引用同一表的外键

这种数据结构称为“自引用表”

SELECt DISTINCT mothers.*FROM person    inner join person mothers on person.mother_id = mothers.id

和

SELECt person.*FROM person    inner join person fathers on person.father_id = fathers.id    inner join person mothers on person.mother_id = mothers.idWHERe     fathers.name='john smith'and     mothers.name='jane'