For some data integrity checks, I needed to know if all SKUs stored in a MySQL database table have been properly imported. That implied making sure each SKU contained a string with 2 underscores as a separator. Knowing that PHP has functions like substr_count() or count_chars(), I was surprised that nothing similar exists in MySQL.
Thankfully, I came across this neat little trick to achieve the same result:
SELECT entity_id, (LENGTH(sku)-LENGTH(REPLACE(sku, '_', ''))) AS char_count WHERE char_count = 2 FROM catalog_product_entity;