Even table
select * from user u join ugroup g on g.id=u.id where u.id=#{id}
Condition 1: connected to the associated field coincides table. The second condition: the routing table have the same sub-field. Under two conditions are met, the binding table to take effect.
If the condition 2, but the condition 1. It will produce a Cartesian product.
If the condition 1, sub-table will be invalid .
When the table is not a sub-table, a table of situations watched table:
select u.* from t_test g join user u on g.id=u.id where u.id=#{id}
t_test is no part table, u is watched table, the results failed sub-table . You must take the band on the front part table.