What is the difference between DELETE and TRUNCATE in SQL?

In SQL, DELETE and TRUNCATE are both used to remove data from tables, but they work differently and serve distinct purposes. DELETE is a Data Manipulation Language (DML) command that removes rows one at a time based on an optional WHERE clause. If no WHERE condition is provided, it deletes all rows, but it still processes each row individually, logging every deletion. This makes it slower for large datasets. In contrast, TRUNCATE is a Data Definition Language (DDL) command that removes all rows by deallocating the data pages used by the table. This operation is faster because it avoids row-by-row processing and minimizes logging. However, TRUNCATE cannot target specific rows—it always clears the entire table.

The transactional behavior and side effects of these commands also differ. DELETE can be executed within a transaction, allowing rollback if needed, and it fires any DELETE triggers defined on the table. For example, if you run DELETE FROM Employees WHERE Department = 'Sales', the database logs each deleted row, and you can undo the operation with ROLLBACK. TRUNCATE, however, cannot be rolled back in some database systems (like MySQL without transaction support) and does not trigger row-level actions. Additionally, TRUNCATE resets identity columns (e.g., auto-incrementing IDs) to their seed value, whereas DELETE preserves the current identity value. Permissions also differ: TRUNCATE typically requires higher privileges, such as ALTER TABLE access, while DELETE only needs standard delete permissions.

Use cases for each command depend on the scenario. Use DELETE when you need to remove specific rows or require transactional safety. For instance, deleting a user’s orders while retaining their account information. Use TRUNCATE when you need to quickly empty a large table, such as resetting a temporary dataset or staging table. However, TRUNCATE won’t work if the table is referenced by a foreign key constraint, whereas DELETE can still remove rows as long as constraints are respected. For example, TRUNCATE TABLE Logs clears the table instantly, but DELETE FROM Logs WHERE CreatedAt < '2023-01-01' selectively removes older entries. Choosing the right tool depends on balancing speed, granularity, and transactional needs.