Why Does My SQL WHERE Clause Ignore a Parameter?

Troubleshooting SQL WHERE Clauses: Why Parameters Are Ignored

SQL queries can sometimes produce unexpected results, particularly when the WHERE clause incorporates multiple conditions. This often stems from incorrect operator precedence and grouping.

Let's examine a scenario where a WHERE clause seemingly ignores a parameter:

SELECT * FROM people
WHERE
  university='2'
  AND MATCH (lname,fname) AGAINST ('+massive' IN BOOLEAN MODE)
  OR (fname LIKE '%box%' OR lname LIKE '%box%')

This query aims to select records from the people table where university is '2' and a full-text search for 'massive' matches or the first or last name contains 'box'. However, the result set might include entries that don't satisfy the university='2' condition.

The problem lies in SQL's operator precedence: AND takes precedence over OR. The query's implicit grouping is:

(university='2') AND (
  (MATCH (lname,fname) AGAINST ('+massive' IN BOOLEAN MODE))
  OR (fname LIKE '%box%' OR lname LIKE '%box%')
)

Notice how university='2' only applies to the full-text search. To correct this, we need explicit parentheses to ensure the AND condition applies to all parts:

SELECT * FROM people
WHERE
  (university='2')
  AND (MATCH (lname,fname) AGAINST ('+massive' IN BOOLEAN MODE)
       OR fname LIKE '%box%'
       OR lname LIKE '%box%')

By grouping all conditions within parentheses, we enforce the intended logic, ensuring only records meeting all specified criteria are returned. This highlights the importance of careful parenthesis usage when constructing complex WHERE clauses in SQL.

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