Tuesday, November 4, 2008

Differences between Delete, Drop and Truncate Statements

DELETE, DROP and TRUNCATE statements differ in their nature itself.

  • DELETE statement is a DML (DATA MANIPULATION LANGUAGE) statement.
  • DROP AND TRUNCATE statements are DDL (DATA definition language) statements.

A DML action can be rolled back if the data is not committed, but DDL actions cannot be rolled back until Oracle 9i. With Oracle 9i FLASH BACK enabled even some of the DDL statements can be rolled back.

Delete

A delete statement deletes the data from a table. A delete statement can have a “where” clause, which need to be satisfied for the data to be deleted from the table.

Actions of Delete statement

  • Deletes the data from the current table space.
  • Modifies the undo table space with the delete records.
  • Executes all the before / after statement and row level triggers.
  • Updates the indexes (makes the index empty if the where clause is omitted).
  • Constraint checks are performed before deleting the rows

Syntax of a delete Statement:

DELETE FROM table [WHERE condition]

Truncate

Truncate drops all the records in a table. But as it is a DDL statement data cannot be retrieved. Usually truncate is faster than delete statement because there is no need to change or update the UNDO table space with the deleted records. Truncate is an implicit commit Statement. Truncate statement deal locates the space.

Actions of Truncate Statement

  • Removes all the records from the current table space.
  • Updates the indexes.
  • High watermark of the truncated table is reset.
  • Integrity Constraint checks are performed

Syntax of a Truncate Statement:

TRUNCATE TABLE table_name;

DROP STATEMENT

A drop statement removes the table object from the database. Structure and the data will be removed from the database. Action cannot be rolled back (but only by FLASH BACK from Oracle 9i).

Actions of a Drop Statement:

  • Constraint checks are performed before dropping.
  • All the data and structure of the table will be removed.
  • Updates the corresponding data dictionary views like dba_objects, dba_tables, user_tables e.t.c.
  • Updates the Status as “INVALID” for all the dependent objects.
  • Deallocates the Space

Syntax of a Drop Statement:

DROP TABLE table_name;
DROP TABLE table_name cascade constraints;

If “cascade constraints” is specified all foreign keys that reference the table are dropped and then table is dropped.

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