Difference between TRUNCATE, DELETE and DROP TABLE in SQL (Transact-Sql)

1. Both Truncate and Delete are used to delete rows from a table.

2. Truncate is like a delete command without a WHERE clause. 

3. Truncate deletes all rows of a table without logging for each row deletion. Truncate is a DDL command whereas Delete is a DML command.

4. Delete allows you to choose the rows to be deleted based on some condition but Truncate clears everything. Delete logs for each row it deletes, also will raise trigger if there is one and also it acquires lock before deleting that row.

5. Truncate removes all the rows in the table but retains its table structure, indexes, columns, constraints etc. Even Delete retains the constraints, structure, columns,etc.

6. Drop table completely removes a table from the data base including its structure, constraints etc. 

7. We cannot use Truncate on table that has Foreign Key referenced and even on a table that has participated in indexed view.

8. Truncate cannot activate Trigger whereas Delete can.

Advantage of Truncate over Delete as explained in MSDN:

Compared to the DELETE statement, TRUNCATE TABLE has the following advantages:

·         Less transaction log space is used.

The DELETE statement removes rows one at a time and records an entry in the transaction log for each deleted row. TRUNCATE TABLE removes the data by deallocating the data pages used to store the table data and records only the page deallocations in the transaction log.

·         Fewer locks are typically used.

When the DELETE statement is executed using a row lock, each row in the table is locked for deletion. TRUNCATE TABLE always locks the table and page but not each row.

·         Without exception, zero pages are left in the table.

After a DELETE statement is executed, the table can still contain empty pages. For example, empty pages in a heap cannot be deallocated without at least an exclusive (LCK_M_X) table lock. If the delete operation does not use a table lock, the table (heap) will contain many empty pages. For indexes, the delete operation can leave empty pages behind, although these pages will be deallocated quickly by a background cleanup process.

Examples:

TRUNCATE TABLE Employee;

DELETE FROM Employee Where Salary> 10000;

DROP TABLE Employee;