SQL Server – Using Equal Sign After Aggregate Functions in Transact-SQL

aggregatesql serversyntaxt-sql

I have encountered a script like this:

set @sum = sum = (case when somecol1 is null then DATEDIFF(d,[somecol2],somecol3) else 0 end)

I can't understand the meaning of the equal sign (=) after the second keyword sum. When I run the query it is not showing any errors both with the equal sign and without.

I want to know the purpose of putting an equal sign after the keyword sum. Is that a mistake or not?

Thanks

Best Answer

This is documented in UPDATE (Transact-SQL):

SET @variable = column = expression sets the variable to the same value as the column. This differs from SET @variable = column, column = expression, which sets the variable to the pre-update value of the column.

In your code example, sum is the (unwise) name of a column, not an aggregate.

db<>fiddle demo

Related Solutions

SQL Server – Meaning of ‘SET’ in Null Value Eliminated Error

I was just looking through the SQL-92 Specification and saw a passage that reminded me of this question.

There is in fact a prescribed warning for this situation as indicated below

b) Otherwise, let TX be the single-column table that is the result of applying the <value expression> to each row of T and eliminating null values. If one or more null values are eliminated, then a completion condition is raised: warning- null value eliminated in set function.

I assume that the SET in the SQL Server Error Message is a reference to the set function of that error message although I'm not sure why it would make a distinction between aggregates and other set functions, as far as I can see they are synonymous. The relevant bit of the grammar is below.

6.5  <set function specification>

         Function

         Specify a value derived by the application of a function to an
         argument.

         Format

         <set function specification> ::=
                COUNT <left paren> <asterisk> <right paren>
              | <general set function>

         <general set function> ::=
                <set function type>
                    <left paren> [ <set quantifier> ] <value expression> <right paren>


         <set function type> ::=
              AVG | MAX | MIN | SUM | COUNT

         <set quantifier> ::= DISTINCT | ALL

SQL Server – Including Nulls During Comparisons

To start off, NULL does not mean "no value" it means "Unknown value" in SQL Server. There is a session setting called ANSI_NULLS that could make your queries behave as you would like them to, however, it's deprecated and will be forced to ON (which you don't seem to like) in a future version: http://msdn.microsoft.com/en-us/library/ms188048.aspx

I get what you're trying to do, and to make a counter point I would ask if you've seen any queries generated by reporting services or something like Cognos? If so, you'll see exactly what you're describing you don't want to do. Why? Well because with a schema that allows nulls, that's the way to do it. I'm not saying it's a super awesome and great idea but it works all of the time.

What your designer could do is check to see if that column could even be null and if so the appropriate logic could step it and create the correct query. You could also have options such as "This column may be null, do you want those values?". I don't know the end-game per-se and writing your own dynamic querying tool is quite the deat when the logical consistencies are all factored in (such as this).

I would continue to do explicit null checking on columns that could possibly be null, sure it doesn't look the best but it works all of the time.

ANSI_NULL set option will work for now but NOT a good idea especially if you don't control the environment, plus it will be forced set ON later and cause errors where you'll need to re-write your application logic anyway. This is the way SQL Server works with NULLs.

Best Answer

Related Solutions

SQL Server – Meaning of ‘SET’ in Null Value Eliminated Error

SQL Server – Including Nulls During Comparisons

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