ALTER TABLE ... ADD COLUMN IF NOT EXISTS
so there is no option to check if some column exists in database or not with standard Joomla! update function.
If you get following problem when updating to version 3.5.4:
orDuplicate column name 'sku'
try to use following installation ZIP:Duplicate column name 'ean'
a) if you know that you run MariaDB database: https://github.com/PhocaCz/PhocaCart/re ... ariadb.zip
or b) if you run MySQL database or you don't know which database you are running: https://github.com/PhocaCz/PhocaCart/re ... update.zip
This should skip adding EAN or SKU columns if they exist in your Phoca Cart instance.