Why Is A Union All Faster Than A Union?
The union operation, you will recall, brings two sets of data together. It will *NOT* however produce duplicate or redundant rows. To perform this feat of magic, a SORT operation is done on both tables. This is obviously computationally intensive, and uses significant memory as well. A UNION ALL conversely just dumps collection of both sets together in random order, not worrying about duplicates.
What Are Some Advantages To Using Oracle's Create Database Statement To Create A New Database Manually?
You can script the process to include it in a set of install scripts you deliver with a product.
You can put your create database script in CVS for version control, so as you make changes or adjustments to it, you can track them like you do changes to software code.
You can log the output and review it for errors. You learn more about the process of database creation, such as what options are available and why.
What Are The Pros And Cons Of Using Triggers?
A trigger is one or more statements of SQL that are being executed in event of data modification in a table to which the trigger belongs.
Triggers enhance the security, efficiency, and standardization of databases.
Triggers can be beneficial when used:
to check or modify values before they are actually updated or inserted in the database. This is useful if you need to transform data from the way the user sees it to some internal database format.
to run other non-database operations coded in user-defined functions
to update data in other tables. This is useful for maintaining relationships between data or in keeping audit trail information.
Switch To Dos Prompt
SQL> host
How To Access The Current Value And Next Value From A Sequence? Is It Possible To Access The Current Value In A Session Before Accessing Next Value?
Sequence name CURRVAL, sequence name NEXTVAL. It is not possible. Only if you access next value in the session, current value can be accessed.
What Is A Database Link?
Database link is a named path through which a remote database can be accessed.
If Unique Key Constraint On Date Column Is Created, Will It Validate The Rows That Are Inserted With Sysdate?
It won’t, Because SYSDATE format contains time attached with it.
How Will You Activate/deactivate Integrity Constraints?
The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE CONSTRAINT / DISABLE CONSTRAINT.
Where The Integrity Constraints Are Stored In Data Dictionary?
The integrity constraints are stored in USER_CONSTRAINTS.
What Are The Pre-requisites To Modify Datatype Of A Column And To Add A Column With Not Null Constraint?
- To modify the datatype of a column the column must be empty.
- To add a column with NOT NULL constrain, the table must be empty.
How Many Long Columns Are Allowed In A Table? Is It Possible To Use Long Columns In Where Clause Or Order By?
Only one LONG column is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause.
What Is Difference Between Char And Varchar2? What Is The Maximum Size Allowed For Each Type?
CHAR pads blank spaces to the maximum length.
VARCHAR2 does not pad blank spaces.
For CHAR the maximum length is 255 and 2000 for VARCHAR2.
What Are The Data Types Allowed In A Table?
CHAR, VARCHAR2, NUMBER, DATE, RAW, LONG and LONG RAW.
What Is On Delete Cascade?
When ON DELETE CASCADE is specified Oracle maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed.
What Is The Usage Of Savepoints?
SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed.
What Is Referential Integrity Constraint?
Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table.
What Is An Integrity Constraint?
Integrity constraint is a rule that restricts values to a column in a table.
What Is Rowid?
ROWID is a pseudo column attached to each row of a table. It is 18 characters long, blockno, rownumber are the components of ROWID.
Explain Union, Minus, Union All And Intersect?
INTERSECT - returns all distinct rows selected by both queries.
MINUS - returns all distinct rows selected by the first query but not by the second.
UNION - returns all distinct rows selected by either query
UNION ALL - returns all rows selected by either query, including all duplicates.
Difference Between Substr And Instr?
INSTR (String1, String2 (n, (m)),
INSTR returns the position of the m-th occurrence of the string 2 in string1. The search begins from nth position of string1.
SUBSTR (String1 n, m)
SUBSTR returns a character string of size m in string1, starting from n-th position of string1.
The union operation, you will recall, brings two sets of data together. It will *NOT* however produce duplicate or redundant rows. To perform this feat of magic, a SORT operation is done on both tables. This is obviously computationally intensive, and uses significant memory as well. A UNION ALL conversely just dumps collection of both sets together in random order, not worrying about duplicates.
What Are Some Advantages To Using Oracle's Create Database Statement To Create A New Database Manually?
You can script the process to include it in a set of install scripts you deliver with a product.
You can put your create database script in CVS for version control, so as you make changes or adjustments to it, you can track them like you do changes to software code.
You can log the output and review it for errors. You learn more about the process of database creation, such as what options are available and why.
What Are The Pros And Cons Of Using Triggers?
A trigger is one or more statements of SQL that are being executed in event of data modification in a table to which the trigger belongs.
Triggers enhance the security, efficiency, and standardization of databases.
Triggers can be beneficial when used:
to check or modify values before they are actually updated or inserted in the database. This is useful if you need to transform data from the way the user sees it to some internal database format.
to run other non-database operations coded in user-defined functions
to update data in other tables. This is useful for maintaining relationships between data or in keeping audit trail information.
to check against other data in the table or in other tables. This is useful to ensure data integrity when referential integrity constraints aren’t appropriate, or when table check constraints limit checking to the current table only.
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| Red Hat Most Frequently Asked Latest Oracle Interview Questions Answers |
Switch To Dos Prompt
SQL> host
How To Access The Current Value And Next Value From A Sequence? Is It Possible To Access The Current Value In A Session Before Accessing Next Value?
Sequence name CURRVAL, sequence name NEXTVAL. It is not possible. Only if you access next value in the session, current value can be accessed.
What Is A Database Link?
Database link is a named path through which a remote database can be accessed.
If Unique Key Constraint On Date Column Is Created, Will It Validate The Rows That Are Inserted With Sysdate?
It won’t, Because SYSDATE format contains time attached with it.
How Will You Activate/deactivate Integrity Constraints?
The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE CONSTRAINT / DISABLE CONSTRAINT.
Where The Integrity Constraints Are Stored In Data Dictionary?
The integrity constraints are stored in USER_CONSTRAINTS.
What Are The Pre-requisites To Modify Datatype Of A Column And To Add A Column With Not Null Constraint?
- To modify the datatype of a column the column must be empty.
- To add a column with NOT NULL constrain, the table must be empty.
How Many Long Columns Are Allowed In A Table? Is It Possible To Use Long Columns In Where Clause Or Order By?
Only one LONG column is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause.
What Is Difference Between Char And Varchar2? What Is The Maximum Size Allowed For Each Type?
CHAR pads blank spaces to the maximum length.
VARCHAR2 does not pad blank spaces.
For CHAR the maximum length is 255 and 2000 for VARCHAR2.
What Are The Data Types Allowed In A Table?
CHAR, VARCHAR2, NUMBER, DATE, RAW, LONG and LONG RAW.
What Is On Delete Cascade?
When ON DELETE CASCADE is specified Oracle maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed.
What Is The Usage Of Savepoints?
SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed.
What Is Referential Integrity Constraint?
Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table.
What Is An Integrity Constraint?
Integrity constraint is a rule that restricts values to a column in a table.
What Is Rowid?
ROWID is a pseudo column attached to each row of a table. It is 18 characters long, blockno, rownumber are the components of ROWID.
Explain Union, Minus, Union All And Intersect?
INTERSECT - returns all distinct rows selected by both queries.
MINUS - returns all distinct rows selected by the first query but not by the second.
UNION - returns all distinct rows selected by either query
UNION ALL - returns all rows selected by either query, including all duplicates.
Difference Between Substr And Instr?
INSTR (String1, String2 (n, (m)),
INSTR returns the position of the m-th occurrence of the string 2 in string1. The search begins from nth position of string1.
SUBSTR (String1 n, m)
SUBSTR returns a character string of size m in string1, starting from n-th position of string1.
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