Oracle Database Questions & Answers 2

What is Network Database link ?
Network database link is created and managed by a network domain service. A network database link can be used when any user of any database in the network specifies a global object name in a SQL statement or object definition.

What is Data Block ?
ORACLE database’s data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk.

How to define Data Block size ?
A data block size is specified for each ORACLE database when the database is created. A database users and allocated free database space in ORACLE datablocks. Block size is specified in INIT.ORA file and can’t be changed latter.

What is Row Chaining ?
In Circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs , the data for the row is stored in a chain of data block (one or more) reserved for that segment.

What is an Extent ?
An Extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information.

What is a Segment ?
A segment is a set of extents allocated for a certain logical structure.

What are the different type of Segments ?
Data Segment, Index Segment, Rollback Segment and Temporary Segment.

What is a Data Segment ?
Each Non-clustered table has a data segment. All of the table’s data is stored in the extents of its data segment. Each cluster has a data segment. The data of every table in the cluster is stored in the cluster’s data segment.

What is an Index Segment ?
Each Index has an Index segment that stores all of its data.

What is Rollback Segment ?
A Database contains one or more Rollback Segments to temporarily store “undo” information.

What are the uses of Rollback Segment ?
Rollback Segments are used :
To generate read-consistent database information during database recovery to rollback uncommitted transactions for users.

What is a Temporary Segment ?
Temporary segments are created by ORACLE when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment extents are released to the system for future use.

What is a Data File ?
Every ORACLE database has one or more physical data files. A database’s data files contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the data files allocated for a database.

What are the Characteristics of Data Files ?
A data file can be associated with only one database. Once created a data file can’t change size. One or more data files form a logical unit of database storage called a table space.

What is a Redo Log?
The set of Redo Log files for a database is collectively known as the database’s redo log.

What is the function of Redo Log ?
The Primary function of the redo log is to record all changes made to data.

What is the use of Redo Log Information ?
The Information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database’s data files.

What does a Control file Contain ?
A Control file records the physical structure of the database. It contains the following information.
Database Name
Names and locations of a database’s files and redolog files.
Time stamp of database creation.

What is the use of Control File ?
When an instance of an ORACLE database is started, its control file is used to identify the database and redo log files that must be opened for database operation to proceed. It is also used in database recovery.

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.

What is an Integrity Constrains ?
An integrity constraint is a declarative way to define a business rule for a column of a table.

Can an Integrity Constraint be enforced on a table if some existing table data does not satisfy the constraint ?
No.

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.

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.

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.

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.

What is self-referential integrity constraint ?
If a foreign key reference a parent key of the same table is called self-referential integrity constraint.

What are the Limitations of a CHECK Constraint ?
The condition must be a Boolean expression evaluated using the values in the row being inserted or updated and can’t contain subqueries, sequence, the SYSDATE,UID,USER or USERENV SQL functions, or the pseudo columns LEVEL or ROWNUM.

What is the maximum number of CHECK constraints that can be defined on a column ?
No Limit.

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