interface Instruction {
   
   public String getId();
   
   public String getDescription();
   public void setDescription(String description);
   
   public Collection getUnits();
   public void addUnit(Unit unit);
   public void removeUnit(Unit unit);
   
   ...
}

interface Unit {
   
   public String getId();
   
   ...
}

class InstructionImpl implements Instruction {
   
   private String id;
   private String description;
   
   public InstructionImpl() {}
   
   public InstructionImpl(String description) {
      this.description = description;
   }
   
   public String getId() {
      return id;
   }
   
   ...
}

class UnitImpl implements Unit {
   ...
}

create table INSTRUCTION (
   id VARCHAR(32) NOT NULL PRIMARY KEY,
   description VARCHAR(255) NOT NULL
)

create table UNIT (
   id VARCHAR(32) NOT NULL PRIMARY KEY,
   description VARCHAR(255) NOT NULL
)