In SQL, both DELETE and TRUNCATE are commands used to remove data from tables, but they function quite differently. DELETE is a Data Manipulation Language (DML) command that removes rows one at a time and can be controlled using conditions. For example, you can delete specific records from a table based on a WHERE clause, like DELETE FROM employees WHERE department = 'Sales'. This command logs individual row deletions and can be rolled back if wrapped in a transaction. On the other hand, TRUNCATE is a Data Definition Language (DDL) command. It removes all rows from a table without logging individual row deletions, making it quicker and more efficient for emptying large tables.
Another key difference is the way they handle triggers and constraints. When a DELETE command is executed, any triggers associated with the table are fired, which can be useful for maintaining data integrity or logging actions. In contrast, TRUNCATE does not activate triggers because it does not operate on individual rows; it simply deallocates the data pages used by the table. This means that if you have dependent objects that rely on triggers or constraints, like foreign keys or cascades, TRUNCATE may not work as intended.
Lastly, the two commands differ in their impact on database structure and performance. Since TRUNCATE resets any identity columns to their seed value and does not return the number of affected rows, it minimizes the overhead involved in row-level logging. Therefore, it's not only faster but also less resource-intensive than DELETE for removing all records in a table. However, because DELETE can be used with specific conditions, it allows for greater control. In practice, developers will choose between DELETE and TRUNCATE based on their need for granularity, performance, and the role of triggers in their database design.