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INNER JOIN and LEFT JOIN (The Two You Need First)

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Lessons in this chapter:

  1. 15.1 What INNER JOIN Keeps (Matching Rows Only)
  2. 15.2 What LEFT JOIN Keeps (Every Left Row, Matched or Not)
  3. 15.3 How to Write JOIN … ON Correctly (Aliases, Filters, Never Without ON)
  4. 15.4 How to Spot Dropped and Repeated Rows After a Join
  5. 15.5 Practice: Shipments with Customer Names and Hubs

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Ravi Sir's Cards for this chapter

What INNER JOIN really keeps. INNER JOIN is the easiest join to write, but it quietly drops rows that have no match.Revision cardWhy did my total double after a join? The key repeats on the other side, so each row was repeated once per match. The SUM then counted the same fee more than onInterview cardINNER JOIN vs LEFT JOIN. INNER JOIN keeps only matching pairs. LEFT JOIN keeps every row of the table after FROM and adds the matches.Comparison card

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