MySQL select with sub-query and LIMIT [duplicate]

问题

This question already has answers here:

LEFT JOIN query not returning all rows in first table (1 answer)

Left Outer Join doesn't return all rows from my left table? (2 answers)

Closed 3 months ago.

I am using the subselect to get the row IDs I need like this:

SELECT
   p.id, c.id as category_id 
FROM
   (SELECT id FROM products p WHERE p.id > 6319055 ORDER BY id LIMIT 1000) prods 
LEFT JOIN 
   products p ON p.id = prods.id 
LEFT JOIN 
   categories c ON (c.id = p.category_id) 
WHERE
   c.active = 1

The ID 6319055 is my last selected ID. I save it after selecting the data.

Now the problem I am having is that I am selecting 1000 rows on each cycle and at some point I select 1000 rows which doesn't meet the

WHERE c.active = 1

requirements. Select returns nothing and I don't have any row ID to continue the subselect.

Any ideas how could I solve this? How can I get the last ID of the sub select, even if it doesn't meet the WHERE clause?

回答1:

When you use WHERE condition on the right-side table of a LEFT JOIN (Outer Join), it effectively becomes an INNER JOIN, because WHERE clause needs to match the conditions. That is why you are only getting cases where c.active = 1.

You need to shift the WHERE condition to LEFT JOIN .. ON .. AND .. condition:

SELECT
   p.id, c.id as category_id 
FROM
   (SELECT id FROM products p WHERE p.id > 6319055 ORDER BY id LIMIT 1000) prods 
LEFT JOIN 
   products p ON p.id = prods.id 
LEFT JOIN 
   categories c ON c.id = p.category_id 
                   AND c.active = 1

来源：https://stackoverflow.com/questions/57912384/mysql-select-with-sub-query-and-limit

标签

mysql

select

subquery

limit