Column References
Semantics
A column-reference expression returns the item stored in the specified column within the context row (the row that an SELECT expression is currently working on). Syntactically, a column-reference expression consists of one identifier, or 2 identifiers separated by a dot. If there are 2 ids, the first is considered to be a table alias and the second the name of a column in that table. We call this form a qualified column name. A single id is resolved to the name of a column in some of the tables referenced inside the FROM clause. However, in this case there must not be more than one tables that participate in the query and have a column with this name. We call this form an unqualified column name.