How to convert empty value to nulll when insert or update

in SQL Server 2005,I want to prevent any empty value in some columns in a table in updating or inserting and if user entered the empty value in a column it must convert to null instead
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you could put a default value on the field...which would apply for inserts only, but you would have to write a function to do it and bind it to the column.  I dont' suggest this option.  I would write a trigger to do it.
A default value won't work because the default constraint is only used if a value is NOT specified.  Even if you supply NULL, it will use NULL.  My FIRST suggestion would be to tell whoever is inserting '' to stop.  Secondly.  If you must use trigger, you have to know whether or not you have existing triggers.  If you have existing triggers.  If you are dealing with inserts only (for now), the below should work.

The below example shows how it can be done if Field1 is your PK and Field3 is your problem field.
create trigger myIns_Trigger on myTable 
instead of insert
if @@rowcount=0
insert into myTable (field1, field2, field3)
select field1, field2, case field3 when '' then null else field3 end
from inserted
create trigger myUpd_Trigger on myTable
instead of update
if @@rowcount=0
update mt
set field2 = field2,
field3 = case field 3 when '' then null else field3 end
from myTable mt
join inserted i
on mt.field1 = i.field1

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Microsoft SQL Server 2005

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