25.5.09

Union operator in sql server

UNION Operator in SQL Server:

Union operator is used to merge the records from first table with the records from the second table.



Some conditions:

1. It's used to fetch the records from two or more tables into a single result set.
2. The union columns should be of same data type.
3. Union operator will remove the duplicates from the result set.

Am trying to put union between the tables with same column types,

drop table venkat1
create table venkat1(id int, rating int, dat datetime,auditrecord int)
insert into venkat1 values(1,1,'7/1/2006',1)
insert into venkat1 values(1,3,'7/1/2006',2)
insert into venkat1 values(1,5,'7/2/2006',3)
insert into venkat1 values(1,2,'8/10/2006',4)
insert into venkat1 values(2,4,'8/7/2006',1)
select * from venkat1
create table venkat2(id int, rating int)
insert into venkat2 values(6,1)
insert into venkat2 values(7,3)
insert into venkat2 values(8,5)
insert into venkat2 values(9,2)
select * from venkat2
select id,rating from venkat1
union
select id,rating from venkat2


Everything seems to be fine. Now, am trying with bit different data type.. replacing integer column with varchar column. Let's see what's happening.


drop table venkat2
create table venkat2(id int, rating decimal(5,2))
insert into venkat2 values(6,1)
insert into venkat2 values(7,3)
insert into venkat2 values(8,5)
insert into venkat2 values(9,2)
select * from venkat2
select id,rating from venkat1
union
select id,rating from venkat2


OOPS, no error instead the entire result set will be associated with decimal column.
By execution, the result set will be taking the highest refining column type to accomodate both
the data types. Nice to see...


drop table venkat2
create table venkat2(id int, rating varchar(10))
insert into venkat2 values(6,'Number 1')
insert into venkat2 values(7,'Number 3')
insert into venkat2 values(8,'Number 5')
insert into venkat2 values(9,'Number 2')

select id,rating from venkat1
union
select id,rating from venkat2

We are getting an error,


Conversion failed when converting the varchar value 'Number 1' to data type int.


Checking for the unique property of the union operator:
I have added a new row, which has duplicate record and am doing an union operator.

drop table venkat2
create table venkat2(id int, rating decimal(5,2))
insert into venkat2 values(6,1)
insert into venkat2 values(7,3)
insert into venkat2 values(8,5)
insert into venkat2 values(9,2)
insert into venkat2 values(9,2)
select id,rating from venkat1
union
select id,rating from venkat2

Difference between Union and Union all operator:
Here comes the difference between the above two, Union all will give all the records including duplicates whereas union operator will remove the duplicates.


Thanks and Regards,
Venkatesan Prabu .J
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Thanks and Regards,
Venkatesan Prabu .J

SQL Server Interview questions - Part 1

What is the significance of NULL value and why should we avoid permitting null values?
Null means no entry has been made. It implies that the value is either unknown or undefined.We should avoid permitting null values because Column with NULL values can't have PRIMARY KEY constraints. Certain calculations can be inaccurate if NULL columns are involved.

What is SQL whats its uses and its component ?
The Structured Query Language (SQL) is foundation for all relational database systems. Most of the large-scale databases use the SQL to define all user and administrator interactions. It enable us to retrieve the data from based on our exact requirement. We will be given a flexibility to store the data in our own format.


The DML component of SQL comprises four basic statements:
* SELECT to get rows from tables
* UPDATE to update the rows of tables
* DELETE to remove rows from tables
* INSERT to add new rows to tables

What is DTS in SQL Server ?
Data Transformation Services is used to transfer the data from one source to our required destination. Considering am having some data in sql server and I need to transfer the data to Excel destination. Its highly possible with dialogue based tool called Data Transformation services. More customization can be achieved using SSIS. A specialized tool used to do such migration works.


What is the difference between SQL and Pl/Sql ?

Straight forward. SQL is a single statement to finish up our work.Considering, I need some data from a particular table. “Select * from table” will fetch the necessary information. Where as I need to do some row by row processing. In that case, we need to go for Procedural Logic / SQL.

What is the significance of NULL value and why should we avoid permitting null values?
Null means no entry has been made. It implies that the value is either unknown or undefined.We should avoid permitting null values because Column with NULL values can't have PRIMARY KEY constraints. Certain calculations can be inaccurate if NULL columns are involved.

Difference between primary key and Unique key?
Both constraints will share a common property called uniqueness. The data in the column should be unique. The basic difference is,
· Primary key won’t allow null value. Whereas, unique key will accept null value but only one null value.
· On creating primary key, it will automatically format the data inturn creates clustered index on the table. Whereas, this characteristics is not associated with unique key.
· Only one primary key can be created for the table. Any number of Unique key can be created for the table.

Select Statement in SQL Server

Select Statement in SQL Server

String Functions in sql server

String Functions in sql server
Substring/Len/replace/Ltrim/Rtrim

SQL Server Interview Question - Part 2

What is normalization?

Normalization is the basic concept used in designing a database. Its nothing but, an advise given to the database to have minimal repetition of data, highly structured, highly secured, easy to retrieve. In high level definition, the Process of organizing data into tables is referred to as normalization.


What is a stored procedure:
Stored procedures are precompiled T-SQL statements combined to perform a single task of several tasks. Its basically like a Macro so when you invoke the Stored procedure, you actually run a set of statements. As, its precompiled statement, execution of Stored procedure is compatatively high when compared to an ordinary T-SQL statement.


What is the difference between UNION ALL Statement and UNION ?
The main difference between UNION ALL statement and UNION is UNION All statement is much faster than UNION,the reason behind this is that because UNION ALL statement does not look for duplicate rows, but on the other hand UNION statement does look for duplicate rows, whether or not they exist.

Example for Stored Procedure?
They are three kinds of stored procedures,1.System stored procedure – Start with sp_2. User defined stored procedure – SP created by the user.3. Extended stored procedure – SP used to invoke a process in the external systems.Example for system stored proceduresp_helpdb - Database and its propertiessp_who2 – Gives details about the current user connected to your system. sp_renamedb – Enable you to rename your database


What is a trigger?

Triggers are precompiled statements similar to Stored Procedure. It will automatically invoke for a particular operation. Triggers are basically used to implement business rules.


What is a view?
If we have several tables in a db and we want to view only specific columns from specific tables we can go for views. It would also suffice the needs of security some times allowing specfic users to see only specific columns based on the permission that we can configure on the view. Views also reduce the effort that is required for writing queries to access specific columns every time.


What is an Index?
When queries are run against a db, an index on that db basically helps in the way the data is sorted to process the query for faster and data retrievals are much faster when we have an index.


What are the types of indexes available with SQL Server?

There are basically two types of indexes that we use with the SQL ServerClustered -

1. It will format the entire table, inturn physically sort the table.

2. Only one clustered index can be created for a table.

3. Data will be located in the leaf level.

4. By default, primary key will create clustered index on the table.

Non-Clustered Index

1. It wont touch the structure of the table.

2. It forms an index table as reference to the exact data.

3. A reference to the data will be located in the leaf level.

4. For a table, we can create 249 non clustered index.

Happy Learning!!!
Regards,
Venkatesan Prabu .J

SQL Interview question

Extent Vs Page?

Pages are low level unit to store the exact data in sql server. Basically, the data will be stored in the mdf, ldf, ndf files. Inturn, pages are logical units available in sql server.The size of the page is 8KB.

Eight consecutive pages will form an extent 8 * 8KB = 64KB.

Thus I/O level operation will be happening at pages level.The pages will hold a template information at the start of each page (header of the page).

They are,

1. page number,

2. page type,

3. the amount of free space on the page,

4. the allocation unit ID of the object that owns the page.

Extents will be classifed into two types,

1. Uniform extents

2. Mixed extents

Uniform Extents:It occupied or used by a single object. Inturn, a single object will hold the entire 8 pages.Mixed

Extents:Mulitple objects will use the same extent. SQL Server will allow a max of eight objects to use a shared extent.

Property of SQL Server :Initally if an object is created, sql server will allocate the object to the mixed extent and once if the size reaches 8 pages and more... immediately, a new uniform extent will be provided for that particular object.

Herecomes, our fragmentation and reindexing concepts.



Best Joke - Enjoy it

Best Joke - Enjoy it