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Does unique constraint allow null?
You can insert NULL values into columns with the UNIQUE constraint because NULL is the absence of a value, so it is never equal to other NULL values and not considered a duplicate value. This means that it’s possible to insert rows that appear to be duplicates if one of the values is NULL .
Does count (*) ignore null values?
COUNT(expression) does not count NULL values. COUNT(*) returns the count of the number of rows in the table as an integer. COUNT(*) counts all rows, regardless of the presence of duplicate field values or NULL values.
What happens when set ANSI Nulls is off in SQL Server?
SQL Server returns an error that lists all SET options that violate the required values. Also, when you execute a SELECT statement, if SET ANSI_NULLS is OFF, SQL Server ignores the index values on computed columns or views and resolve the select operation as if there were no such indexes on the tables or views.
How to use where column name = Null in SQL?
A SELECT statement that uses WHERE column_name = NULL returns the rows that have null values in column_name. A SELECT statement that uses WHERE column_name <> NULL returns the rows that have nonnull values in the column. Also, a SELECT statement that uses WHERE column_name <> XYZ_value returns all rows that are not XYZ_value and that are not NULL.
How to return Null on an empty set in SQL?
The ANSI SQL standard defines (chapter 6.5, set function specification) the following behaviour for aggregate functions on empty result sets: Returning NULL for AVG, MIN and MAX makes perfect sense, since the average, minimum and maximum of an empty set is undefined.
When to return NULL for Avg, min and Max?
Returning NULL for AVG, MIN and MAX makes perfect sense, since the average, minimum and maximum of an empty set is undefined. The last one, however, bothers me: Mathematically, the SUM of an empty set is well-defined: 0. Using 0, the neutral element of addition, as the base case makes everything consistent: