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Difference between count(*) and count(column_name)
Ashok Nalam
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
By: Ashok Nalam | 28 Dec 2012 | SQL Server | Views: 10854 | Comments: 0 | Tags:difference  tsql 
Does UNIQUE KEY supports more than one NULL Value?
Ashok Nalam

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.


By: Ashok Nalam | 12 Dec 2012 | SQL Server | Views: 6260 | Comments: 1 | Tags:Constraints  unique key 
 
 
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