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sql/flashcards/sql-cards-joins

Combining data — flashcards

One card per lesson — 6 in this module. Recall the key points, then check. 6 cards. Click a card to flip, or use the arrow keys and space bar.

  1. Join concepts — what are the key points?

    A one-to-many join multiplies rows on the "one" side · Always compare row counts before and after a join · Pre-aggregate the many side to avoid a fan-out Read the lesson

  2. INNER JOIN — what are the key points?

    INNER JOIN keeps only rows matching on both sides · Unmatched rows disappear with no warning · Compare row counts to detect silent data loss Read the lesson

  3. LEFT and RIGHT JOIN — what are the key points?

    LEFT JOIN keeps all rows from the left table · A WHERE filter on the right table turns a LEFT JOIN into an INNER JOIN · Use COUNT(right.column), not COUNT(*), when counting after a LEFT JOIN Read the lesson

  4. FULL OUTER and CROSS JOIN — what are the key points?

    FULL OUTER JOIN keeps unmatched rows from both sides · MySQL has no FULL OUTER JOIN — emulate with LEFT UNION RIGHT · A missing ON clause produces an accidental Cartesian product Read the lesson

  5. Joining three or more tables — what are the key points?

    Joins apply in order; each works on the accumulated result · An INNER JOIN after a LEFT JOIN can undo the outer join · Add joins one at a time and count rows to locate a fan-out Read the lesson

  6. UNION and UNION ALL — what are the key points?

    UNION removes duplicates; UNION ALL keeps them and is faster · Column count, order and types must be compatible · ORDER BY applies once, at the end of the whole union Read the lesson