Select from one table where not in another
SELECT `pm`.`id`
FROM `R2R`.`partmaster` `pm`
WHERE NOT EXISTS (
    SELECT *
    FROM `wpsapi4`.`product_details` `pd`
    WHERE `pm`.`id` = `pd`.`part_num`
)

SELECT pm.id FROM r2r.partmaster pm
WHERE pm.id NOT IN (SELECT pd.part_num FROM wpsapi4.product_details pd)

SELECT id FROM partmaster LEFT JOIN product_details ON (...) WHERE product_details.part_num IS NULL

CREATE TEMPORARY TABLE

`database1`.`newRows`

SELECT

`t1`.`id` AS `columnID`

FROM

`database2`.`table` AS `t1`

CREATE INDEX `columnID` ON `database1`.`newRows`(`columnID`)

DELETE FROM `database1`.`newRows`

WHERE

EXISTS(
    SELECT `columnID` FROM `database1`.`product_details` WHERE `columnID`=`database1`.`newRows`.`columnID`
)