Understanding the Issue with Converting nvarchar to smalldatetime Data Types in SQL Server

Introduction

In this article, we will explore a common issue that arises when trying to convert an nvarchar data type to a smalldatetime data type in Microsoft SQL Server. The problem is often encountered when using queries to filter dates and times in the database. We’ll dive into the details of why this issue occurs and how to resolve it.

Background

The nvarchar data type represents a sequence of Unicode characters, while smalldatetime represents a date and time with a maximum precision of 1 minute. When converting an nvarchar data type to a smalldatetime data type, SQL Server uses a system-defined conversion function called CAST().

The Problem

The problem arises when the input nvarchar value is not in a format that can be converted to a smalldatetime. For example, if the input string represents a date and time in the format “YYYYMMDD” (e.g., “20140810”), SQL Server will attempt to convert it to a smalldatetime.

SELECT CAST(LAST_UPDATED AS smalldatetime) > (CAST(@updateTime AS smalldatetime))

In this case, the conversion function may not be able to accurately represent the date and time because it is missing hours or minutes.

The Out-of-Range Value Error

When SQL Server encounters an input value that cannot be accurately represented as a smalldatetime, it will return an out-of-range value. This error occurs because the system-defined conversion function is unable to make an accurate conversion between the two data types.

The conversion of a nvarchar data type to a smalldatetime data type resulted in an out-of-range value.

The Role of Substring() and GetLastUpdate()

In the provided Stack Overflow question, the author was using Substring() to extract the date portion from an nvarchar string. However, this approach may not always produce accurate results.

string updatedTime = strUpdateTime.Substring(0, 11);

In addition, the GetLastUpdate() function is used to retrieve a formatted date and time string. This function may return a value in a format that cannot be accurately converted to a smalldatetime.

Resolving the Issue

To resolve this issue, we need to ensure that the input data types are consistent and accurate. Here are some steps you can take:

1. Verify Input Data Types

Ensure that the input data types for both LAST_UPDATED and @updateTime are consistent and accurate.

SELECT LAST_updated, @updateTime

2. Use System-Defined Conversion Functions

Use system-defined conversion functions to convert between data types. For example:

CAST(LAST_UPDATED AS DATE)

This will ensure that the date portion is accurately represented without hours or minutes.

3. Format Date and Time Strings Correctly

Format date and time strings correctly to avoid conversion errors. For example, if you need to use a specific format (e.g., “YYYYMMDD”), consider using an application-level formatting function instead of relying on the system-defined conversion functions.

string strUpdateTime = GetLastUpdate();
string updatedTime = strUpdateTime.Substring(0, 11);

Conclusion

Converting an nvarchar data type to a smalldatetime data type can be challenging due to the differences in data types and formatting. By understanding the issues and using system-defined conversion functions correctly, you can ensure accurate results and avoid the out-of-range value error.

Code Example

Here’s an example of how to use the CAST() function to convert between data types:

SELECT 
    LAST_UPDATED,
    @updateTime
FROM Employees
WHERE 
    (CAST(LAST_UPDATED AS DATE) > (CAST(@updateTime AS DATE)))

This code snippet demonstrates the correct usage of system-defined conversion functions to filter dates and times in a SQL Server query.

Note: The above code example assumes that you want to compare only date portions. If you need to compare dates and times, modify the query accordingly.

Last modified on 2025-04-11