Can we put an equal sign (=) after aggregate functions in Transact-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
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&lt;>fiddle demo