前言:
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。
example代码:
两张表
Employees
Dept
关系图:
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。onUpgrade(SQLiteDatabse db, int oldVersion, int newVersion)
: 更新的时候可以删除表和创建新的表。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";
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 ); }
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())});
}
{
SQLiteDatabase db=this.getWritableDatabase();
db.delete(employeeTable,colID+"=?", new String [] {String.valueOf(emp.getID())});
db.close();
}
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。