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Describe integrity constraints?
Integrity Constraints - A database is just as excellent as the information stored in it, and a DBMS must therefore help avoid the entry of incorrect information. An integrity constraint is a condition specified on a database schema and restricts the data that can be stored in an instance of the database. If a database instance satisfies all the integrity constraints specified on the database schema, it is a legal instance. A DBMS enforces integrity constraints, in that it permits only legal instances to be stored within the database.
Integrity constraints are specified and enforced at various times:
When the DBA or end user describes a database schema, he or she specifies the integrity constraints which must carry on any instance of this database. While a database application is run, the DBMS checks for violations and disallows changes to the data in which violate the specified integrity constraints. Several types of integrity constraints can be specified in the relational model, such as, Not Null, Check, Unique, Primary Key, etc.
CREATE TABLE prodtrans ( pno number (4) ptype char (1) CHECK (ptype in ('I','R','S')), qty number (5) FOREIGN KEY pno REFERENCES product (pno) ON DELETE CASCADE);
How does a query tree represent a relational algebra expression? Discuss any three rules for query optimisation, giving example as to when should each rule be applied. Ans
Sub query in the UPDATE command : Example : //Double the commission for employees, who have got at least 2 increments. UPDATE EMP SET COMM = COMM * 2 WHERE
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