Why Does My SQL Query Match with a Trailing Space Despite Using the Equality Operator?

Trapped by Trailing Spaces: A SQL Comparison Conundrum

Consider the following SQL query, where we attempt to match a value in the Zone table:

select '"' + ZoneReference + '"' as QuotedZoneReference
from Zone
where ZoneReference = 'WF11XU'

To our surprise, the result shows an unexpected trailing space:

"WF11XU "

Despite the trailing whitespace, SQL Server returns a match, leaving us puzzled.

This seemingly counterintuitive behavior can be explained by diving into the standards that guide SQL Server's string comparisons. As per ANSI/ISO SQL-92 specifications, SQL Server pads character strings used in comparisons to ensure they have the same length before performing the check. This rule extends to most comparison operations, making strings like 'abc' and 'abc ' equivalent for string comparisons.

However, the exception to this rule is the LIKE predicate. Only when the LIKE operator is involved does SQL Server refrain from padding. This distinction stems from the LIKE predicate's primary purpose of pattern matching rather than direct string equality tests.

In our case, since we're using the simple equality operator (=), the trailing space is considered by SQL Server, leading to a successful match. If our intent were to search for exact matches, we could utilize the LIKE predicate, which would ignore the trailing space and provide a more accurate result.

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