DDL

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DDL stands for “Data Definition Language.” It’s a subset of SQL (Structured Query Language) used to define and manage the structure and schema of a relational database. DDL statements are used to create, alter, and drop database objects such as tables, indexes, views, and constraints. Here are some common DDL statements:

1. CREATE: Used to create new database objects, such as tables, views, indexes, and constraints. For example:

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

2. ALTER: Used to modify the structure of existing database objects. For example, you can add, modify, or drop columns from a table:

   ALTER TABLE employees
   ADD email VARCHAR(100);

3. DROP: Used to delete existing database objects. For example:

   DROP TABLE employees;

4. TRUNCATE: Used to remove all data from a table, while keeping its structure intact. It’s faster than using DELETE to remove all rows:

   TRUNCATE TABLE employees;

5. COMMENT: Used to add comments or descriptions to database objects:

   COMMENT ON TABLE employees
   IS 'Stores information about company employees.';

6. CREATE INDEX: Used to create an index on one or more columns of a table to improve query performance:

   CREATE INDEX idx_last_name ON employees (last_name);

7. CREATE VIEW: Used to create a virtual table based on the result of a query. Views provide a way to simplify complex queries and present data in a more user-friendly way:

   CREATE VIEW employee_details AS
   SELECT first_name, last_name, department_name
   FROM employees
   JOIN departments ON employees.department_id = departments.department_id;

8. CREATE CONSTRAINT: Used to create integrity constraints on a table, such as primary keys, foreign keys, and unique constraints.

DDL statements are powerful tools for defining and managing the structure of a database. They play a crucial role in ensuring the consistency, integrity, and organization of the data stored in the database.

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