By Balamurugan G on Sep 12, 2008 | Reply

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The difference in TRUNCATE and DELETE in Sql Server
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I’ve answered this question many times, and answered it again this weekend. What is the difference when doing a DELETE TableA instead of TRUNCATE TableA? A common misconception is that they do the same thing. Not so. In fact, there are many differences between the two.

DELETE is a logged operation on a per row basis. This means that the deletion of each row gets logged and physically deleted.

You can DELETE any row that will not violate a constraint, while leaving the foreign key or any other contraint in place.

TRUNCATE is also a logged operation, but in a different way. TRUNCATE logs the deallocation of the data pages in which the data exists. The deallocation of data pages means that your data rows still actually exist in the data pages, but the extents have been marked as empty for reuse. This is what makes TRUNCATE a faster operation to perform over DELETE.

You cannot TRUNCATE a table that has any foreign key constraints. You will have to remove the contraints, TRUNCATE the table, and reapply the contraints.

TRUNCATE will reset any identity columns to the default seed value. This means if you have a table with an identity column and you have 264 rows with a seed value of 1, your last record will have the value 264 (assuming you started with value 1) in its identity columns. After TRUNCATEing your table, when you insert a new record into the empty table, the identity column will have a value of 1. DELETE will not do this. In the same scenario, if you DELETEd your rows, when inserting a new row into the empty table, the identity column will have a value of 265.