When we are using the function in the SELECT statement it returns data depends on the number of records retrieve by the select statement and what type of value is passed in the scalar function. The performance of the MS SQL Query is little bit degraded using the scalar function. But developer often used it for encapsulate frequently performed logic. But if we saw some SELECT statement, we can see that the functions can be used in WHERE conditions also. Using scalar or User defines function in WHERE clause is a good idea? Is this hampering the performance? This article is related to it.