Mysql count left join strange result

Question

Can someone help me to understand those results ? (For me all 3 should return 6455).

(Using RDS mysql-8.0.13)

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SELECT COUNT(p.product_id) FROM product p LEFT JOIN product_attributes pa ON p.pdt_id = pa.pdt_id WHERE pa.code = 'season';

Results : 6332

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SELECT COUNT(*) FROM product p;

Results : 6455

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SELECT COUNT(p.product_id) FROM product p LEFT JOIN product_attributes pa ON p.pdt_id = pa.pdt_id AND pa.code = 'season';

Results : 6455

Answer 1

Your first join uses the WHERE clause, this mean sit selected all the rows, including those with a null join and then filters out those WHERE the pa.code = season, ie the null joins.

The last one joins on both, but because it is a left join you still get the full table of results, and nothing is filtered because you remove the WHERE clause. If you were to use an INNER JOIN in the last query you should get the same result (6332).

This link might be useful What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN?