Issues In Sql server

1) how to change datatype of column to uniqueidentifier from bigint in sql server?

 Follow the following Steps:-

1).Firstly delete old data from the table.

2). Use ALTER TABLE dbo.tbltest
ALTER COLUMN ID varchar(200)

3). Now again ALTER TABLE dbo.tbltest
ALTER COLUMN ID uniqueidentifier

To be clear, you can't convert a column from numeric to uniqueidentifier directly,

but you can convert numeric -> varchar -> uniqueidentifier.