It is clear to me that only allowing a single null value will not violate the explanation below. It would be equally true that allowing multiple null values would not violate it. -----Original Message----- From: pgsql-general-owner@xxxxxxxxxxxxxx [mailto:pgsql-general-owner@xxxxxxxxxxxxxx] On Behalf Of Tom Lane Sent: Thursday, January 20, 2005 8:03 AM To: Greg Stark Cc: pgsql-general@xxxxxxxxxxxxxx Subject: Re: Unique Index Greg Stark <gsstark@xxxxxxx> writes: > Tom Lane <tgl@xxxxxxxxxxxxx> writes: >> Not for UNIQUE constraints. SQL92 section 4.10 "Integrity constraints": >> >> A unique constraint is satisfied if and only if no two rows in >> a table have the same non-null values in the unique columns. > That's ambiguous. Does it mean no two rows have all non-null columns that are > all identical? Or does it mean no two rows have columns that excluding any > null columns are identical. OK, try the more formal definition in 8.9 <unique predicate> 2) If there are no two rows in T such that the value of each column in one row is non-null and is equal to the value of the cor- responding column in the other row according to Subclause 8.2, "<comparison predicate>", then the result of the <unique predi- cate> is true; otherwise, the result of the <unique predicate> is false. (11.7 defines the UNIQUE constraint in terms of the unique predicate) regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to majordomo@xxxxxxxxxxxxxx ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faqs/FAQ.html
