{"id":92554,"date":"2024-04-17T12:15:35","date_gmt":"2024-04-17T06:45:35","guid":{"rendered":"https:\/\/www.mygreatlearning.com\/blog\/sql-commands\/"},"modified":"2025-05-17T01:22:49","modified_gmt":"2025-05-16T19:52:49","slug":"sql-commands","status":"publish","type":"post","link":"https:\/\/www.mygreatlearning.com\/blog\/sql-commands\/","title":{"rendered":"SQL Commands (DDL, DML, DQL, DCL, TCL) with Examples"},"content":{"rendered":"\n

SQL<\/a>(Structured Query Language)<\/strong> is the standard language for managing and manipulating relational databases. SQL commands are the instructions used to communicate with a Relational Database Management System (RDBMS<\/a>) to create database structures, manipulate data, and control access.<\/p>\n\n\n\n

Whether you need to build a new table, update customer records, or retrieve a specific dataset, you will use specific subsets of SQL. This comprehensive guide covers the five main categories of SQL commands, s<\/span>uch as DDL, DML, DQL, DCL, and TCL, complete with definitions, syntax, and practical code examples.<\/p>\n\n\n\n

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SQL Commands Cheat Sheet<\/h2>\n\n\n\n

A quick reference for the most common commands.<\/p>\n\n\n\n

Command<\/th>Category<\/th>Description<\/th><\/tr><\/thead>
ALTER<\/td>DDL<\/td>Adds or modifies columns in a table.<\/td><\/tr>
COMMIT<\/td>TCL<\/td>Saves changes permanently.<\/td><\/tr>
CREATE<\/td>DDL<\/td>Creates a new table or database.<\/td><\/tr>
DELETE<\/td>DML<\/td>Removes rows from a table.<\/td><\/tr>
DROP<\/td>DDL<\/td>Deletes a table entirely.<\/td><\/tr>
GRANT<\/td>DCL<\/td>Gives a user access permissions.<\/td><\/tr>
INSERT<\/td>DML<\/td>Adds new rows to a table.<\/td><\/tr>
MERGE <\/td>DML <\/td>Conditionally inserts, updates, or deletes data (Upsert)<\/td><\/tr>
ROLLBACK<\/td>TCL<\/td>Undoes recent changes.<\/td><\/tr>
SELECT<\/td>DQL<\/td>Retrieves data.<\/td><\/tr>
TRUNCATE<\/td>DDL<\/td>Wipes all data but keeps the table.<\/td><\/tr>
UPDATE<\/td>DML<\/td>Edits existing data.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
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Data Definition Language (DDL)<\/h2>\n\n\n\n

Data Definition Language (DDL)<\/strong> commands define and manage the structure (schema) of your database. They allow you to create, modify, or delete database objects such as tables, indexes, and views.<\/p>\n\n\n\n

Note<\/strong>: In most relational databases (like Oracle and MySQL), DDL commands are auto-committed<\/strong>. This means once you execute a DROP<\/code> <\/strong>or TRUNCATE<\/code> <\/strong>command, the change is permanent and cannot be undone using a ROLLBACK<\/code> <\/strong>command.<\/p>\n\n\n

\"Data<\/figure>\n\n\n\n

1. CREATE:<\/strong><\/p>\n\n\n\n

Creates a new database or table.<\/p>\n\n\n\n

Example:<\/p>\n\n\n

\nCREATE TABLE Employees (\n    ID INT PRIMARY KEY,\n    Name VARCHAR(100),\n    Age INT,\n    Department VARCHAR(50)\n);\n<\/pre><\/div>\n\n\n
2. ALTER:<\/strong><\/p>\n\n\n\n
Modifies the structure of an existing table (e.g., adding, deleting, or modifying columns).<\/p>\n\n\n\n
Example:<\/p>\n\n\n
\nALTER TABLE Employees\nADD Salary DECIMAL(10, 2);\n<\/pre><\/div>\n\n\n
3. DROP:<\/strong><\/p>\n\n\n\n
Deletes a table and all its data permanently.<\/p>\n\n\n\n
Example:<\/p>\n\n\n
\nDROP TABLE Employees;\n<\/pre><\/div>\n\n\n
4. TRUNCATE:<\/strong><\/p>\n\n\n\n
Deletes all data inside a table but keeps the structure.<\/p>\n\n\n\n
Example:<\/p>\n\n\n
\nTRUNCATE TABLE Employees;\n<\/pre><\/div>\n\n\n
5. RENAME:<\/strong><\/p>\n\n\n\n
Changes the name of a database object (like a table or column).<\/p>\n\n\n\n
Example:<\/p>\n\n\n
\nRENAME TABLE Employees TO Staff;\n<\/pre><\/div>\n\n\n
6. COMMENT:<\/strong><\/p>\n\n\n\n
Adds or modifies a comment on a database object (like a table or column).<\/p>\n\n\n\n
Example:<\/p>\n\n\n
\nCOMMENT ON COLUMN Employees.Name IS 'Employee Name';\n<\/pre><\/div>\n\n\n
Essential SQL Constraints<\/strong><\/h3>\n\n\n\n
When using the CREATE TABLE DDL command, you will often apply constraints to specify rules for the data in a table. Common constraints include:<\/p>\n\n\n\n