Android數據庫SQLite表內設置外鍵
介紹
Android默認的數據是SQLite，但SQLite3.6.19之前(在2.2版本中使用的是3.6.22，因此如果你的應用只兼容到2.2版本就可以放心使用外鍵功能)是不支持外鍵的，如果有兩張表需要關聯，用外鍵是最省事的，但不支持的話怎麼辦呢？這裏就有一個解決辦法，就是用事務將兩張表關聯起來，並且最後生成一張視圖。
現有兩張表
- Employees
- Dept
視圖
ViewEmps:顯示僱員信息和他所在的部門
創建數據庫
自定義一個輔助類繼承SQLiteOpenHelper類.
- onCreate(SQLiteDatabase db): 當數據庫被創建的時候，能夠生成表，並創建視圖跟觸發器。
- onUpgrade(SQLiteDatabse db, int oldVersion, int newVersion): 更新的時候可以刪除表和創建新的表。
代碼如下：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 | 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"; public DatabaseHelper(Context context) { super(context, dbName, null,33); } // 創建庫中的表，視圖和觸發器 public void onCreate(SQLiteDatabase db) { 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+"));"); //創建觸發器 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;"); //創建視圖 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 ); } // 更新庫中的表 public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 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); } } |
加入數據
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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();
|
更新數據
1 2 3 4 5 6 7 8 9 10 11 | 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())}); } |
刪除數據
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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();
}
|
取得所有部門信息
1 2 3 4 5 6 7 8 9 | Cursor getAllDepts() { SQLiteDatabase db=this.getReadableDatabase(); Cursor cur=db.rawQuery("SELECT "+colDeptID+" as _id, "+colDeptName+" from "+deptTable,new String [] {}); return cur; } |
取得部門內僱員信息
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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;
}
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取得部門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},
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"));
}
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上面部門和僱員信息的表因爲實現的關聯，所以更新和刪除都會對對應的信息更新。
原文鏈接 。