Why are these two mysql queries different? - mysql

Question

select distinct p.product_id from cscart_products p
left join product_bikes pb on p.product_id = pb.product_id
left join cscart_product_options po on po.product_id = p.product_id
left join cscart_product_option_variants pov on pov.option_id = po.option_id
left join variant_bikes vb on vb.variant_id = pov.variant_id
where pb.bike_id = 111 or vb.bike_id = 111
And:
select distinct p.product_id from cscart_products p
left join product_bikes pb on p.product_id = pb.product_id and pb.bike_id = 111
left join cscart_product_options po on po.product_id = p.product_id
left join cscart_product_option_variants pov on pov.option_id = po.option_id
left join variant_bikes vb on vb.variant_id = pov.variant_id and vb.bike_id = 111
Return different result sets, why?

Answer 1

The first query has an OR in the WHERE clause:
WHERE pb.bike_id = 111 OR vb.bike_id = 111
The second query effectively has AND instead, via the conditions:
LEFT JOIN product_bikes pb ON p.product_id = pb.product_id AND pb.bike_id = 111
...
LEFT JOIN variant_bikes vb ON vb.variant_id = pov.variant_id AND vb.bike_id = 111
Bonus question: is there a way with joins to make it behave the same and benefit from having smaller joins for performance?
There is a way to write the query, but it isn't necessarily any faster because the method (that I'm thinking of) uses UNION:
select distinct p.product_id from cscart_products p
left join product_bikes pb on p.product_id = pb.product_id and pb.bike_id = 111
left join cscart_product_options po on po.product_id = p.product_id
left join cscart_product_option_variants pov on pov.option_id = po.option_id
left join variant_bikes vb on vb.variant_id = pov.variant_id -- and vb.bike_id = 111
UNION
select distinct p.product_id from cscart_products p
left join product_bikes pb on p.product_id = pb.product_id -- and pb.bike_id = 111
left join cscart_product_options po on po.product_id = p.product_id
left join cscart_product_option_variants pov on pov.option_id = po.option_id
left join variant_bikes vb on vb.variant_id = pov.variant_id and vb.bike_id = 111
There probably is a better way of doing it, such that you have a UNION sub-query, along the lines of:
SELECT DISTINCT p.product_id
FROM cscart_products AS p
LEFT JOIN cscart_product_options AS po ON po.product_id = p.product_id
LEFT JOIN cscart_product_option_variants AS pov ON pov.option_id = po.option_id
LEFT JOIN (SELECT vb.product_id FROM variant_bikes AS vb WHERE vb.bike_id = 111
UNION
SELECT pb.product_id FROM product_bikes AS pb WHERE pb.bike_id = 111
) AS pv ON pv.product_id = p.product_id
Since you aren't (in the example) selecting data from the cscart_product_options or cscart_product_options_variants tables, you could eliminate those from the query. You should also look at whether the LEFT JOIN with the sub-query is appropriate; I think it more likely that you want an inner join. There may well be more work that can be done to improve the performance.

Answer 2

In addition to what Jonathan said. In the first query, the WHERE forces it that you don't get ANY results UNLESS (pb.bike_id = 111 or vb.bike_id = 111) is true. In the second query, you will get all DISTINCT [product_id]s even though only one row will be able to join via the LEFT JOINs.
If you are getting a lot of results from the second query vs. the first, that's way. The easier way to see this is by putting more in your SELECT so:
SELECT p.product_id, pb.bike_id ...
You'll notice if you do that, that the first query will have 111 in every product it displays, but the second query will have a lot of NULL values for pb.bike_id.
Make sense?