Classification of SQL Queries

SQL (Structured Query Language) is a standard language for managing and manipulating databases. SQL is divided into different languages or subcategories based on the type of operation it performs. Here's a classification of SQL languages along with examples of queries:

1. **Data Query Language (DQL)**:

   - **Purpose**: Used to retrieve data from a database.

   - **Key Statement**: `SELECT`

Example:

   SELECT * FROM students;

   In this example, the `SELECT` statement retrieves all rows from the `students` table.

2. **Data Definition Language (DDL)**:

   - **Purpose**: Defines the structure of the database, such as creating, altering, and dropping tables and other database objects.

   - **Key Statements**: `CREATE`, `ALTER`, `DROP`, `TRUNCATE`

 Example 1: `CREATE TABLE`

   CREATE TABLE students (

      student_id INT PRIMARY KEY,

      student_name VARCHAR(100),

      age INT

   );

   This creates a `students` table with columns for `student_id`, `student_name`, and `age`.

 Example 2: `ALTER TABLE`

   ALTER TABLE students ADD COLUMN gender VARCHAR(10);

   This adds a `gender` column to the `students` table.

Example 3: `DROP TABLE`

   DROP TABLE students;

   This statement deletes the `students` table along with all its data.

---

3. **Data Manipulation Language (DML)**:

   - **Purpose**: Used to manipulate data within the database. It covers inserting, updating, and deleting records.

   - **Key Statements**: `INSERT`, `UPDATE`, `DELETE`

Example 1: `INSERT`

   INSERT INTO students (student_id, student_name, age) 

   VALUES (1, 'John Doe', 20);

   This inserts a new record into the `students` table.

Example 2: `UPDATE`

   UPDATE students 

   SET age = 21 

   WHERE student_id = 1;

   This updates the `age` of the student with `student_id` 1 to 21.

Example 3: `DELETE`

   DELETE FROM students WHERE student_id = 1;

   This deletes the record of the student with `student_id` 1 from the `students` table.

4. **Data Control Language (DCL)**:

   - **Purpose**: Used to control access to data in the database, typically through permission management.

   - **Key Statements**: `GRANT`, `REVOKE`

Example 1: `GRANT`

   GRANT SELECT, INSERT ON students TO 'username';

   This grants the user `username` permission to `SELECT` and `INSERT` records in the `students` table.

Example 2: `REVOKE`

   REVOKE INSERT ON students FROM 'username';

   This revokes the `INSERT` permission from the user `username` on the `students` table.

---

5. **Transaction Control Language (TCL)**:

   - **Purpose**: Used to manage transactions in the database. Transactions allow groups of SQL statements to be executed in a way that ensures consistency and atomicity.

   - **Key Statements**: `COMMIT`, `ROLLBACK`, `SAVEPOINT`

 Example 1: `COMMIT`

   COMMIT;

   This commits the current transaction, making all changes made permanent.

 Example 2: `ROLLBACK`

   ```sql

   ROLLBACK;

   ```

   This rolls back the current transaction, undoing all changes since the last `COMMIT`.

 Example 3: `SAVEPOINT`

   SAVEPOINT save1;

   This creates a savepoint named `save1`, which allows partial rollback to this specific point

Classification of SQL Queries:

Each of these languages plays a critical role in working with relational databases, and depending on the specific use case, you would use different combinations of them to interact with your data effectively.