used to grant specific privileges to users or roles.
– REVOKE: Used to revoke previously granted privileges.
Syntax example:
GRANT SELECT, INSERT ON employees TO user1;
4. Transaction Control Language (TCL):
TCL commands are used to manage transactions within the database. The main TCL commands are:
– COMMIT: Used to save changes made within a transaction.
– ROLLBACK: Used to undo changes made within a transaction.
– SAVEPOINT: Used to set a savepoint within a transaction.
Syntax example:
BEGIN TRANSACTION;
UPDATE employees SET age = 35 WHERE id = 1;
SAVEPOINT sp1;
UPDATE employees SET age = 40 WHERE id = 1;
ROLLBACK TO sp1;
COMMIT;
5. Data Query Language (DQL):
DQL commands are used to retrieve data from the database. The main DQL command is:
– SELECT: Used to retrieve data from one or more tables based on specified conditions.
Syntax example:
SELECT * FROM employees WHERE age > 30;
In conclusion, SQL provides a comprehensive set of commands for managing and manipulating relational databases. Understanding the different types of SQL commands, their syntax, and examples is essential for effectively working with databases. Whether you need to create, modify, retrieve, control access, or manage transactions, SQL offers a wide range of commands to meet your needs.
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