User Defined Function in SQL

Table of Contents

  • Introduction
  • Why we use function
  • Type of function
  • System defined function
  • User defined function

Introduction

We all are from a programming background and we know what function is and why we need it. But I am going to recap what function is. Function provides us with encapsulated reusable logic. In other words function hides the steps and complexity of code and we use that function where we want.

sql function

Why we use function

There are benefits of using function. Some of them are following.

  • We can use user defined function in select as well as where class.
  • We can use user defined function used as rowset.
  • We can use user defined function as parameterize views.

Type of function

SQL provide two type of functions.

  • System defined function
  • User defined function

System defined function

System defined functions are provided by SQL itself. You can find the details of these functions on the following. For more details click here.

User defined function

User defined functions are two types. First is Scalar and second is Table value.

Scalar Function

These functions return only a single value. These functions are any data type;  it could be return int , varchar or datetime. Let’s understand by example. Let’s create a table and name it myTable.

  1. CREATE TABLE [dbo].[myTable](  
  2.    [id] [int] IDENTITY(1,1) NOT NULL,  
  3.    [myDate] [dateNOT NULL,  
  4.    [data] [intNOT NULL  
  5. )  

 

Let’s insert some records into it.
  1. INSERT [dbo].[myTable] ( [myDate], [data]) VALUES (‘2015-01-01’, 10)  
  2. INSERT [dbo].[myTable] ( [myDate], [data]) VALUES (‘2015-02-01’, 15)  
  3. INSERT [dbo].[myTable] ( [myDate], [data]) VALUES (‘2015-03-01’, 20)  
  4. INSERT [dbo].[myTable] ( [myDate], [data]) VALUES (‘2015-04-01’, 15)  
Let’s run a select statement to check if it’s working or not.
  1. select * from myTable  
Output

see output

Let’s create a scalar function. This function will do some calculations. This function performs some calculations like it checks the date on the behalf of id and if that date contains the first month of the year it returns 0 else it performs this formula.

Change value = (current month data / previous month data) * 100
  1. Create Function dbo.MyFunction(@id int)  
  2. returns decimal  
  3. as  
  4. begin  
  5. declare @currentMonth int  
  6. declare @currentYear int  
  7. declare @returnValue float  
  8. declare @currentMonthData decimal  
  9. declare @previousMonthData decimal  
  10. set @currentMonth = (select MONTH(myDate) from myTable where id=@id)  
  11. set @currentYear = (select Year(myDate) from myTable where id=@id)  
  12. if @currentMonth=1  
  13. begin  
  14. set @returnValue=0  
  15. end  
  16. else  
  17. begin  
  18. set @currentMonthData = (select data from myTable where id=@id)  
  19. set @previousMonthData = (select data from myTable where month(myDate)=@currentMonth-1 and Year(myDate)=@currentYear)  
  20. set @returnValue = (@currentMonthData/@previousMonthData)*100  
  21. end  
  22. return cast(@returnValue as decimal(10,2))  
  23. end  
  24. -- Call that function  
  25. select *,dbo.MyFunction(id) as 'Quote change' from myTable  
Output

run

Table value function

As the  name suggests it returns a table. A table valued function can be used anywhere a table can be used like in select statement or in procedure or view.
  1. // Normal table value function  
  2. Create Function GetDetail()  
  3. returns table  
  4. as  
  5. return (select * from myTable)  
  6. // Parameterize table value function  
  7. Create Function GetDetailById(@id int)  
  8. returns table  
  9. as  
  10. return (select * from myTable where id=@id)  
  11. select * from GetDetail()  
Output

result
  1. select * from GetDetailById(1)  
Output

Output

I hope this blog was useful to you.
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