create table BaseUnit ( BaseUnitID integer not null primary key ... ) ;

create table DataType ( 
  DataTypeID integer not null primary key,
  BaseUnitID integer not null references BaseUnit ( BaseUnitID )) ;

alter table DataType add constraint DataType_AK unique ( DataTypeID, BaseUnitID ) ;

create table Unit ( 
  UnitID integer not null primary key,
  BaseUnitID integer not null references BaseUnit ( BaseUnitID )) ;

alter table Unit add constraint Units_AK unique ( UnitID, BaseUnitID ) ;

create table MainData ( 
  ....
  DataTypeID integer not null,
  UnitID integer not null, 
  BaseUnitID integer not null
  .... ) ;

alter table MainData add constraint MainData_Unit_FK
  foreign key ( UnitID, BaseUnitID ) references Unit ( UnitID, BaseUnitID ) ;

alter table MainData add constraint MainData_DataType_FK
  foreign key ( DataTypeID, BaseUnitID ) references DataType ( DataTypeID, BaseUnitID ) ;