Android[高级教程] Android数据库SQLite表内设置外键
介绍：
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
Employees
Dept
视图
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
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public DatabaseHelper(Context context) {
super(context, dbName, null,33);
}
创建库中的表，视图和触发器
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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
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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
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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
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public int GetDeptID(String Dept)
{
SQLiteDatabase db=this.getReadableDatabase();
Cursor c=db.query(deptTable, new String[]{colDeptID+" as _id",colDeptName},
&n
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