2011年5月11日 星期三

Distinct vs Single


Select Distinct vs Select Single

SINGLE

The result of the selection should be a single entry. If it is not possible to identify a unique entry, the system uses the first line of the selection. If you use the FOR UPDATE addition, the selected entry is protected against parallel updates from other transactions until the next database commit (see LUW and database lock mechanism). If the database system identifies a deadlock, a runtime error occurs.

DISTINCT

Duplicate entries in the result set are automatically deleted.