SQL Update with inner join - mysql

Question

I have a table of records, which has a self-relationship.
Additionally - to make searching easier - I have a flag which determines that a record has been referenced and hence that row is now "obsolete" and is only there for audit purposes:
CREATE TABLE Records
(
RecordID INT(5) NOT NULL,
Replaces INT(5) NULL,
Obsolete INT(1) NOT NULL
)
RecordID is the PK, Replaces links to a previous RecordID which has now been replaced, and Obsolete is redundant information which just says that another record has replaced this one. It just makes searching a lot easier. The table is very large. These are just 3 of the columns.
The only problem is: there was a typo in one of the queries in the system so for a small set of rows, the Obsolete value was not set to 1 (true).
This query will show all the records with Obsolete equal to 0 which should be equal to 1:
SELECT *
FROM Records AS rec1
LEFT JOIN Records AS rec2
ON rec1.Replaces = rec2.RecordID
WHERE rec2.RecordID IS NOT NULL
AND rec2.Obsolete = 0;
Now I need to run an UPDATE to change all those req2.Obsolete from 0 to 1, but I'm not sure how to write a query with an INNER JOIN.

Answer 1

You don't need an inner join. Since your query already returns the records that need to be updated, just do this:
Update Records
set Obsolete=1 where
RecordID in (
SELECT rec2.RecordID
FROM Records AS rec1
LEFT JOIN Records AS rec2
ON rec1.Replaces = rec2.RecordID
WHERE rec2.RecordID IS NOT NULL
AND rec2.Obsolete = 0
)

Answer 2

UPDATE Records
SET obsolete = 1
WHERE recordID in (
SELECT rec1.recordid
FROM Records AS rec1
LEFT JOIN Records AS rec2
ON rec1.Replaces = rec2.RecordID
WHERE rec2.RecordID IS NOT NULL
AND rec2.Obsolete = 0
)

Answer 3

I would suggest doing this in two steps using a temporary table:
-- Create temporary table for holding RecordIDs to be marked as obsolete
CREATE TEMPORARY TABLE `mark_obsolete` (`RecordID` INT NOT NULL);
-- Insert RecordIDs to mark as obsolete into temp table
INSERT INTO `mark_obsolete` (`RecordID`)
SELECT `rec2`.`RecordID`
FROM
`Records` AS `rec1`
INNER JOIN `Records` AS `rec2`
ON `rec1`.`Replaces` = `rec2`.`RecordID`
WHERE `rec2`.`Obsolete` = 0;
-- Update records using inner join to temp table
UPDATE
`Records` AS `r`
INNER JOIN `mark_obsolete` AS `o`
ON `r`.`RecordID` = `o`.`RecordID`
SET `r`.`Obsolete` = 1;
DROP TEMPORARY TABLE `mark_obsolete`;
Note that using a LEFT JOIN with WHERE rec2.RecordID IS NOT NULL is the same as an INNER JOIN.
The reason for using a temporary table is to avoid locking issues when updating the same table used in the sub-query. And it might also give you better performance than using the IN clause.