我有一个查询，该查询在我的SQL数据库中找到重复项-现在如何删除所说的重复项？

Question

I have an sql query that finds and groups these duplicates using very complicated conditions:

SELECT right(post_url, LOCATE('-', REVERSE(post_url),LOCATE('-',REVERSE(post_url))+1) -1) as name,
left(post_name,LOCATE('-',post_url,LOCATE('-',post_url)+1) - 1) as city,
post_title as original,ID,post_name,count(*) 
FROM table WHERE post_type='finder' 
GROUP BY name,city having count(*) > 1

To explain the query, post_url is basically a url name, ending with the name of someone, eg : new-jersey-something-something-donald-t

I go to the second dash from the right and get the name that way. Then I get the city/state which is in the second dash from the left. In this manner, I've successfully found the duplicates in this database-but I'm having trouble thinking of a way to isolate the duplicate and delete it. In addition, I only want to delete the copy that does not have %near% in post_url. my question is, using the query here, how would I change this to delete the duplicate?

Answer 1

You're not going to be able to do it in one query. That's because you need to write a query that looks something like this:

DELETE FROM table
WHERE id IN (SELECT ... FROM table WHERE ...)

MySQL specifically prohibits this. You can't delete based on a subquery that references the same table. You also can't rewrite this query using JOIN s.

There is an easy solution, though: use a temporary table and two queries.

-- build the list of IDs to delete
CREATE TEMPORARY TABLE temp
SELECT ... FROM table WHERE ...

-- now delete those items
DELETE FROM table
WHERE id IN (SELECT id FROM temp);

You can improve performance with JOIN s and indexes.

The key to "isolating" the duplicates is to ensure that every item you want to delete has a primary key - that way you can easily build a list of IDs to delete. If your table don't have primary keys, you are reduced to doing WHERE clauses and JOIN s on multiple columns - that gets messy very quickly.