For example , in a three - table join , only one table can be used to bridge from one of the other tables to the third table , and none of the columns from the middle table have to be referenced in the select list 选择列表不必包含联接中每个表的列。例如,在三表联接中,只能用一个表作为中间表来联接另外两个表,而选择列表不必引用该中间表的任何列。