SQL



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SQL Interview Questions And Answers

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SQL can be divided into two parts: 
> The Data Manipulation Language (DML)
> The Data Definition Language (DDL).
The query and update commands form the DML part of SQL:
> SELECT - extracts data from a database
> UPDATE - updates data in a database
> DELETE - deletes data from a database
> INSERT INTO - inserts new data into a database
The DDL part of SQL permits database tables to be created or deleted. It also define indexes (keys), specify links between tables, and impose constraints between tables. The most important DDL statements in SQL are:
> CREATE DATABASE - creates a new database
> ALTER DATABASE - modifies a database
> CREATE TABLE - creates a new table
> ALTER TABLE - modifies a table
> DROP TABLE - deletes a table
> CREATE INDEX - creates an index (search key)
> DROP INDEX - deletes an index
Data Control Language (DCL) statements. Some examples:
> GRANT - gives user's access privileges to database
> REVOKE - withdraw access privileges given with the GRANT command

Truncate is more faster than delete because when we delete the records from the database, database has to perform 2 actions.
> Delete from the database
> Write the deleted records into "rollback"  
segments. But incase of "Truncate" the second activity is not required.It is faster than becouse truncate is a ddl command so it does not produce any rollback information and the storage space is released while the delete command is a dml command and it produces rollback information too and space is not deallocated using delete command.

RDBMS stands for Relational Database Management
System. RDBMS is the basis for SQL, and for all
modern database systems like MS SQL Server, IBM
DB2, Oracle, MySQL, and Microsoft Access. The data in RDBMS is stored in database objects
called tables. A table is a collections of
related data entries and it consists of columns
and rows.Relational Data Base Management Systems
(RDBMS) are database management systems that maintain data records and indices in tables. Relationships may be created and maintained across and among the data and tables. In a relational database, relationships between data items are expressed by means of tables. Interdependencies among these tables are expressed by data values rather than by pointers. This allows a high degree of data independence. An RDBMS has the capability to recombine the data items from different files, providing powerful tools for data usage.

 normalization have a se veral forms that a database structure can be subject to, each with rules that constrain the database further and each creating what is called a Normal Form. These are, in order:
> First Normal Form (1NF)
> Second Normal Form (2NF)
> Third Normal Form (3NF)
> Boyce Codd Normal Form (BCNF)
> Fourth Normal Form (4NF)
> Fifth Normal Form (5NF)
> Optimal Normal Form (ONF)
> Domain-Key Normal Form (DKNF)
Theses are defined as :
> 1NF: Eliminate Repeating Groups : Make a separate table for each set of related attributes, and give each table a primary key. Each field contains at most one value from its attribute domain.
> 2NF: Eliminate Redundant Data : If an attribute depends on only part of a multi-valued key, remove it to a separate table.
> 3NF: Eliminate Columns Not Dependent On Key : If attributes do not contribute to a description of the key, remove them to a separate table. All attributes must be directly dependent on the primary key.
> BCNF: Boyce-Codd Normal Form : If there are non-trivial dependencies between candidate key attributes, separate them out into distinct tables.
> 4NF: Isolate Independent Multiple Relationships : No table may contain two or more 1:n or n:m relationships that are not directly related.
> 5NF: Isolate Semantically Related Multiple : Relationships There may be practical constrains on information that justify separating logically related many-to-many relationships.
> ONF: Optimal Normal Form : A model limited to only simple (elemental) facts, as expressed in Object Role Model notation.
> DKNF: Domain-Key Normal Form : A model free from all modification anomalies.

Normalization is a process which is the tables in a database are optimized to remove the potential for redundancy. Two main problems may arise if this is not done:
> Repeated data makes a database bigger.
> Multiple instances of the same values make
maintaining the data more difficult and can
create anomalies.


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SQL Objective Questions And Answers

SQL Objective Questions And Answers

SQL Interview Questions And Answers

SQL Interview Questions And Answers


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