Why Does SQL Server Ignore Trailing Spaces in WHERE Clause Comparisons?

Trailing Spaces Discrepancy in SQL WHERE Clause Comparisons

In SQL Server, the WHERE clause comparison using the = operator often poses a discrepancy that can be perplexing. Specifically, trailing spaces in string values are ignored when making the comparison. This behavior deviates from what one might expect, leading to unexpected results.

Example Demonstration

Consider the following example in SQL Server 2008:

SELECT '"' + ZoneReference + '"' AS QuotedZoneReference
FROM Zone
WHERE ZoneReference = 'WF11XU'

This query retrieves data from a table called "Zone," where "ZoneReference" is the primary key column. Running the query would return the following result:

"WF11XU "

Notice the trailing space appended to the value in the result. This is despite the fact that the comparison criteria do not include any trailing space.

Cause of the Discrepancy

The discrepancy arises due to the SQL ANSI/ISO SQL-92 specification, specifically Section 8.2, which dictates how string comparisons with spaces are handled. As per the specification, SQL Server pads character strings used in comparisons to match in length before making the comparison. This means that trailing spaces are automatically added or removed to align the lengths.

Impact on Results

In most cases, the inclusion of trailing spaces in comparison matches produces expected results. However, when comparing values where trailing spaces are significant, it can lead to incorrect or unexpected outcomes. For instance, when searching for exact matches that include spaces, the additional trailing space can cause false matches.

Resolution

To resolve the issue and ensure precise comparison, one can use the TRIM() function to remove any leading or trailing spaces from the strings involved in the comparison. This will force an exact match without padding.

Example Solution

SELECT '"' + TRIM(ZoneReference) + '"' AS QuotedZoneReference
FROM Zone
WHERE TRIM(ZoneReference) = 'WF11XU'

Using the TRIM() function as demonstrated ensures an exact match of the string, including spaces.

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