Scalar-Valued Function Returning NULL: A Deep Dive into SQL Server Functionality

Introduction

SQL Server functions are an essential part of any database-driven application. They allow developers to encapsulate complex logic within a reusable block of code, making it easier to maintain and update their applications over time. In this article, we will explore the intricacies of scalar-valued functions in SQL Server, focusing on the common issue of returning NULL values.

Understanding Scalar-Valued Functions

Scalar-valued functions are a type of user-defined function (UDF) that returns a single value or a table with one row. They are declared using the ALTER FUNCTION statement and can be used to perform various operations, such as data transformations, calculations, and aggregations.

A scalar-valued function typically has four parts:

1. Parameter list: This is where you specify the input parameters of your function.
2. Return type: This specifies the data type that the function will return.
3. Function body: This contains the logic of your function, which can include SELECT statements, arithmetic operations, and conditional statements.
4. End statement: This marks the end of the function body.

The Problem with Returning NULL

In SQL Server, when a scalar-valued function is executed, it will return either a single value or NULL if no valid result is produced. In the context of the provided example, the getFiatProfit function returns NULL because one of its branches does not produce a valid result.

To illustrate this issue, let’s break down the logic of the original getFiatProfit function:

IF (@fiatInvestedCurrency = 'USD')
    BEGIN
        select @tmp = [dbo].[usdtPairs].[Value] from [dbo].[usdtPairs] where usdtPairs.ID = @currency;
        select @usdtgbp = [dbo].[usdtPairs].[Value] from [dbo].[usdtPairs] where usdtPairs.ID = 'GBP';
        set @result = (((@quantity * @tmp) - @fiatInvested) / @usdtgbp);
    END
ELSE
    BEGIN
        select @tmp = [dbo].[usdtPairs].[Value] from [dbo].[usdtPairs] where usdtPairs.ID = @currency;
        set @result = ((@quantity * @tmp) - @fiatInvested);
    END
return (@result)
END

In this example, the function returns NULL if @fiatInvestedCurrency is not ‘USD’. The reason for this behavior lies in the default value of nvarchar data type.

The Issue with Default Values

When a parameter is declared without specifying a length or precision, SQL Server will use its default value. For nvarchar, the default length is 1 character. This can lead to unexpected behavior if you expect a longer string value.

To resolve this issue, it’s recommended to use fixed-length data types like char(3) instead of nvarchar. Here’s an updated version of the getFiatProfit function that addresses this problem:

CREATE OR ALTER FUNCTION dbo.getFiatProfit (
    @fiatInvested float,
    @fiatInvestedCurrency char(3),
    @quantity float,
    @currency char(3)
)
RETURNS TABLE
AS RETURN

SELECT
  result = ((@quantity * u.Value) - @fiatInvested)
    / (CASE WHEN @fiatInvestedCurrency = 'USD'
        THEN 1
        ELSE
          (SELECT u2.Value FROM dbo.usdtPairs u2 WHERE u2.ID = 'GBP')
       END)
FROM dbo.usdtPairs u
WHERE u.ID = @currency;

In this updated version, both @fiatInvestedCurrency and @currency are declared with a fixed length of 3 characters. This ensures that the function receives the correct string values.

Converting to Table-Valued Functions

Another significant improvement is to convert the scalar-valued function to a table-valued function. Table-valued functions return a result set, which can be used like any other table in your query.

Here’s an updated version of the getFiatProfit function that uses a table-valued approach:

CREATE OR ALTER FUNCTION dbo.getFiatProfit (
    @fiatInvested float,
    @fiatInvestedCurrency char(3),
    @quantity float,
    @currency char(3)
)
RETURNS TABLE
AS RETURN

SELECT
  result = ((@quantity * u.Value) - @fiatInvested)
    / (CASE WHEN @fiatInvestedCurrency = 'USD'
        THEN 1
        ELSE
          (SELECT u2.Value FROM dbo.usdtPairs u2 WHERE u2.ID = 'GBP')
       END)
FROM dbo.usdtPairs u
WHERE u.ID = @currency;

In this example, the getFiatProfit function returns a table with one row and one column. This allows you to use the result set like any other table in your query.

Best Practices

To avoid common issues when creating scalar-valued functions:

• Use fixed-length data types for string parameters.
• Specify return types clearly and consistently.
• Keep function logic concise and readable.
• Test your functions thoroughly with different input scenarios.

By following these guidelines and addressing the common problem of returning NULL values, you can write effective scalar-valued functions that meet your application’s requirements.

Last modified on 2024-01-06