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Is is possible in SQL Server 2008 to create such a constraint that would restrict two columns to have NULL values at the same time? So that
Column1 Column2
NULL NULL -- not allowed
1 NULL -- allowed
NULL 2 -- allowed
2 3 -- allowed
909
SQL Server ISNULL() FunctionThe ISNULL() function returns a specified value if the expression is NULL. If the expression is NOT NULL, this function returns the expression.
A primary key must be unique and non-null, so they're the same from that standpoint. However, a table can only have one primary key, while you can have multiple unique non-null keys. Most systems also use metadata to tag primary keys separately so that they can be identified by designers, etc.
Unlike PRIMARY KEY constraints, UNIQUE constraints allow for the value NULL. However, as with any value participating in a UNIQUE constraint, only one null value is allowed per column. A UNIQUE constraint can be referenced by a FOREIGN KEY constraint.
Primary key will not accept NULL values whereas Unique key can accept NULL values. A table can have only one primary key whereas there can be multiple unique key on a table. A Clustered index automatically created when a primary key is defined whereas Unique key generates the non-clustered index.
ALTER TABLE MyTable WITH CHECK
ADD CONSTRAINT CK_MyTable_ColumNulls CHECK (Column1 IS NOT NULL OR Column2 IS NOT NULL)
As part of the create
CREATE TABLE MyTable (
Column1 int NULL,
Column2 int NULL,
CONSTRAINT CK_MyTable_ColumNulls CHECK (Column1 IS NOT NULL OR Column2 IS NOT NULL)
)
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