如何从MySQL中的两个不同表中删除重复的字段

Question

Imagine that you have two tables.

Table bar:

+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| name  | varchar(255) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+

Table foo:

+------------------+--------------+------+-----+---------+-------+
| Field            | Type         | Null | Key | Default | Extra |
+------------------+--------------+------+-----+---------+-------+
| name_alternative | varchar(255) | YES  |     | NULL    |       |
+------------------+--------------+------+-----+---------+-------+

And you want to remove the rows from table bar that have the same name value as the field name_alternative from table foo

I could use the following query:

DELETE FROM foo WHERE name IN (SELECT name_alternative FROM bar);

But this takes a very long time to execute having a large amount of records in both tables.

So I was wondering if there is a better alternative to this query.

Answer 1

This is another way that would work, and I think it would be more efficient:

DELETE FROM bar
    USING foo, bar
    WHERE name_alternative = name

Here is a working SQLFiddle example .

See the documentation for delete .

Answer 2

Besides @dan1{4} answer, my guess is that it takes a long time because there is no index on bar(name) .

You don't need an index covering the whole 255 chars, depending on your strings, ie if the first characters are different on a lot of rows, create an index on a few characters

CREATE INDEX indexname ON bar(name(8))

8 is a suggested value that you may want to refine. The index will help mysql to locate each bar row that matches a foo.namealternative

Answer 3

delete from bar
using (
    select foo.name_alternative
    from 
        bar
        inner join
        foo on bar.name = foo.name_alternative
) s
where bar.name = s.name_alternative