DDL Commands in Database Management System.pptx
- 1. DDL Commands â˘FIRST COMMANDS TO DEAL WITH DATABASE â˘CREATE, ALTER, RENAME, DROP, TRUNCATE
- 2. ALTER TABLE Statement ⢠The ALTER TABLE statement is used to add, delete, or modify columns in an existing table. ⢠The ALTER TABLE statement is also used to add and drop various constraints on an existing table.
- 3. âşTo add a column in a table, use the following syntax: âş ALTER TABLE table_name ADD column_name datatype; ALTERTABLE Statement
- 4. âşTo delete a column in a table, use the following syntax : âşALTER TABLE table_name DROP COLUMN column_name; ALTERTABLE Statement
- 5. ALTER TABLE Statement âşTo change the data type of a column in a table, use the following syntax: âşALTER TABLE table_name MODIFY column_name datatype;
- 6. EXAMPLES : âşALTER TABLE Persons ADD DateOfBirth date; âşALTER TABLE Persons MODIFY name varchar2(30); âşALTER TABLE Persons DROP COLUMN DateOfBirth;
- 7. TRUNCATE COMMAND âşThe Truncate SQL command is used to delete all records from an existing oracle table. âşRemoving rows with the TRUNCATE TABLE statement can be faster than removing all rows with the DELETE statement, especially if the table has numerous triggers, indexes, and other dependencies.
- 8. âşTruncate drops the table first and then recreates only the table structure which is much faster than deleting the records one by one. âş Hence in case a table contains large number of records, it works much faster than delete statement. TRUNCATE COMMAND
- 9. âşSyntax : TRUNCATE TABLE TABLE_NAME; âşExample: 1. TRUNCATE TABLE EMP; 2. TRUNCATE TABLE DEPARTMENT; TRUNCATE COMMAND
- 10. DROP COMMAND âş The DROP command is used to remove an object from the database. âş If u drop a table, all the rows in the table is deleted and the table structure is removed from the database. âş Once a table is dropped we cannot get it back, so be careful while using drop command. âş When a table is dropped all the references to the table will not be valid.
- 11. âş Syntax : DROP TABLE TABLE_NAME; âş Example: DROP TABLE EMP; DROP TABLE DEPARTMENT; DROP COMMAND
- 12. Difference TRUNCATE DROP Truncate command removes all records from the specified table, but keeps structure of the table intact. Drop command removes all records from the table along with the table structure and puts it in recycle bin. The records once deleted with truncate command cannot be recovered with the help of rollback. The table removed with the drop command can be recovered from the recycle bin with the help of flashback command.
- 13. Difference TRUNCATE DELETE Truncate command removes all records from the specified table. Delete command can delete one or more records from the table. The records once deleted with truncate command cannot be recovered with the help of rollback. The records once deleted with delete command can be recovered with the help of rollback.