Oracle 8 Database Administration volume 2 instruction guide phần 3

Tham khảo tài liệu 'oracle 8 database administration volume 2 instruction guide phần 3', công nghệ thông tin, cơ sở dữ liệu phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Maintaining Constraints and Triggers Disabling and Enabling Triggers Use ALTER TRIGGER to disable or enable one trigger. ALTER TRIGGER DISABLE Use ALTER TABLE to disable or enable all triggers. ALTER TABLE ENABLE ALL TRIGGERS 14-16 Copyright Oracle Corporation 1998. All rights reserved. ORACLE By default a trigger is enabled when created. Disabling Triggers Disabling a trigger may be necessary in the following conditions An object that the trigger references is not available but the DML activity on the table cannot be stopped. To speed up a large data load on the table that has the trigger. Use the following command to disable a trigger ALTER TRIGGER schema. trigger DISABLE Enabling Triggers Use the following command to enable a trigger ALTER TRIGGER schema. trigger ENABLE What follows is the syntax for enabling or disabling all triggers in a table ALTER TABLE schema. table DISABLE ENABLE ALL TRIGGERS Oracle8 Database Administration 14-27 Lesson 14 Maintaining Data Integrity Dropping Constraints Drop constraints using this command ALTER TABLE DROP CONSTRAINT emp_ln_uk Drop a table and any referencing foreign key using this command DROP TABLE departments CASCADE CONSTRAINTS 14-17 Copyright Oracle Corporation 1998. All rights reserved. ORACLE Constraints may be dropped when They are no longer necessary. They need to be modified but because they cannot be modified directly they must be dropped and added. Syntax Use the following command to drop a constraint ALTER TABLE schema. table DROP CONSTRAINT constraint PRIMARY KEY UNIQUE column column . CASCADE Note Use the CASCADE option to drop any constraint that is referenced by a foreign key. When a primary key or a unique constraint is dropped any associated unique index is also dropped. If a primary key or unique constraint is implemented using a nonunique index the associated index is not dropped and must be dropped manually if not required. 14-28 Oracle8 Database .