Sunday, January 31, 2016

SQL - COALESCE() Function

SQL - COALESCE() - This function plays a very important role in SQL because it will take any number of parameters, and return the first value encountered that isn't NULL. All the passed parameters into the function must have the same data type. If all parameters are NULL, COALESCE returns NULL. At least one of the null values must be a typed NULL.
COALESCE is ANSI standard function which is internally translated to a CASE expression and there is an advantage that we aware what to expect when using it. 
COALESCE is harder to spell, but at least it doesn't lead to incorrect.
SQL Syntax
COALESCE ( expression [ ,...n ] )
Arguments
Expression: Is an expression of any type.
Return Types: Returns the data type of expression with the highest data type precedence.
Note: COALESCE requires at least minimum of two expressions. If all the passed parameters are not the same data-type then they get implicitly cast to an appropriate data-type using data-type order of precedence.


To understand the powerful features of COALESCE function; we are taking an example to explain it. In this example, we are going to create a temporary table to store the user comments on the various topics and their contact numbers.

USE tempdb
GO

---- CREATE TEMP TABLE FOR COMMENTS
CREATE TABLE #UserComments
(
UserId Int,
Topic varchar(200),
Comments varchar(500),
Phone1 Varchar(12),
Phone2 Varchar(12),
Phone3 Varchar(12)
)

---- insert data into comments table
INSERT INTO #UserComments(UserId,Topic,Comments,Phone1,Phone2,Phone3)
VALUES(1,'Population','Population could be the biggest problem for the world',1245679870,NULL,9852631450),
(2,'Pollution','Now a day, people are suffering lots of disease due to Pollution',NULL,6758901243,NULL),
(3,'Population','India and China is the biggest country for the Population',9321456235,NULL,NULL),
(4,'Pollution','There are many type of the Pollution',NULL,NULL,8856321456)


----- pull the records from the comments table
SELECT UserId,Topic,Comments,
Phone1,Phone2,Phone3
FROM #UserComments;

UserId
Topic
Comments
Phone1
Phone2
Phone3
1
Population
Population could be the biggest problem for the world.
1245679870
NULL
9852631450
2
Pollution
Now a day, people are suffering lots of disease due to Pollution.
NULL
6758901243
NULL
3
Population
India and China is the biggest country for the Population.
9321456235
NULL
NULL
4
Pollution
There are many type of the Pollutions.
NULL
NULL
8856321456


---- drop temp table
DROP TABLE #UserComments;
Return first Non-able Value
On the basis of the User Comments table, we can get the any available contact number against any user Id because COALESCE function can take n-numbers of the arguments and will return the first not null able value from them as given below:
---- Pull the contact number for the user
select
UserId,
Topic,
ContactNumber=Coalesce(Phone1,Phone2, Phone3)
from
#UserComments

UserId
Topic
ContactNumber
1
Population
1245679870
2
Pollution
6758901243
3
Population
9321456235
4
Pollution
8856321456
Before coalesce function is run, it checks what data types it is comparing. i.e. INT, INT, INT and DATETIME. It decides that for this function they should all be processed as DATETIME and implicitly converts them before it calls the function.

Using Coalesce to comma separated values
In the above table, we can use the coalesce function to get the comma separated values as given below:
---- declare local variable
DECLARE @UserComments VARCHAR(1000)

----- get the vales into variable
SELECT @UserComments = COALESCE(@UserComments,'') + Comments + ',' 
FROM #UserComments
WHERE (Topic = 'Population')

----- pull value from variable
SELECT @UserComments AS UserComments
UserComments
Population could be the biggest problem for the world.,India and China is the biggest country for the Population.,

Conclusion
Coalesce allows multiple items to be compared in one statement. COALESCE () most often appears within a very specific content, such as in a query or view or stored procedure. COALESCE is a powerful tool if you are returning numerous values to a user and want to substitute occurrences of NULL with values from a different column or with an expression.

Thursday, January 28, 2016

SQL - Derived Tables

Derived tables is the most outstanding feature of SQL which provides a momentous help to the developers because they are always created on the fly with the help of SELECT and FROM statements in parenthesis and can also be used in queries such as Sub-Queries, Common Table Expressions, and Views. The amazing point is that no need to drop it because SQL Server is the responsible to create and populate the table in the memory which can be referenced by the outer Select query who created.

To simplify it, a derived table is best thought of as a table within a table that exists only as long as the outer query does. They can assumed as on the fly views also in SQL.

Drawback Points: A derived table only exists in the query in which it is created. So, derived tables are not actually part of the database schema because they are not real tables.
SQL server will eliminate it forever from the memory when the outer query is finished.
Reusable Options: If you want to re-use the output of the Derived table in the SQL query then you must need to be store its output into the table variable or temporary table because they are available till the end  of the query session.


Due to reside in the memory, it would be faster the temporary tables which are created in the temp database but in case of if you are fetching few records from the database. SQL server will eliminate it when the outer query is finished.
Explanation of Derived tables: Use of Derived tables are depend on the requirements and we are explaining some of them here. An employee table is used to store all the necessary information of the employees and department master is used to store department related details

USE TEMPDB
GO
SET NOCOUNT ON
---- Create table employee
CREATE TABLE Employees
(
EmpId Int NOT NULL,
EmpName Varchar(20) NOT Null,
EmpSalary money not null,
DeptId Int Not Null,
EmpAge Int Not Null
)
GO
----- Insert values into table
INSERT INTO Employees (EmpId, EmpName, EmpSalary, DeptId,EmpAge)
VALUES (101, 'Ryan Arjun', 50000, 51, 23),
(102, 'Kimmy Wang', 42000, 52, 29),
(103, 'Bill Smith', 45000, 52, 35),
(104, 'Rose White', 25000, 53, 21),
(105, 'Mac Johnson', 30000, 51, 32)  
GO
----- Output from the table
select * from Employees;
EmpId
EmpName
EmpSalary
DeptId
EmpAge
101
Ryan Arjun
50000
51
23
102
Kimmy Wang
42000
52
29
103
Bill Smith
45000
52
35
104
Rose White
25000
53
21
105
Mac Johnson
30000
51
32
----- create another table department
CREATE TABLE Department
(
DeptID INT Not Null,
DeptName Varchar(20) Not Null
)
GO
---- Insert Values into table
INSERT Department VALUES (51, 'Accounts'),
(52, 'Sales'), (53, 'Marketing'), (54, 'Admin')
GO
----- pull values from department table
select * from Department;
DeptID
DeptName
51
Accounts
52
Sales
53
Marketing
54
Admin
---- pull values
select e1.EmpId, e1.EmpName, e1.EmpSalary, dpt.DeptName
from Department dpt
----- derived table
inner join
(
select EmpId, EmpName, DeptId, EmpSalary from Employees
where EmpSalary>42000
and EmpName like 'R%'
)E1 on dpt.DeptID=E1.DeptId
EmpId
EmpName
EmpSalary
DeptName
101
Ryan Arjun
50000
Accounts
---- DROP TABLES
DROP TABLE Employees, Department

Referencing Multiple Derived Tables- As we are working with the views, the same way, you need to pull the required columns to put them into SELECT and FROM statements in parenthesis and join them together on the basis of data key relations. In this case, they are best way to provide on the fly views like structures.

Conclusion
Derived tables are the best option to avoid to create views which are not going to be used again and again. They provide a simple way to improve the performance of the SQL Query and SQL Server will take care of the Derived Tables because they stored in the memory and refreshed by the itself. 

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