Unknown column 'category_name' in 'field list' code example

Example 1: unknown column in field list

Try using different quotes for "y" as the identifier quote character is the backtick (“`”). Otherwise MySQL "thinks" that you point to a column named "y".

See also MySQL 5 Documentation

Example 2: Unknown column '' in 'field list'

use qoutes when you add a string, otherwise mysql will thinks the added value is a column name