Data Definition Language (DDL) queries in SQL are used to define, modify, and manage the structure of a database and its objects. DDL statements allow you to create, alter, and delete database schemas, tables, indexes, and other database objects. Here are the key aspects of DDL queries:

1. CREATE Statement:

   • The CREATE statement is used to create new database objects, such as tables, indexes, views, and schemas.

   • Example: Creating a new table.

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

2. ALTER Statement:

   • The ALTER statement is used to modify existing database objects, such as adding, modifying, or deleting columns from a table.

   • Example: Adding a new column to an existing table.

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     ALTER TABLE employees
     ADD email VARCHAR(100);
     
     

3. DROP Statement:

   • The DROP statement is used to delete database objects, including tables, indexes, views, and schemas.

   • Example: Deleting a table.

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     DROP TABLE employees;
     
     

4. TRUNCATE Statement:

   • The TRUNCATE statement is used to remove all rows from a table but retains the table structure. It is faster than the DELETE statement.

   • Example: Truncating a table.

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     TRUNCATE TABLE employees;
     
     

5. RENAME Statement:

   • Some database systems support the RENAME statement, which is used to rename database objects.

   • Example: Renaming a table (specific syntax may vary by database system).

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     RENAME TABLE old_table TO new_table;
     
     

6. Constraints:

   • DDL queries allow you to define constraints on tables, such as primary keys, unique constraints, foreign keys, and check constraints.

   • Example: Adding a primary key constraint.

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     ALTER TABLE employees
     ADD CONSTRAINT pk_employee_id PRIMARY KEY (employee_id);
     
     

7. Indexes:

   • You can create and manage indexes on columns to improve query performance.

   • Example: Creating an index on a column.

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     CREATE INDEX idx_last_name ON employees (last_name);
     
     

8. Views:

   • DDL queries are used to create and modify views, which are virtual tables representing data from one or more tables.

   • Example: Creating a view.

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     CREATE VIEW employee_names AS
     SELECT employee_id, CONCAT(first_name, ' ', last_name) AS full_name
     FROM employees;
     
     

9. Schemas:

   • DDL queries allow you to create and manage database schemas, which are containers for organizing database objects.

   • Example: Creating a schema.

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     CREATE SCHEMA hr;
     
     

10. Transactions:

    • DDL statements are often part of transactions to ensure data consistency and integrity. You can use COMMIT and ROLLBACK statements to manage transactions.

DQL queries are used to define and manipulate the structure of a database, ensuring that it meets the requirements of the application and adheres to data integrity constraints. Properly defining and maintaining the database schema is crucial for data management and application development.