Question

Pretty straightforward question: Why the output of the following SQL is not null?

SELECT * FROM table WHERE 
(`FirstName` LIKE 'D%' OR `LastName` LIKE 'D%' OR `City` LIKE 'D%')
AND
(`FirstName` NOT LIKE 'D%' OR `LastName` NOT LIKE 'D%' OR `City` NOT LIKE 'D%')

I am using MySQL 5.1. The result is ignoring the “NOT LIKE” part all together.

Answer 1

If you have a FirstName that of ‘David” then the first parenthetical condition is true. If you have a LastName of “Smith” then the second parenthetical condition is true and your query will return that result.

What you probably mean to be doing is something like this:

SELECT * FROM table WHERE 
(`FirstName` LIKE 'D%' OR `LastName` LIKE 'D%' OR `City` LIKE 'D%')
AND
(`FirstName` NOT LIKE 'D%' AND `LastName` NOT LIKE 'D%' AND `City` NOT LIKE 'D%')

(Note the ORs that have been changed to ANDs.)