* in PhpStorm:
# comment out the `di:OnAfterConfigRead` event
# change `ItemName` calculated field (of `di` unit) declaration into `'IF(p.Name IS NULL,c.l%2$s_Name,p.l%2$s_Name)'`
* in Admin Console:
# go to {nav E-commerce > Discounts & Certificates} section
# create discount, that has 1 product
# confirm, that product name is displayed correctly on `Items` tab during discount editing
# use `Item Name` column filter to search for letter present in product name
# confirm, that product is displayed and no SQL error present in Debugger
# use `Item Name` column filter to search for letter not present in product name
# confirm, that product isn't displayed and no SQL error present in Debugger
# remove grid search filter
* in PhpStorm:
# create the `DiscountItemsEventHandler::SetCustomQuery` method in which apply HAVING filter to the `ItemName` column (e.g. `$object->addFilter('test_filter', 'ItemName LIKE ' . $this->Conn->qstr('z'), kDBList::HAVING_FILTER);`)
# confirm, that no SQL error happens and data is filtered as expected