![]() SQL uses two different concepts to tell if two values are the “same”: equals ( =) and distinct. Null-Aware Equals Comparison: is not distinct from This returns true if the value of expression is null, and false otherwise. The SQL predicate is null tests whether the result of an expression is null or not. The SQL Server setting SET ANSI_NULLS OFF changes this behavior. The following example removes all rows-even those where col is actually null-because the where clause treats unknown like false. Null in Distinct, Group by, Partition by, Union, etc.Ĭomparisons (, =, …) to null are neither true nor false but instead return the third logical value of SQL: unknown.Null in Aggregate Functions (count, sum, …). ![]() ![]() Null values just propagate through expressions without aborting execution. Another important use case is error handling: Contrary to other programming languages, it is not an error to process null values in SQL. A very common use case is to allow optional attributes without introducing an extra table. Users can use the null value for any reason they like. 2 Although exceptions exist, 3 it is not generally possible to tell why a value is null. For example, the SQL language itself uses the null value for the absent values resulting from an outer join. The null value does not indicate why a value is absent-it simply marks the places that do not have a data value. “Every data type includes a special value, called the null value,” 0 “that is used to indicate the absence of any data value”.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |