Qualified column names
When a query involves multiple tables, it is sometimes difficult
to know which column belongs to which table, especially if the tables
have the same column names. To help distinguish among column names,
you can use SQL to fully qualify column names by specifying the column
as database.schema.table.column.
When you are referring to column names in cross-database access,
the system expects the qualified column name to be in the form of exposed-table-reference.column-name;
where the exposed-table-reference is any of the acceptable
table references in the FROM clause.
For example,
emp, admin.emp, dev.admin.emp,
and dev..emp are all equivalent forms for the same
table:FROM emp WHERE dev.admin.emp.id = 10;
FROM dev.admin.emp WHERE emp.id = 10;
FROM emp WHERE admin.emp.id = 10;
FROM emp WHERE dev.admin.emp.id = 10;
FROM dev..emp WHERE admin.id = 10;