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Oracle Interview Questions And Answers
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Page 6 of 95
51. What is a Data Dictionary ? The data dictionary of an ORACLE database is a set of tables and views that are used as a read-only reference about the database. It stores information about both the logical and physical structure of the database, the valid users of an ORACLE database, integrity constraints defined for tables in the database and space allocated for a schema object and how much of it is being used. 52. What is an Integrity Constrains ? An integrity constraint is a declarative way to define a business rule for a column of a table. 53. Can an Integrity Constraint be enforced on a table if some existing table data does not satisfy the constraint ? No. 54. Describe the different type of Integrity Constraints supported by ORACLE ? NOT NULL Constraint - Disallows NULLs in a table's column. UNIQUE Constraint - Disallows duplicate values in a column or set of columns. PRIMARY KEY Constraint - Disallows duplicate values and NULLs in a column or set of columns. FOREIGN KEY Constrain - Require each value in a column or set of columns match a value in a related table's UNIQUE or PRIMARY KEY. CHECK Constraint - Disallows values that do not satisfy the logical expression of the constraint. 55. What is difference between UNIQUE constraint and PRIMARY KEY constraint ? A column defined as UNIQUE can contain NULLs while a column defined as PRIMARY KEY can't contain Nulls. 56. Describe Referential Integrity ? A rule defined on a column (or set of columns) in one table that allows the insert or update of a row only if the value for the column or set of columns (the dependent value) matches a value in a column of a related table (the referenced value). It also specifies the type of data manipulation allowed on referenced data and the action to be performed on dependent data as a result of any action on referenced data. 57. What are the Referential actions supported by FOREIGN KEY integrity constraint ? UPDATE and DELETE Restrict - A referential integrity rule that disallows the update or deletion of referenced data. DELETE Cascade - When a referenced row is deleted all associated dependent rows are deleted. 58. What is self-referential integrity constraint ? If a foreign key reference a parent key of the same table is called self-referential integrity constraint. |