Solving SQL Conversion Errors: Handling VARCHAR to INT Conversions

Introduction

When working with databases, particularly SQL Server, it’s common to encounter issues when converting data types. One frequent error is when attempting to convert a VARCHAR value to an integer (INT). This tutorial will explore the causes of this conversion error and provide solutions using best practices in SQL queries.

Understanding the Problem

The error message "Conversion failed when converting the varchar value ‘…’ to data type int" typically occurs when trying to directly convert a string that is not purely numeric into an integer. For example, if you attempt to convert a VARCHAR field containing non-numeric characters or decimal points (e.g., '12345.0', 'simple,') into an INT, SQL Server will throw this error.

Common Causes

1. Non-Numeric Characters: The string contains letters or special symbols, making it impossible to convert directly.
2. Decimal Points: Strings ending in a decimal point (e.g., '12345.').
3. NULL Values: Attempting to convert NULL can also lead to conversion errors.

Solutions

1. Use ISNUMERIC and CASE Statements

A robust way to handle potential conversion issues is by using the ISNUMERIC function combined with a CASE statement. This approach checks if the value can be safely converted before attempting it:

SELECT 
    a.name, 
    CONVERT(INT,
        CASE 
            WHEN ISNUMERIC(CONVERT(VARCHAR(12), a.value)) = 1 THEN CONVERT(VARCHAR(12), a.value)
            ELSE NULL -- or a default integer like 0
        END) AS value, 
    COUNT(*) AS pocet   
FROM 
(
    SELECT item.name, value.value 
    FROM mdl_feedback AS feedback 
    INNER JOIN mdl_feedback_item AS item ON feedback.id = item.feedback
    INNER JOIN mdl_feedback_value AS value ON item.id = value.item 
    WHERE item.typ = 'multichoicerated' AND item.feedback IN (43)
) AS a 
INNER JOIN #myTempTable on 
    CONVERT(INT,
        CASE 
            WHEN ISNUMERIC(CONVERT(VARCHAR(12), a.value)) = 1 THEN CONVERT(VARCHAR(12), a.value)
            ELSE NULL
        END) = #myTempTable.num
GROUP BY a.name, value ORDER BY a.name;

2. Remove Decimal Points

If your VARCHAR values contain decimal points but should be treated as integers (e.g., '12345.0'), you can remove the decimal point:

SELECT 
    a.name,
    CONVERT(INT, REPLACE(CONVERT(VARCHAR(12), a.value), '.0', '')) AS value, 
    COUNT(*) AS pocet   
FROM 
(
    SELECT item.name, value.value 
    FROM mdl_feedback AS feedback 
    INNER JOIN mdl_feedback_item AS item ON feedback.id = item.feedback
    INNER JOIN mdl_feedback_value AS value ON item.id = value.item 
    WHERE item.typ = 'multichoicerated' AND item.feedback IN (43)
) AS a 
INNER JOIN #myTempTable on 
    CONVERT(INT, REPLACE(CONVERT(VARCHAR(12), a.value), '.0', '')) = #myTempTable.num
GROUP BY a.name, value ORDER BY a.name;

3. Change Data Type in Temporary Table

Align the data types of the temporary table and target column to avoid conversion:

CREATE TABLE #myTempTable
(
    num VARCHAR(12)
);
INSERT INTO #myTempTable (num) VALUES ('1'), ('2'), ('3'), ('4'), ('5');

SELECT 
    a.name, 
    a.value AS value, 
    COUNT(*) AS pocet   
FROM 
(
    SELECT item.name, value.value 
    FROM mdl_feedback AS feedback 
    INNER JOIN mdl_feedback_item AS item ON feedback.id = item.feedback
    INNER JOIN mdl_feedback_value AS value ON item.id = value.item 
    WHERE item.typ = 'multichoicerated' AND item.feedback IN (43)
) AS a 
INNER JOIN #myTempTable on a.value = #myTempTable.num
GROUP BY a.name, value ORDER BY a.name;

Conclusion

Handling conversion errors in SQL requires understanding the nature of your data and applying appropriate checks or transformations. By using ISNUMERIC, handling decimal points, or aligning column types, you can prevent these common errors and ensure your queries run smoothly.