(3)altertableteacheraddteasexchar(2)nullconstraintteasex_ckcheck(teasexin('男','女'))(4)altertablecla_teaaddconstraintclaid_fkforiegnkey(claid)referencesclass(claid)altertablecla_teaaddconstraintteaid_fkforiegnkey(teaid)referencesteacher(teaid)