Writing Conditional JOIN and WHERE Clauses

Erik Darling has Blondie stuck in my head:

The OR operator is a perfectly valid one to use in SQL statements. If you use an IN clause, there’s a reasonable chance that the optimizer will convert it to a series of OR statements.

For example, IN(1, 2, 3) could end up being = 1 OR = 2 OR = 3 without you doing a darn thing. Optimizers are funny like that. Funny little bunnies.

The problem generally isn’t when asking for IN or OR for a single column, with a list of literal values, the problem is usually when you:

• Use OR across multiple where clause columns
• Use OR in a join clause of any variety
• Use OR to handle NULL parameters or variables

This is an excellent, detailed article and well worth the read.