SQL JOINs Explained with Simple Examples
Understand INNER, LEFT, RIGHT, and FULL joins, unmatched rows, and NULL values.
A SQL join combines rows from two tables based on a condition. The join type determines what happens to rows without a match.
INNER JOIN
Returns only rows that match the join condition on both sides.
Duplicate keys can produce multiple output rows for one input row.
LEFT, RIGHT, and FULL JOIN
LEFT JOIN keeps every row from the left table and fills unmatched right-side columns with NULL.
RIGHT JOIN does the mirror image. FULL JOIN keeps unmatched rows from both sides where supported.
Debugging a join
Check key types and whitespace, inspect duplicate keys, and count unmatched rows. A join condition that is too broad can multiply records unexpectedly.
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