Database Languages – DDL (Data Definition Language)
Comprehensive Oracle SQL examples and explanations for database structure management.
Database languages allow interaction with a DBMS. DDL (Data Definition Language) is used to define, create, modify, and delete database objects such as tables, indexes, views, and sequences.
1. What is DDL?
- Defines database schema and structure.
- Changes are permanent (auto-committed in Oracle).
- Does not manipulate data (use DML for that).
- Requires proper privileges (CREATE, ALTER, DROP).
2. Common DDL Commands
| Command | Purpose | Oracle Example |
|---|---|---|
| CREATE | Create a new database object | CREATE TABLE student (student_id NUMBER PRIMARY KEY, name VARCHAR2(50), course VARCHAR2(50)); |
| ALTER | Modify an existing object | ALTER TABLE student ADD (email VARCHAR2(50)); |
| DROP | Delete a database object | DROP TABLE student; |
| TRUNCATE | Remove all rows, keep structure | TRUNCATE TABLE student; |
| RENAME | Rename an object | RENAME student TO student_old; |
3. Oracle Examples in Detail
CREATE Table
CREATE TABLE employee (
emp_id NUMBER PRIMARY KEY,
emp_name VARCHAR2(50),
dept_id NUMBER
);
ALTER Table
ALTER TABLE employee ADD (salary NUMBER(10,2)); ALTER TABLE employee MODIFY (emp_name VARCHAR2(100));
DROP Table
DROP TABLE employee;
TRUNCATE Table
TRUNCATE TABLE employee;
RENAME Table
RENAME employee TO emp_backup;
4. Key Points and Best Practices
- DDL commands are auto-committed – changes are permanent.
- Ensure proper privileges before executing DDL.
- Take backups before DROP or TRUNCATE commands.
- Use ALTER carefully to avoid data loss.
5. Summary
DDL is essential for database design, schema creation, and maintenance. Commands include CREATE, ALTER, DROP, TRUNCATE, RENAME. DDL affects the structure of database objects but not the data itself.
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