Conversion Failed When Converting the Varchar Value to Int Inside CONCAT

The CONCAT function in SQL Server allows you to concatenate multiple strings into a single string. However, when using this function with a CAST statement to convert a string to an integer, things can get tricky.

In this blog post, we’ll delve into the world of SQL Server concatenation and explore why using the + operator inside CONCAT can lead to unexpected results.

Understanding Concatenation in SQL Server

When you use the + operator with strings in SQL Server, it performs an implicit conversion. This means that if one of the operands is a string containing whitespace characters (such as spaces or tabs), the result will also contain those whitespace characters.

For example:

SELECT 'Hello' + 'World' AS Result;

Result: Hello World

In this case, the + operator concatenates the two strings without any issues. However, when using CONCAT with a string containing whitespace characters, things get complicated.

The Problem with CONCAT and Implicit Conversion

When you use CONCAT inside a query like this:

SELECT CAST(CONCAT('BL.', IdBL,' - lgn.', LgnBL, ' \ BE.', BE_Numero_carnet) AS varchar(1000)) AS chemin 
FROM ...

The SQL Server engine performs an implicit conversion on the string ' \ BE.'. This means that the resulting string will contain a whitespace character (' ').

However, when you use CAST to convert this string to an integer, SQL Server fails because it cannot implicitly convert a string containing whitespace characters to an integer.

The Solution: Avoid Using + Inside CONCAT

To avoid this problem, don’t use the + operator inside CONCAT. Instead, use a space character (' ') or a string literal that represents no whitespace (such as '.').

For example:

SELECT CAST(CONCAT('BL.', IdBL,' - lgn.', LgnBL, ' . BE.', BE_Numero_carnet) AS varchar(1000)) AS chemin 
FROM ...

By using the space character (' '), we ensure that the resulting string does not contain any whitespace characters.

Alternatively, you can use a string literal like '.' to separate the strings:

SELECT CAST(CONCAT('BL.', IdBL,' - lgn.', LgnBL, '. BE.', BE_Numero_carnet) AS varchar(1000)) AS chemin 
FROM ...

Additional Considerations

When working with concatenation and implicit conversions in SQL Server, it’s essential to keep the following tips in mind:

• Always use CONCAT instead of the + operator when concatenating strings.
• Avoid using whitespace characters (' ' or \t) as part of a string literal.
• Use string literals like '.' or other non-whitespace characters to separate strings.

Real-World Example

Suppose we have a table called Customers with columns IdBL, LgnBL, and BE_Numero_carnet. We want to concatenate these values into a single string that represents the customer’s information, including their branch name, login details, and ID card number.

We can use the following query:

SELECT CAST(CONCAT('Branch:', IdBL,' - Login:', LgnBL, '. Card Number:', BE_Numero_carnet) AS varchar(1000)) AS CustomerInfo 
FROM Customers;

Result:

In this example, we use CONCAT to concatenate the values in a single string. By avoiding the use of whitespace characters (' '), we ensure that the resulting string does not contain any issues.

Conclusion

When working with concatenation and implicit conversions in SQL Server, it’s essential to be aware of the pitfalls and best practices. By using CONCAT instead of the + operator and avoiding whitespace characters, you can avoid unexpected results and write more reliable code.

Remember to always use string literals like '.' or other non-whitespace characters to separate strings, and keep your queries well-structured and readable.

Last modified on 2023-06-16