SQL Summary(2)

SQL Summary(2)

View
     A view is a virtual table which consists of a subset of data contained in a table. Views are not virtually present, and it takes less space to store. View can have data of one or more tables combined, and it is depending on the relationship.
Index
     An index is performance tuning method of allowing faster retrieval of records from the table. An index creates an entry for each value and it will be faster to retrieve data.

Indexes

Unique Index

This indexing does not allow the field to have duplicate values if the column is unique indexed. Unique index can be applied automatically when primary key is defined.

• Clustered Index

This type of index reorders the physical order of the table and search based on the key values. Each table can have only one clustered index.

• NonClustered Index

NonClustered Index does not alter the physical order of the table and maintains logical order of data. Each table can have 999 nonclustered indexes.

Cursor

     A database Cursor is a control which enables traversal over the rows or records in the table. This can be viewed as a pointer to one row in a set of rows. Cursor is very much useful for traversing such as retrieval, addition and removal of database records.

Database Relationships

• One to One Relationship.
• One to Many Relationship.
• Many to One Relationship.
• Self-Referencing Relationship.

DB query

     A DB query is a code written in order to get the information back from the database. Query can be designed in such a way that it matched with our expectation of the result set. Simply, a question to the Database.

Subquery

     A subquery is a query within another query. The outer query is called as main query, and inner query is called subquery. SubQuery is always executed first, and the result of subquery is passed on to the main query.

• A correlated subquery cannot be considered as independent query, but it can refer the column in a table listed in the FROM the list of the main query.
• A Non-Correlated subquery can be considered as independent query and the output of subquery are substituted in the main query.

Stored procedure

     Stored Procedure is a function consists of many SQL statement to access the database system. Several SQL statements are consolidated into a stored procedure and execute them whenever and wherever required.

DB trigger

     A DB trigger is a code or programs that automatically execute with response to some event on a table or view in a database. Mainly, trigger helps to maintain the integrity of the database.

What is the difference between DELETE and TRUNCATE commands?

     DELETE command is used to remove rows from the table, and WHERE clause can be used for conditional set of parameters. Commit and Rollback can be performed after delete statement.

     TRUNCATE removes all rows from the table. Truncate operation cannot be rolled back.

What are local and global variables and their differences?

     Local variables are the variables which can be used or exist inside the function. They are not known to the other functions and those variables cannot be referred or used. Variables can be created whenever that function is called.

     Global variables are the variables which can be used or exist throughout the program. Same variable declared in global cannot be used in functions. Global variables cannot be created whenever that function is called.

Constraint

Constraint can be used to specify the limit on the data type of table. Constraint can be specified while creating or altering the table statement. Sample of constraint are.

• NOT NULL.
• CHECK.
• DEFAULT.
• UNIQUE.
• PRIMARY KEY.
• FOREIGN KEY.

What I'll be doing tomorrow?
I will be continuing doing the SQL summary and reasons will be explained why I  am doing this summary.