If a constraint is de~ned within the create table command or added using the alter table command, the constraint is automatically enabled. A constraint can be disabled using the command
disable
. and set the column to au_lname° . and set the column to state° 4, 315 Constraint in the Pubs Table online errata only, John Kauffman here, with my thanks to poster "rsearing" who discovered a constraint in the Pubs table that was unknown to me. This is not a correction per se, but an additional few sentences I should have included.
Thinkpad constraint j primary key j unique[] [cascade];
According to the indictment, Jones would steal various IBM and Penguin computer servers from Verisign's warehouse in Virginia and sell them to Johnson. Johnson would then sell the servers to several individuals, who would sometimes place them for sale on eBay. As a result of this scheme, the indictment alleges that Jones and Johnson caused Verisign to lose more than $120, 000 worth of computer equipment. In the indictment, Jones and Johnson are charged in three counts with causing the interstate transportation of stolen property, namely IBM 330 and 335 servers, in violation of 18 U.S.C.
Microsoft To disable a primary key, one must disable all foreign key constraints that depend on this primary key. The clause cascade automatically disables foreign key constraints that depend on the (disabled) primary key.
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Laptop Computers Example: Disable the primary key of the table DEPT and disable the foreign key constraint in the table EMP:
41 Error in Concept "(Unlike rules, constraints are not really objects unto themselves, but rather pieces of meta data describing a particular table.)" Top of page 41, "There is the default that is an object unto itself, and the default that is not really an object, but rather meta data describing a particular column in a table (in much the same way as we have constraints, which are objects, and rules, which are not objects but meta data)."
Laptop Computer alter table DEPT disable primary key cascade;
Under the second bullet point (text starting "Use CREATE TABLE to define constraints..."), the Oracle statement has two errors in the line "PRODUCT_ID)": see detailed description The data type for the PRODUCT_ID column is missing. It should probably be NUMBER, and possibly a foreign key constraint to a table with the products. The semicolon may or may not be required depending on the SQL tool being used, but for consistency's sake, a semicolon is good here.
Desktop Computer In order to enable an integrity constraint, the clause enable is used instead of disable. A constraint can only be enabled successfully if no tuple in the table violates the constraint. Otherwise an error message is displayed. Note that for enabling/disabling an integrity constraint it is important that you have named the constraints.
Notebooks In order to identify those tuples that violate an integrity constraint whose activation failed, one can use the clause exceptions into EXCEPTIONS with the alter table statement. EXCEPTIONS is a table that stores information about violating tuples.3 Each tuple in this table is identi~ed by the attribute ROWID. Every tuple in a database has a pseudo-column ROWID that is used to identify tuples. Besides the rowid, the name of the table, the table owner as well as the name of the violated constraint are stored.
Lenovo Example: Assume we want to add an integrity constraint to our table EMP which requires that each manager must earn more than 4000:
Hard Drive alter table EMP add constraint managersal
Travelstar check(JOB != 'MANAGER' or SAL >= 4000) exceptions into EXCEPTIONS;
Gateway If the table EMP already contains tuples that violate the constraint, the constraint cannot be activated and information about violating tuples is automatically inserted into the table
Laptop Parts EXCEPTIONS.
Detailed information about the violating tuples can be obtained by joining the tables EMP and EXCEPTIONS, based on the join attribute ROWID:
Software select EMP.~, CONSTRAINT from EMP, EXCEPTIONS where EMP.ROWID = EXCEPTIONS.ROWID;
Hard Drives 3Before this table can be used, it must be created using the SQL script utlexcept.sql which can be found in the directory $ORACLEHOME/rdbms/admin.
Tuples contained in the query result now can be modi~ed (e.g., by increasing the salary of managers) such that adding the constraint can be performed successfully. Note that it is important to delete \old" violations from the relation EXCEPTIONS before it is used again.
Electronics If a table is used as a reference of a foreign key, this table can only be dropped using the command drop table
