Column level vs table level constraints in sql server?

Question

a. Column Level

GO

CREATE TABLE Products
(
ProductID INT CONSTRAINT pk_products_pid PRIMARY KEY,
ProductName VARCHAR(25)
);

GO

b.

Answer 1

Your first example declares the constraint in line, the second does not. Only simple keys (involve one attribute) can be declared in line, compound keys (involving multiple columns) cannot. But both are table-level constraints!

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There are four logical levels of constraint:

1) Column level:

CHECK ( ProductID > 0 )

2) Row level:

CHECK ( Product_start_date < Product_end_date )

3) Table level (the following example is not yet supported in SQL Server):

CHECK ( NOT EXISTS ( SELECT *
                       FROM ( SELECT ROW_NUMBER() OVER ( PARTITION BY ProductID ) AS Tally
                                FROM Products AS P ) AS DT1 
                      WHERE Tally > 1 ) )

4) Database level (not yet supported in SQL Server):

CREATE ASSERTION EnterpriseUniqueIds
   CHECK ( NOT EXISTS ( SELECT *
                          FROM ProductID AS P
                               JOIN Components AS C
                                  ON C.ComponentID = P.ProductID ) );

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A key constraint involves comparing different rows within the same table, therefore it is a table-level constraint.