ForeignKeyConstraint 外键约束的使用及作用的学习[转]

   da.SelectCommand.CommandText="select au_id,au_fname,au_lname from authors";
    da.Fill(ds,"Author");
    da.SelectCommand.CommandText="select au_id,title_id from titleauthor";
    da.Fill(ds,"TitleAuthor");
    da.SelectCommand.CommandText="select title_id,title from titles";
    da.Fill(ds,"Titles");
    //
    ds.Tables["Titles"].Columns["title_id"].Unique=true;
    ds.Tables["Titles"].Columns["title_id"].AllowDBNull=false;
    ds.Tables["Titles"].PrimaryKey=new DataColumn[]{ds.Tables["Titles"].Columns["title_id"]};
    //
    ds.Tables["Author"].Columns["au_id"].Unique=true;
    ds.Tables["Author"].Columns["au_id"].AllowDBNull=false;
    ds.Tables["Author"].PrimaryKey=new DataColumn[]{ds.Tables["Author"].Columns["au_id"]};
    //
    ds.Tables["TitleAuthor"].PrimaryKey=new DataColumn[]{ds.Tables["TitleAuthor"].Columns["au_id"],
     ds.Tables["TitleAuthor"].Columns["title_id"]};
    //定义约束
    ForeignKeyConstraint fk1=new ForeignKeyConstraint("authorstitleauthor",ds.Tables["Author"].Columns["au_id"],
     ds.Tables["TitleAuthor"].Columns["au_id"]);
    ds.Tables["TitleAuthor"].Constraints.Add(fk1);
    ForeignKeyConstraint fk2=new ForeignKeyConstraint(
     "titlestitleauthor",ds.Tables["Titles"].Columns["title_id"],
     ds.Tables["TitleAuthor"].Columns["title_id"]);
    ds.Tables["TitleAuthor"].Constraints.Add(fk2);
    this.dataGrid1.DataSource=ds;
    this.dataGrid1.DataMember="TitleAuthor";