"First, I used an iSight report to evaluate the two queries. And I quickly saw that although they were similar, they weren’t actually identical. Whereas Jan’s query was accurate, Mike’s query had a semantic error.

Both Jan and Mike pulled data from the "customer" table, but Mike had used the wrong customer table.    See the report

It was easy to understand Mike’s mistake because our database actually has two different "customer" tables: one in the Finance schema and the other in the Orders schema."