SQL Delete Table Delete drop truncate difference

(1) The DELETE statement performs the deletion by deleting one row from the table at a time and saving the row's delete operation as a transaction record in the log for the rollback operation. TRUNCATE table deletes all the data from the table at once and does not record the individual deletion records in the log, and deleting rows is not recoverable. Delete triggers related to the table are not activated during the removal process. Execution speed is fast.

(2) The space occupied by the table and index. When the table is truncate, the space occupied by the table and index is restored to its original size, and the delete operation does not reduce the space occupied by the table or index. The drop statement frees all the space occupied by the table.

(3) Generally, drop > truncate > Delete

(4) Scope of application. TRUNCATE can only be table;delete to a table and view

(5) TRUNCATE and delete delete data only, and drop deletes the entire table (structure and data).

(6) Truncate and without where Delete: delete data only, without deleting the structure of the table (definition) The DROP statement will delete the structure of the table that is dependent on the constraint (constrain), the trigger (trigger) index (index); The stored procedure/function that relies on the table will be preserved, but its state will change to: invalid.

(7) The DELETE statement is DML (data maintain Language), which is placed in rollback segment and is not valid until the transaction is committed. If there is a corresponding Tigger, the execution time will be triggered.

(8) Truncate, drop is a DLL (data define language), the operation takes effect immediately, the original data is not placed in the rollback segment, can not be rolled back

(9) In the absence of backup, use the drop and truncate sparingly. To delete some data rows, use Delete and pay attention to where to constrain the extent of the impact. The rollback segment should be large enough. To delete a table with drop, if you want to preserve the table and delete the data in the table, you can do it with truncate if the transaction is irrelevant. If it is related to a transaction, or if the teacher wants to trigger trigger, use Delete.

Truncate table name is fast and efficient because:
TRUNCATE TABLE is functionally the same as a DELETE statement without a WHERE clause: Both delete all rows in the table. However, TRUNCATE TABLE is faster than DELETE and uses less system and transaction log resources. The DELETE statement deletes one row at a time and records an entry in the transaction log for each row that is deleted. TRUNCATE table deletes data by releasing the data page used to store the table data, and records the release of the page only in the transaction log.

TRUNCATE table deletes all rows in the table, but the table structure and its columns, constraints, indexes, and so on, remain unchanged. The count value used for the new row identity is reset to the seed of the column. If you want to preserve the identity count value, use DELETE instead. If you want to delete the table definition and its data, use the DROP table statement.

(12) For a table referenced by the FOREIGN KEY constraint, you cannot use TRUNCATE table and you should use a DELETE statement without a WHERE clause. Because TRUNCATE TABLE is not recorded in the log, it cannot activate the trigger.

SQL Delete Table Delete drop truncate difference (GO)