PL/SQL Interview Preparation Questions with Answers Part(2)

This is the second episode of PL/SQL Interview Questions with Answers series.Thanks to all for appreciating this series.Hope you learn from it and share it..

What's Data types
Order of SQL statement execution
What's Transaction
What's Commit
What's Posting
What's Rollback
What's Savepoint
Set Transaction
What's Locking
What's Consistency
What's Integrity
What's Deadlock
What's Mutating Table
What's SQL*Loader
The most important DDL statements in SQL are:
Operators used in SELECT statements.
SELECT statements:
The SELECT INTO Statement is most often used to create backup copies of tables or for archiving records.
The INSERT INTO Statements:
The Update Statement:
What's Data types
Max. columns in a table is 255. Max. Char size is 255, Long is 64K & Number is 38 digits.
Cannot Query on a long column.
Char, Varchar2 Max. size is 2000 & default is 1 byte.
Number(p,s) p is precision range 1 to 38, s is scale -84 to 127.
Long Character data of variable length upto 2GB.
Date Range from Jan 4712 BC to Dec 4712 AD.
Raw Stores Binary data (Graphics Image & Digitized Sound). Max. is 255 bytes.
Mslabel Binary format of an OS label. Used primarily with Trusted Oracle.
Order of SQL statement execution
Where clause, Group By clause, Having clause, Order By clause & Select.
What's Transaction
Transaction is defined as all changes made to the database between successive commits.
What's Commit
Commit is an event that attempts to make data in the database identical to the data in the form. It involves writing or posting data to the database and committing data to the database. Forms check the validity of the data in fields and records during a commit. Validity check are uniqueness, consistency and db restrictions.
What's Posting
Posting is an event that writes Inserts, Updates & Deletes in the forms to the database but not committing these transactions to the database.
What's Rollback
Rollback causes work in the current transaction to be undone.
What's Savepoint
Savepoint is a point within a particular transaction to which you may rollback without rolling back the entire transaction.
Set Transaction
Set Transaction is to establish properties for the current transaction.
What's Locking
Locking are mechanisms intended to prevent destructive interaction between users accessing data. Locks are used to achieve.
What's Consistency
Consistency : Assures users that the data they are changing or viewing is not changed until the are thro' with it.
What's Integrity
Assures database data and structures reflects all changes made to them in the correct sequence. Locks ensure data integrity and maximum concurrent access to data. Commit statement releases all locks. Types of locks are given below.
Data Locks protects data i.e. Table or Row lock.
Dictionary Locks protects the structure of database object i.e. ensures table's structure does not change for the duration of the transaction.
Internal Locks & Latches protects the internal database structures. They are automatic.
Exclusive Lock allows queries on locked table but no other activity is allowed.
Share Lock allows concurrent queries but prohibits updates to the locked tables.
Row Share allows concurrent access to the locked table but prohibits for a exclusive table lock.
Row Exclusive same as Row Share but prohibits locking in shared mode.
Shared Row Exclusive locks the whole table and allows users to look at rows in the table but prohibit others from locking the table in share or updating them.
Share Update are synonymous with Row Share.
What's Deadlock
Deadlock is a unique situation in a multi user system that causes two or more users to wait indefinitely for a locked resource. First user needs a resource locked by the second user and the second user needs a resource locked by the first user. To avoid dead locks, avoid using exclusive table lock and if using, use it in the same sequence and use Commit frequently to release locks.
What's Mutating Table
Mutating Table is a table that is currently being modified by an Insert, Update or Delete statement. Constraining Table is a table that a triggering statement might need to read either directly for a SQL statement or indirectly for a declarative Referential Integrity constraints. Pseudo Columns behaves like a column in a table but are not actually stored in the table. E.g. Currval, Nextval, Rowid, Rownum, Level etc.
What's SQL*Loader
SQL*Loader is a product for moving data in external files into tables in an Oracle database. To load data from external files into an Oracle database, two types of input must be provided to SQL*Loader : the data itself and the control file. The control file describes the data to be loaded. It describes the Names and format of the data files, Specifications for loading data and the Data to be loaded (optional). Invoking the loader sqlload username/password controlfilename .
The most important DDL statements in SQL are:
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CREATE TABLE - creates a new database table
 
ALTER TABLE - alters (changes) a database table
 
DROP TABLE - deletes a database table 

CREATE INDEX - creates an index (search key) 

DROP INDEX - deletes an index 
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Operators used in SELECT statements.
= Equal 
<> or != Not equal 
> Greater than 
< Less than 
>= Greater than or equal 
<= Less than or equal 
BETWEEN Between an inclusive range 
LIKE Search for a pattern
SELECT statements:
SELECT column_name(s) FROM table_name 
SELECT DISTINCT column_name(s) FROM table_name 
SELECT column FROM table WHERE column operator value 
SELECT column FROM table WHERE column LIKE pattern 
SELECT column,SUM(column) FROM table GROUP BY column 
SELECT column,SUM(column) FROM table GROUP BY column HAVING SUM(column) condition value
Note that single quotes around text values and numeric values should not be enclosed in quotes. Double quotes may be acceptable in some databases.
The SELECT INTO Statement is most often used to create backup copies of tables or for archiving records.
SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source 
SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source WHERE column_name operator value
The INSERT INTO Statements:
INSERT INTO table_name VALUES (value1, value2,....) 
INSERT INTO table_name (column1, column2,...) VALUES (value1, value2,....)
The Update Statement:
UPDATE table_name SET column_name = new_value WHERE column_name = some_value

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