Difference between TRUNCATE, DELETE and DROP?

DELETE and TRUNCATE are two SQL commands used to remove records from a particular table. But they differ in how they execute and operate.
–> DELETE:

1. Removes Some or All rows from a table.

2. A WHERE clause can be used to remove some rows. If no WHERE condition is specified, all rows will be removed.

3. Causes all DELETE triggers on the table to fire.

4. It removes rows row-by-row one at a time and records an entry in the Transaction logs, thus is slower than TRUNCATE.

5. Every deleted row in locked, thus it requires more number of locks and database resources.

6. According to MS BOL, if a table is a Heap or no Clustered index is defined than the row-pages emptied are not de-allocated instantly and remain allocated in the heap. Thus, no other object can reuse this associated space. Thus to de-allocate the space a Clustered index is required or TABLOCK hint should be applied in the DELETE statement.

7. This is a DML command as it is just used to manipulate/modify the table data. It does not change any property of a table.


 

–> TRUNCATE:

1. Removes All rows from a table.

2. Does not require a WHERE clause, so you can not filter rows while Truncating.

3. With SQL Server 2016 you can Truncate a Table Partition, for more details check [here].

4. IDENTITY columns are re-seeded on this operation, if no seed was defined then the default value 1 is used.

5. No Triggers are fired on this operation because it does not operate on individual rows.

6. It de-allocates Data Pages instead of Rows and records Data Pages instead of Rows in Transaction logs, thus is faster than DELETE.

7. While de-allocating Pages it locks Pages and not Rows, thus it requires less number of locks and few resources.

8. TRUNCATE is not possible when a table:
a. is reference by a Foreign Key or tables used in replication or with Indexed views.
b. participates in an Indexed/Materialized View.
c. published by using Transactional/Merge replication.

9. This is a DDL command as it resets IDENTITY columns, de-allocates Data Pages and empty them for use of other objects in the database.

Note: It is a misconception among some people that TRUNCATE cannot be roll-backed. But in reality both DELETE and TRUNCATE operations can be COMMITTED AND ROLL-BACKED if provided inside a Transaction. The only method to Rollback a committed transaction after DELETE/TRUNCATE is to restore the last backup and run transactions logs till the time when DELETE/TRUNCATE is about to happen.


 

–> DROP:

1. The DROP TABLE command removes one or more table(s) from the database.

2. All related Data, Indexes, Triggers, Constraints, and Permission specifications for the Table are dropped by this operation.

3. Some objects like Views, Stored Procedures that references the dropped table are not dropped and must be explicitly dropped.

4. Cannot drop a table that is referenced by any Foreign Key constraint.

5. According to MS BOL, Large tables and indexes that use more than 128 extents are dropped in two separate phases: Logical and Physical. In the Logical phase, the existing allocation units used by the table are marked for de-allocation and locked until the transaction commits. In the physical phase, the IAM pages marked for de-allocation are physically dropped in batches.

 

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DELETE
1. DELETE is a DML Command.
2. DELETE statement is executed using a row lock, each row in the table is locked for deletion.
3. We can specify filters in where clause
4. It deletes specified data if where condition exists.
5. Delete activates a trigger because the operation are logged individually.
6. Slower than truncate because, it keeps logs.
7. Rollback is possible.

TRUNCATE
1. TRUNCATE is a DDL command.
2. TRUNCATE TABLE always locks the table and page but not each row.
3. Cannot use Where Condition.
4. It Removes all the data.
5. TRUNCATE TABLE cannot activate a trigger because the operation does not log individual row deletions.
6. Faster in performance wise, because it doesn’t keep any logs.
7. Rollback is not possible.

DELETE and TRUNCATE both can be rolled back when used with TRANSACTION.

If Transaction is done, means COMMITED, then we can not rollback TRUNCATE command, but we can still rollback DELETE command from LOG files, as DELETE write records them in Log file in case it is needed to rollback in future from LOG files.

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