Data Definition Language (DDL)

Command Syntax Guide and Reference

1. Overview of DDL Commands

Command Purpose
CREATE Creates new database objects
ALTER Modifies existing objects
DROP Deletes objects completely
TRUNCATE Removes all rows from a table
RENAME Renames database objects

2. Database Operations

Create & Select Database Syntax 

CREATE DATABASE database_name;

USE database_name;

Drop Database Syntax 

DROP DATABASE database_name;

3. Table Creation & Constraints

Create Table Syntax 

CREATE TABLE table_name
(
    column1_name data_type constraint,
    column2_name data_type constraint,
    column3_name data_type,
    FOREIGN KEY (column3_name) REFERENCES parent_table(parent_column)
);

Common Column Constraints

  • PRIMARY KEY — Uniquely identifies a record
  • FOREIGN KEY — Links to a primary key in another table
  • NOT NULL — Ensures column cannot have NULL values
  • UNIQUE — Ensures all values in a column are distinct
  • CHECK — Ensures values meet a specific condition
  • DEFAULT — Sets a fallback value when none is provided

4. Structure Modification (ALTER)

Add Column Syntax 

ALTER TABLE table_name 
ADD column_name data_type;

Modify Column Data Type Syntax 

ALTER TABLE table_name 
MODIFY column_name new_data_type;

Rename Column Syntax 

ALTER TABLE table_name 
RENAME COLUMN old_column_name TO new_column_name;

Drop Column Syntax 

ALTER TABLE table_name 
DROP COLUMN column_name;

5. Table Renaming, Truncating, and Dropping

Rename Table Syntax 

RENAME TABLE old_table_name TO new_table_name;

Truncate Table Syntax 

TRUNCATE TABLE table_name;

Drop Table Syntax 

DROP TABLE table_name;

View Table Structure Syntax 

DESCRIBE table_name;