LECTURE #30|
NOTES LINK:https://drive.google.com/file/d/1cppg...
#CHECK CONSTRAINT IN MYSQL WORKBENCH
#MySQL WORKBENCH DIRECTLY DOESNOT SUPPORT #CHECK CONSTRAINT
Specifying Constraints in SQL
1. Specifying Attribute Constraints and Attribute Defaults:
Because SQL allows NULLs as attribute values, a constraint NOT NULL may be specified if NULL is not permitted for a particular attribute. This is always implicitly specified for the attributes that are part of the primary key of each relation, but it can be specified for any other attributes whose values are required not to be NULL.
It is also possible to define a default value for an attribute by appending the clause DEFAULT value to an attribute definition. The default value is included in any new tuple if an explicit value is not provided for that attribute. If no default clause is specified, the default default value is NULL for attributes that do not have the NOT NULL constraint.
CREATE TABLE DEPARTMENT
( … ,
Mgr_ssn CHAR(9) NOT NULL DEFAULT ‘888665555’,
… ,
CONSTRAINT DEPTPK
PRIMARY KEY(Dnumber),
CONSTRAINT DEPTSK
UNIQUE (Dname),
CONSTRAINT DEPTMGRFK
FOREIGN KEY (Mgr_ssn) REFERENCES EMPLOYEE(Ssn)
ON DELETE SET DEFAULT ON UPDATE CASCADE);
Above code assigns a Default Manager. The Default Mgr_ssn is ‘888665555’
Another type of constraint can restrict attribute or domain values using the CHECK clause following an attribute or domain definition. For example, suppose that department numbers are restricted to integer numbers between 1 and 20; then, we can change the attribute declaration of Dnumber in the DEPARTMENT table to the following:
Referential integrity is specified via the FOREIGN KEY clause. A referential integrity constraint can be
violated when tuples are inserted or deleted, or when a foreign key or primary key attribute value is updated. The default action that SQL takes for an integrity violation is to reject the update operation that will cause a violation, which is known as the RESTRICT option. However, the schema designer can specify an alternative action to be taken by attaching a referential triggered action clause to any foreign key constraint. The options include SET NULL, CASCADE, and SET DEFAULT. An
option must be qualified with either ON DELETE or ON UPDATE. For e.g.
CREATE TABLE EMPLOYEE
( … ,
Dno INT NOT NULL DEFAULT 1,
CONSTRAINT EMPPK
PRIMARY KEY (Ssn),
CONSTRAINT EMPSUPERFK
FOREIGN KEY (Super_ssn) REFERENCES EMPLOYEE(Ssn)
ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT EMPDEPTFK
FOREIGN KEY(Dno) REFERENCES DEPARTMENT(Dnumber)
ON DELETE SET DEFAULT ON UPDATE CASCADE);
Here, the database designer chooses ON DELETE SET NULL and ON UPDATE CASCADE for the foreign key Super_ssn of EMPLOYEE. This means that if the tuple for a supervising employee is deleted, the value of Super_ssn is automatically set to NULL for all employee tuples that were referencing the deleted employee tuple. On the other hand, if the Ssn value for a supervising employee is updated (say, because it was entered incorrectly), the new value is cascaded to Super_ssn for all employee tuples referencing the updated employee tuple.
In general, the action taken by the DBMS for SET NULL or SET DEFAULT is the same for both ON DELETE and ON UPDATE: The value of the affected referencing attributes is changed to NULL for SET NULL and to the specified default value of the referencing attribute for SET DEFAULT. The action for CASCADE ON DELETE is to delete all the referencing tuples, whereas the action for CASCADE ON UPDATE is to change the value of the referencing foreign key attribute(s) to the updated (new)
primary key value for all the referencing tuples. It is the responsibility of the database designer to choose the appropriate action and to specify it in the database schema. As a general rule, the CASCADE option is suitable for “relationship” relations(To be Discussed Further) , such as WORKS_ON; for relations that represent multivalued attributes, such as DEPT_LOCATIONS; and for relations that represent weak entity types, such as DEPENDENT.
3. Giving Names to Constraints:
A constra