SQL：在外键引用不存在的外键列上添加默认约束

Question

What will happen if we try to add a default value constraint on a foreign key where that default value does not exist in the table ?

Example:

• Table_Person has Gender column as foreign key
• Table_Gender has rows with values '1' and '2'

What will happen if I make default constraint in Table_Person for Gender column with default value = '3' ?

Will it be created or throw an error? And reason for that?

Answer 1

It will depend highly on what concrete database system (RDBMS) you're using.

For SQL Server :

• You can create both tables, including the default constraint in Table_Person

• You can insert rows into Table_Person that explicitly define a value for Gender which is 1 or 2

• You can not however insert rows into Table_Person that omit the value for Gender and thus would get the default value of 3 inserted - since that violates the FK constraint