Error - Magento 1: SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`databasename_magento`.`catalog_category_product_index`, CONSTRAINT `FK_CAT_CTGR_PRD_IDX_CTGR_ID_CAT_CTGR_ENTT_ENTT_ID` FOREIGN KEY (`category_id`) REFERENCES `catalog_category_entity` (`entity_) Print

This error occurs when trying to run the Category Products index, and is due to the indexing process attempting to insert a row into the catalog_category_product_index table, where either the product_id or the category_id don't exist. This causes the "Intergrity Constraint Violation".

This can sometimes happen when a database gets corrupt, often from 3rd party desktop type software or a badly written extension.

To fix this, first check that this is indeed the issue. Backup your database and then run the following SQL to see if any product and/or category associations are incorrect:

# Determine incorrect product associations
SELECT * FROM `catalog_category_product` WHERE
product_id not in (select entity_id from catalog_product_entity);

# Determine incorrect categories
SELECT * FROM `catalog_category_product` WHERE 
category_id not in (select entity_id from catalog_category_entity);

If that returns a record, you'll know exactly which product is in a non-existent category or which category has a non-existent product. Run the following SQL to then remove those invalid records:

# Delete incorrect product associations
DELETE FROM `catalog_category_product` WHERE 
product_id not in (select entity_id from catalog_product_entity);

# Delete incorrect categories
DELETE FROM `catalog_category_product` WHERE 
category_id not in (select entity_id from catalog_category_entity);

Now log back into Magento Admin and reindex the Category Products index.