Anilrgowda

In a SQL Server database, can you create multiple foreign keys on a table and all the column involved have some null values? If the table already has a primary key and you need to create foreign keys, do you have to make the new column the primary key in order to create a foreign key? Do you have some examples or illustrations (T-SQL Syntax) on how to create multiple foreign keys on two tables as a unique constraint while some of the values in these new foreign keys will have some null values?

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Anilrgowda

First error means there are duplicate records in your os_systems column of apps table. Naturally, you can't put a unique constraint on it.

Second error is because of the absence of the unique constraint that we tried to create in first statement (and failed).

If you can't remove duplicate values from os_systems column of apps table, you can't do this activity.

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