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SQL Server may be a special case: a parent table in the reference table, as not allow null values, and the "referential integrity statement" may not be mandatory. Even if the parent table does not contain a null value, in the child table reference the parent table primary key constraint or unique constraint column may contain null values. If the value from the parent table is currently unknown, there will be any problems. For example, the parent table may be an address table, and the child table may include contact information. For many reasons, may be temporarily passed to the parent table do not know the contact address. This is a time-based problems, which may be a null value is appropriate. The following example, we create the parent table and insert the two values. SET NOCOUNT ONGOCREATE TABLE ParentGOINSERT Parent VALUES GOINSERT Parent VALUES GO