Du ska aldrig behöva uppdatera flera tabeller samtidigt åtminstone för en enda ändring, t.ex. elevens namn har ändrats (elevtabellen skulle ändras) eller säg ett betyg ändrat (ändra elevens respektive betygsinmatning).
Så jag tror att det som skulle passa bäst är metoder för att uppdatera en elevs uppgifter (för- eller efternamn eller båda) och en metod för att ändra betygen (igen endera eller båda).
När det gäller radering kan du göra detta sekventiellt ta bort från betygstabellen och från elevtabellen eller vice versa.
Som sådan tror jag att följande kod inkluderar metoderna deleteStudentInfo , changeStudentName , changeStudentFirstName , changeStudentLastName och changeStudentGrade och inkluderar även exempelanvändning tillsammans med att skapa och fylla i tabellerna (du kanske vill överväga det reviderade schemat):-
public class MainActivity extends AppCompatActivity {
public static final String DBNAME = "study";
public static final String STUDENT_TABLE_NAME = "STUDENT_TABLE";
public static final String COL_STUDENT_ID = "studentid";
public static final String COL_STUDENT_FIRSTNAME = "fname";
public static final String COL_STUDENT_LASTNAME = "lname";
public static final String CLASS_TABLE_NAME = "CLASS_TABLE";
public static final String COL_CLASS_ID = "classid";
public static final String COL_CLASS_NAME = "classname";
public static final String GRADE_TABLE_NAME = "GRADE_TABLE";
public static final String COL_GRADE_POINTGRADE = "pointgrade";
public static final String COL_GRADE_LETTERGRADE = "lettergrade";
public static final String BY_STUDENTID = COL_STUDENT_ID + "=?";
public static final String BY_CLASSID = COL_CLASS_ID + "=?";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
SQLiteDatabase db = openOrCreateDatabase(DBNAME,Context.MODE_PRIVATE,null);
db.execSQL("CREATE TABLE IF NOT EXISTS " + STUDENT_TABLE_NAME + " (" +
COL_STUDENT_ID + " TEXT PRIMARY KEY, " +
COL_STUDENT_FIRSTNAME + " TEXT," +
COL_STUDENT_LASTNAME + " TEXT)"
);
db.execSQL("CREATE TABLE IF NOT EXISTS " + CLASS_TABLE_NAME + "(" +
COL_CLASS_ID + " TEXT PRIMARY KEY," +
COL_CLASS_NAME + " TEXT UNIQUE " +
")"
);
db.execSQL("CREATE TABLE IF NOT EXISTS " + GRADE_TABLE_NAME + "(" +
COL_STUDENT_ID + " TEXT, " +
COL_CLASS_ID + " TEXT, " +
COL_GRADE_POINTGRADE + " INTEGER, " +
COL_GRADE_LETTERGRADE + " TEXT" +
")"
);
db.execSQL("INSERT OR IGNORE INTO " + STUDENT_TABLE_NAME +
" VALUES" +
"('00001','Fred','Smith')," +
"('00010','Mary','Thomas')," +
"('00910','Angela','Jones')"
);
db.execSQL("INSERT OR IGNORE INTO " + CLASS_TABLE_NAME +
" VALUES" +
"('001','English')," +
"('101','Mathematics')," +
"('201','Chemistry')"
);
db.execSQL("INSERT OR IGNORE INTO " + GRADE_TABLE_NAME +
" VALUES" +
" ('00001','001',99,'A'), -- Fred Smith has 99 point grade as an A in English\n" +
" ('00001','101',25,'F'), -- Fred Smith has 25 point grade as an F on Mathematics\n" +
" ('00010','201',76,'B'), -- Angela Jones 76 a B in Chemistry\n" +
" ('00910','101',50,'C'), \n" +
" ('00910','201',63,'C'),\n" +
" ('00910','001',89,'A')\n" +
";"
);
changeStudentName(db,"00001","Joe","Bloggs");
changeStudentFirstName(db,"00001","Harry");
changeStudentLastName(db,"00001","Hoffmann");
// e.g. won't change due to -1 (skip pointsgrade) and null (skip lettergrade)
changeStudentGrade(db,"00001","001",-1,null);
// Change both
changeStudentGrade(db,"00001","001",25,"D");
changeStudentGrade(db,"00001","001",27,null);
// Ooops invalid student id
if (deleteStudentInfo(db,"001")) {
Log.d("DELETION","Student 001 deleted.");
} else {
Log.d("DELETION","Ooops Student 001 not deleted?????");
}
// Corrected Student ID
if (deleteStudentInfo(db,"00001")) {
Log.d("DELETION","Student 001 deleted.");
} else {
Log.d("DELETION","Ooops Student 001 not deleted?????");
}
}
private boolean deleteStudentInfo(SQLiteDatabase db, String studentid) {
String tag = "STUDENT_DELETE";
String student_table = "STUDENT_TABLE";
String grade_table = "GRADE_TABLE";
long pre_delete_student_count = DatabaseUtils.queryNumEntries(db,student_table);
long pre_delete_grade_count = DatabaseUtils.queryNumEntries(db,grade_table);
String whereclause = "studentid =?";
String[] whereargs = {studentid};
db.delete(student_table,whereclause,whereargs);
db.delete(grade_table,whereclause,whereargs);
long post_delete_student_count = DatabaseUtils.queryNumEntries(db,student_table);
long post_delete_grade_count = DatabaseUtils.queryNumEntries(db,grade_table);
Log.d(
tag,
"Number of Students deleted from " +
student_table + " is " +
String.valueOf(
pre_delete_student_count - post_delete_student_count
));
Log.d(
tag,
"Number of Grades deleted from " + grade_table + " is " +
String.valueOf(
pre_delete_grade_count - post_delete_grade_count
)
);
if ((pre_delete_student_count + pre_delete_grade_count) != (post_delete_student_count + post_delete_grade_count)) {
return true;
}
return false;
}
/**
* Flexible Student Name Change
*
* @param db The SQliteDatabase
* @param studentid The studentid (String)
* @param newfirstname The new firstname, null or blank to leave as is
* @param newlastname the new lastname, null or blank to leave as is
*/
private void changeStudentName(SQLiteDatabase db, String studentid, String newfirstname, String newlastname ) {
//Anything to do? if not do nothing
if ((newfirstname == null || newfirstname.length() < 1) && (newlastname == null || newlastname.length() < 1)) {
return;
}
ContentValues cv = new ContentValues();
if (newfirstname != null && newfirstname.length() > 0) {
cv.put(COL_STUDENT_FIRSTNAME,newfirstname);
}
if (newlastname != null && newlastname.length() > 0) {
cv.put(COL_STUDENT_LASTNAME,newlastname);
}
// Overcautious check
if (cv.size() < 1) {
return;
}
db.update(STUDENT_TABLE_NAME,cv,BY_STUDENTID,new String[]{studentid});
}
/**
* Change a Student's First Name (invokes changeStudentName method)
* @param db The SQLiteDatabase
* @param studentid The student's id (String)
* @param newfirstname The new first name to apply
*/
private void changeStudentFirstName(SQLiteDatabase db, String studentid, String newfirstname) {
changeStudentName(db,studentid,newfirstname,null);
}
/**
* Change a Student's Last Name (invokes changeStudentName method)
* @param db
* @param studentid
* @param newlastname
*/
private void changeStudentLastName(SQLiteDatabase db, String studentid, String newlastname) {
changeStudentName(db,studentid,null,newlastname);
}
/**
* Change a students grade (allowing just one (points/letter))
* @param db
* @param studentid
* @param classid
* @param newpointsgrade
* @param newlettergrade
*/
private void changeStudentGrade(SQLiteDatabase db, String studentid, String classid, int newpointsgrade, String newlettergrade) {
// Anything to do? if not do nothing
if (newpointsgrade < 0 && (newlettergrade == null || newlettergrade.length() < 1)) {
return;
}
ContentValues cv = new ContentValues();
if (newpointsgrade >= 0) {
cv.put(COL_GRADE_POINTGRADE,newpointsgrade);
}
if (newlettergrade != null && newlettergrade.length() > 0) {
cv.put(COL_GRADE_LETTERGRADE,newlettergrade);
}
String whereclause = COL_STUDENT_ID + "=? AND " + COL_CLASS_ID + "=?";
String[] whereargs = new String[]{studentid,classid};
db.update(GRADE_TABLE_NAME,cv,whereclause,whereargs);
}
}