Why SQL Full Text Indexing Doesn’t Return Results for Words Containing #

When running SQL queries, you might have encountered situations where your expected results aren’t returned. One common scenario involves using the FREETEXT function to search for words containing special characters, like the hash # symbol. If you’ve been using SQL Server 2005 and are puzzled by missing results for queries like SELECT * FROM Table WHERE FREETEXT(SearchField, 'c#'), you’re not alone. This blog post will help clarify why this happens and provide effective solutions to address the issue.

Understanding the Problem

In SQL Server, especially versions like SQL Server 2005, the way certain characters are processed can greatly impact search results:

• Special Characters as Punctuation: The # character is treated as punctuation by SQL’s full-text indexing. As a result, it is ignored during searching.
• Difference between FREETEXT and LIKE: While FREETEXT ignores special characters, other methods like LIKE can still function and return results, as seen in the following query:

  SELECT * FROM Table WHERE SearchField LIKE '%c#%'
  

  This query captures instances of c# successfully, as it matches the text pattern directly without being hindered by punctuation rules.

Why is # Treated Differently?

SQL Server employs a set of predefined rules for indexing, filtering out certain noise words and punctuation. Specifically, here’s what happens with terms that include #:

• Lowercase vs. Uppercase: According to SQL documentation, the term c# is indexed simply as c if c isn’t in the noise word list. However, C# is indexed as C# if it begins with an uppercase letter, ignoring ‘c’ from the noise word considerations.
• General Rule: A lowercase letter followed by a special character (like + or #) often results in the letter being ignored in indexing, whereas an uppercase letter maintains the special character.

Solutions to Retrieve Desired Results

If you find your FREETEXT query not returning results, here are a few strategies to consider:

1. Adjust Noise Word List

• Modify the Noise Word List: You may want to remove C from your noise word list. This adjustment can potentially allow for better indexing and retrieval for terms such as c#.
• Rebuild the Indexes: After making changes to the noise list, remember to rebuild the indexes to apply these modifications properly.

2. Explore Alternative Word Breakers

• Use Different Linguistic Options: SQL Server allows for different word breakers based on the language used. By utilizing an appropriate word breaker, special characters may be treated differently, allowing for comprehensive search results.

Example Adjusted Query

After addressing the noise word list and rebuilding your indexes, try running your FREETEXT query once more:

SELECT * FROM Table WHERE FREETEXT(SearchField, 'c#')

With this adjustment, you should start to see results that include terms with #.

Conclusion

Handling special characters in SQL Server’s Full Text Indexing can be tricky, especially when searching for terms containing punctuation like #. By understanding how SQL processes these characters, adjusting your noise word list, and exploring alternate word break settings, you can improve your query results significantly.

This knowledge will enable you to perform more effective searches and effectively harness SQL Server’s capabilities for your data needs.