Android[高级教程] Android数据库SQLite表内设置外键

本文出自:http://blog.csdn.net/kangkangz4/article/details/7003659

安卓默认的数据是SQLite，但SQLite3.6.19之前是不支持外键的，如果有两张表需要关联，用外键是最省事的，但不支持的话怎么办呢？这里就有一个解决办法，就是用事务将两张表关联起来，并且最后生成一张视图。

现有两张表

1. Employees
2. Dept

视图

1. ViewEmps:显示雇员信息和他所在的部门

创建数据库

自定义一个辅助类继承SQLiteOpenHelper类

1. onCreate(SQLiteDatabase db): 当数据库被创建的时候，能够生成表，并创建视图跟触发器。

2. onUpgrade(SQLiteDatabse db, int oldVersion, int newVersion): 更新的时候可以删除表和创建新的表。

代码如下：

public class DatabaseHelper extends SQLiteOpenHelper {

static final String dbName="demoDB";
static final String employeeTable="Employees";
static final String colID="EmployeeID";
static final String colName="EmployeeName";
static final String colAge="Age";
static final String colDept="Dept";

static final String deptTable="Dept";
static final String colDeptID="DeptID";
static final String colDeptName="DeptName";

static final String viewEmps="ViewEmps";

构造器

public DatabaseHelper(Context context) {
  super(context, dbName, null,33);
  }

创建库中的表，视图和触发器

public void onCreate(SQLiteDatabase db) {
  // TODO Auto-generated method stub

  db.execSQL("CREATE TABLE "+deptTable+" ("+colDeptID+ " INTEGER PRIMARY KEY , "+
    colDeptName+ " TEXT)");

  db.execSQL("CREATE TABLE "+employeeTable+"
    ("+colID+" INTEGER PRIMARY KEY AUTOINCREMENT, "+
        colName+" TEXT, "+colAge+" Integer, "+colDept+"
    INTEGER NOT NULL ,FOREIGN KEY ("+colDept+") REFERENCES
    "+deptTable+" ("+colDeptID+"));");

  //创建触发器
  db.execSQL("CREATE TRIGGER fk_empdept_deptid " +
    " BEFORE INSERT "+
    " ON "+employeeTable+

    " FOR EACH ROW BEGIN"+
    " SELECT CASE WHEN ((SELECT "+colDeptID+" FROM "+deptTable+"
    WHERE "+colDeptID+"=new."+colDept+" ) IS NULL)"+
    " THEN RAISE (ABORT,'Foreign Key Violation') END;"+
    "  END;");

 //创建视图
  db.execSQL("CREATE VIEW "+viewEmps+
    " AS SELECT "+employeeTable+"."+colID+" AS _id,"+
    " "+employeeTable+"."+colName+","+
    " "+employeeTable+"."+colAge+","+
    " "+deptTable+"."+colDeptName+""+
    " FROM "+employeeTable+" JOIN "+deptTable+
    " ON "+employeeTable+"."+colDept+" ="+deptTable+"."+colDeptID
    );
 }

更新库中的表

public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
  // TODO Auto-generated method stub

  db.execSQL("DROP TABLE IF EXISTS "+employeeTable);
  db.execSQL("DROP TABLE IF EXISTS "+deptTable);

  db.execSQL("DROP TRIGGER IF EXISTS fk_empdept_deptid");
  db.execSQL("DROP VIEW IF EXISTS "+viewEmps);
  onCreate(db);
 }

加入数据

SQLiteDatabase db=this.getWritableDatabase();
 ContentValues cv=new ContentValues();
   cv.put(colDeptID, 1);
   cv.put(colDeptName, "Sales");
   db.insert(deptTable, colDeptID, cv);

   cv.put(colDeptID, 2);
   cv.put(colDeptName, "IT");
   db.insert(deptTable, colDeptID, cv);
                     db.close();

更新数据

public int UpdateEmp(Employee emp)
  {
   SQLiteDatabase db=this.getWritableDatabase();
   ContentValues cv=new ContentValues();
   cv.put(colName, emp.getName());
   cv.put(colAge, emp.getAge());
   cv.put(colDept, emp.getDept());
   return db.update(employeeTable, cv, colID+"=?",
    new String []{String.valueOf(emp.getID())});
  }

删除数据

public void DeleteEmp(Employee emp)
  {
   SQLiteDatabase db=this.getWritableDatabase();
   db.delete(employeeTable,colID+"=?", new String [] {String.valueOf(emp.getID())});
   db.close();
  }

取得所有部门信息

Cursor getAllDepts()
  {
   SQLiteDatabase db=this.getReadableDatabase();
   Cursor cur=db.rawQuery("SELECT "+colDeptID+" as _id,
    "+colDeptName+" from "+deptTable,new String [] {});

   return cur;
  }

取得部门内雇员信息

public Cursor getEmpByDept(String Dept)
  {
   SQLiteDatabase db=this.getReadableDatabase();
   String [] columns=new String[]{"_id",colName,colAge,colDeptName};
   Cursor c=db.query(viewEmps, columns, colDeptName+"=?",
    new String[]{Dept}, null, null, null);
   return c;
  }

取得部门ID

public int GetDeptID(String Dept)
  {
   SQLiteDatabase db=this.getReadableDatabase();
   Cursor c=db.query(deptTable, new String[]{colDeptID+" as _id",colDeptName},
    colDeptName+"=?", new String[]{Dept}, null, null, null);
   //Cursor c=db.rawQuery("SELECT "+colDeptID+" as _id FROM "+deptTable+"
   //WHERE "+colDeptName+"=?", new String []{Dept});
   c.moveToFirst();
   return c.getInt(c.getColumnIndex("_id"));
  }

上面部门和雇员信息的表因为实现的关联，所以更新和删除都会对对应的信息更新。

原网页： http://www.codeproject.com/KB/android/AndroidSQLite.aspx#