SQL JOINs Without the Confusion
by Froga
INNER, LEFT, RIGHT, FULL and CROSS — what each one actually returns, why unmatched rows come back as NULL, and how to stop guessing which one you need.
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Study material
Picture two overlapping circles. INNER JOIN returns only the overlap — a row appears in the result only if the join condition finds a partner on the other side. Rows without a match are dropped entirely, which is the single most common cause of "my query lost rows".
An INNER JOIN between two tables returns which rows?
multiple choice
Every row from both tables
Only rows with a match in both tables
Every row from the first table only
Only rows with no match in either table
INNER keeps the intersection.
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