How can I add new field in my relational database table

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Hi..
I have a relational database in which I have demand table. Now I want to add one more field into my table but if I unchecked the 'Allow Null' checkbox then the table save and if I checked the 'Allow Null' it will show me a dialog box containing the error like below..

What I have tried:

C#

'Employees' table saved successfully
'Items' table saved successfully
'Demands' table
- Unable to modify table.  
Cannot insert the value NULL into column 'Purchased_Date', table 'Inventory_Aamir.dbo.Tmp_Demands'; column does not allow nulls. INSERT fails.
The statement has been terminated.

C#

TITLE: Microsoft SQL Server Management Studio
------------------------------

User canceled out of save dialog
 (MS Visual Database Tools)

------------------------------
BUTTONS:

OK
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Posted 22-Dec-16 23:02pm

Muhammd Aamir

Updated 22-Dec-16 23:31pm

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Tomas Takac 23-Dec-16 5:06am

I don't quite follow but when you are adding a new column to a table then it either has to allow nulls or have a default value specified.

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OriginalGriff · Answer 1 · 2016-12-22T23:31:00

When you add a column to a table which has existing data, the resulting table must be consistent: what SQL does is effectively create a new table, insert all the old records, then delete the old table and rename the new. If your new columns cannot take a null value, then the insert to the new table fails - because SQL has no other data to insert in that column - so the "add column" operation itself fails to maintain the DB consistency.

You either need to allow null values on the new column, or set the DEFAULT value for the column to a reasonable value so SQL can use that for existing rows.