vliet (10/8/2009)

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Since the 'or' operator has lower precedence as the 'and' operator, when you ommit the parentheses the join condition means something completely different. Without parentheses the predicate behind the 'or' is evaluated separately, which leaves out the 'a.id = b.id' part completely, thus returning all records from 'a' for each and every record where 'b.date2' is empty. This might blow up the size of the result set a lot when a has many rows and b has many rows without a date in this column. Never ommit any parenthenses whenever you use both 'and' and 'or' in a predicate. You can never use too many parenthenses in these cases, computers don't know what you mean, they just do what you tell them to, but that's generaly not what you want.

I've seen predicates with both the 'and' and 'or' operator being the cause of 'human' errors in SQL so often, it's almost worth an article.