Copy data from one table to another table

The following is the query to copy the data from one table to another table.

select * into employee2 from employee1

Here Table employee1 is existing table and take backup in another table named : employee1. It will automatically will create the another table employee1 with the same structure. 

What's the difference between a primary key and a unique key?

Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.

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