Multi-Column Anti-Joins and Composite Keys

Concepts covered: sqlExists

The escalation: 'find users who visited page A but never page B on the same day.' One condition is no longer enough; the anti-match has to consider multiple columns at once. The form choice matters again, and the same NULL trap reappears in a slightly different shape. The multi-column version Express the absence with NOT EXISTS, joining the subquery on every column that defines the match. The subquery joins on three columns: user_id, visit_date, and page. The visit_date join is what enforces 'on the same day.' Without it, a user who visited page A today and page B last month would be excluded, which is wrong. The interviewer will sometimes ask you to walk through a test case with one user who visited both pages on different days; the multi-column join is what handles that correctly. The LE

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