The problem: We have created an attribute with the "Multiple Select" property and now want to use it in Magento in the category view for the sorting function, i.e. sort the product list according to the values of this attribute. In the standard settings of a multiple-select attribute, this functionality ("Use in Layered Navigation") cannot be enabled.
So we take the database approach:
In Magento 1.9, we first check which attribute_id our attribute has (via the attribute management section in the backend, the "eav_attribute" table, etc.). We then look for our attribute_id in the "catalog_eav_attribute" table and change the value in the "used_for_sort_by" column to "1" (true). For older Magento versions, it may also be worth taking a look at this (English) thread (referring to Magento 1.3.2.4), which led us to the solution described above.
