A column specification ( column_spec) specifies a column in a table.
<column_spec>::= <column_name> | <table_name>.<column_name> | <reference_name>.<column_name> | <result_table_name>.<column_name>
Explanation
For all names consisting of several identifiers that are separated by a period ("."), any number of blanks can be specified before and after the period.
A column specification is used in the following SQL syntax elements: