SQL, DELETE, DROP, and ALTER are commands used for different purposes:

DELETE

1. Use Case: When you want to remove specific rows from a table based on a condition, but retain the structure of the table for future use.
2. Important Notes :

• You can delete all rows in a table if you don't specify a WHERE clause.
• DELETE can be rolled back if wrapped in a transaction (if the database supports transactions).
• Deleting rows doesn’t free up the space occupied by the data immediately; it’s managed by the database.
• Can be slower than TRUNCATE because it logs each row deletion (in most systems).

Example (deleting all rows):

DELETE FROM employees;        

Example (deleting specific rows):

DELETE FROM employees WHERE department = 'HR';        

DROP

1. Use Case: When you need to completely remove a database object (e.g., table, view, index) from the database, including all its data and structure.
2. Important Notes:

• It removes the object permanently and cannot be rolled back without a backup.
• Dropping a table will remove all its indexes, constraints, triggers, and data associated with that table.
• Usually faster than DELETE since it doesn’t log each row but just removes the object from the schema.

Example (dropping a table):

DROP TABLE employees;        

Example (dropping a view):

DROP VIEW employee_view;        

ALTER

1. Use Case: When you need to modify the structure or schema of an existing table without losing the existing data.
2. Important Notes :

• Allows modification of column definitions, constraints, renaming tables/columns, adding/removing columns, and more.
• Unlike DROP, it does not remove or destroy data.
• Changes made with ALTER may lock the table or cause performance overhead depending on the system and the type of modification.

Common ALTER operations:

• Adding a column:

ALTER TABLE employees ADD COLUMN salary DECIMAL(10, 2);        

• Dropping a column:

ALTER TABLE employees DROP COLUMN salary;        

• Renaming a column:

ALTER TABLE employees RENAME COLUMN old_name TO new_name;        

• Changing a column data type:

ALTER TABLE employees ALTER COLUMN salary TYPE INT;        

Key Differences (Summary):

Additional Notes:

• Performance: DROP is typically faster than DELETE because it removes the table entirely, whereas DELETE might involve more operations like logging each row removal.
• Space: DROP immediately frees up space, while DELETE may leave unused space in the table, which could require further maintenance (e.g., vacuuming in some databases).

These clarifications and examples should give you a more complete understanding of the differences between DELETE, DROP, and ALTER in SQL!