MySQL, 5th Edition

2.8.2. Qualifying References to Columns from Joined Tables

References to each table column throughout a SELECT statement must resolve unambiguously to a single table named in the FROM clause. If only one table is named, there is no ambiguity; all columns must be columns of that table. If multiple tables are named, any column name that appears in only one table is similarly unambiguous. However, if a column name appears in multiple tables, references to the column must be qualified with a table identifier using tbl_name.col_name syntax to specify which table you mean. Suppose that a table mytbl1 contains columns a and b, and a table mytbl2 contains columns b and c. References to columns a or c are unambiguous, but references to b must be qualified ...