postgresql - 可空的自然 id？

Question

我能否向自然 ID 字段添加唯一约束，以便只有非空值应该是唯一的，但允许多个空值？

Answer 1

是。根据文档，这听起来像是默认行为

In general, a unique constraint is violated when there are two or more rows in the table where the values of all of the columns included in the constraint are equal. However, null values are not considered equal in this comparison. That means even in the presence of a unique constraint it is possible to store duplicate rows that contain a null value in at least one of the constrained columns. This behavior conforms to the SQL standard, but we have heard that other SQL databases may not follow this rule. So be careful when developing applications that are intended to be portable.

http://www.postgresql.org/docs/8.1/static/ddl-constraints.html