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Does UNIQUE KEY supports more than one NULL Value?
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No. In SQL server we can insert only one NULL value to column which has UNIQUE KEY constraint.
Example:CREATE TABLE SamepleUnique
(
Id INT PRIMARY KEY,
Name varchar(20) UNIQUE
)
INSERT INTO SamepleUnique VALUES(1,'Ram')
INSERT INTO SamepleUnique VALUES(2,'Krish')
INSERT INTO SamepleUnique VALUES(3,Null)
INSERT INTO SamepleUnique VALUES(4,Null) --Error as Violation of UNIQUE KEY constraint 'UQ__SamepleU__737584F603317E3D'. Cannot insert duplicate key in object 'dbo.SamepleUnique'.
--The statement has been terminated.
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Difference between count(*) and count(column_name)
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We might assume that count(*) and count(column_name) will return same result count. But NO, in case of column holds any null values.
Count (*) – returns all values (including nulls and duplicates)
Count (Column_Name) – returns all Non-NULL values(including duplicates)
In the below script we will see how it works. So that it will be easy for us to understand.
create table #tempTable(Name char(1))
insert into #tempTable values("A")
insert into #tempTable values("B")
insert into #tempTable values(Null)
insert into #tempTable values("C")
insert into #tempTable values(Null)
insert into #tempTable values("C")
select COUNT(*) from #tempTable
select COUNT(Name) from #tempTable
drop table #tempTable Output: 6 and 4
The table #temptable has total 6 rows. Count(*) returns all the rows including null/duplicates but where as count(name) returns only 4 rows which includes duplicates("C") but not null values.
If you want to remove the duplicates from count(Name) then use distinct keyword in it.
select COUNT(distinct Name) from #tempTable –-returns 3
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What is managed and unmanaged code in .NET
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Manged Code - code which is executed under Common Language Runtime (CLR). Due to this, the code has many benefits like memory management, support of version control, type safety and security. The code which targets CLR and written in .NET framework is a managed code. E.g: C#,VB.NET
Unmanaged code : code which is not executed under .NET runtime (CLR). CLR is does not have control over execution of code.memory management, security and type safety should be taken care by developer. E.g: VB6, C,C++.
managed code typically compiled to Intermediate Language(IL) code where as unmanged code directly compiles to native code.
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Constructors in .NET
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In this article we will see about constructors in .net. and types of constructors with sample example.
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