如何在 SQLite 中使用 android 中的多个数据库助手类进行内部连接

Question

我的 java android 应用程序中有 2 个数据库表。

我有 2 个帮手，每张桌子一个。

public class SyncStudentsHelperextends SQLiteOpenHelper {

    private static final String TAG = "SyncStudentsHelper";

    private static final String TABLE_NAME = "students";
    private static final String COL1 = "field";
    private static final String COL2 = "field";
    private static final String COL3 = "field";
    private static final String COL4 = "field";
    private static final String COL5 = "field";

    public SyncStudentsHelper (Context context) {
        super(context, TABLE_NAME, null, 2);
    }
}    

我有一张类似的教师表。

我的问题是如何在 2 个表之间进行内部连接查询。

这是我的尝试。

public Cursor getinfo() {
    SQLiteDatabase db = this.getWritableDatabase();
    String query = "SELECT * FROM students Inner Join teachers ON student.matchingfield = teachers.matchingfield" ;
    Cursor data = db.rawQuery(query, null);
    return data;
}

我收到此错误：

no such table: teachers(code 1): , while compiling: 

我的问题是，因为我有 2 个不同的数据库帮助文件，如何从 2 个表进行内部 SQLite 内部联接。

编辑

我的查询是关于学生的助手 class，如下所示。

public class SyncStudentsHelperextends SQLiteOpenHelper {

    private static final String TAG = "SyncStudentsHelper";

    private static final String TABLE_NAME = "students";
    private static final String COL1 = "field";
    private static final String COL2 = "field";
    private static final String COL3 = "field";
    private static final String COL4 = "field";
    private static final String COL5 = "field";

    public SyncStudentsHelper (Context context) {
        super(context, TABLE_NAME, null, 2);
    }
}  

//Return Cursor type
public Cursor getinfo() {
    SQLiteDatabase db = this.getWritableDatabase();
    String  attached = "attached_teacher";
    db.execSQL("ATTACH DATABASE  SyncTeachers.teachers AS teachersdatabase ");
     String query = "SELECT * FROM students Inner Join teachersdatabase.teachers ON students.matchingfield = teachersdatabase.teachers.matchingfield " ;
    Cursor data = db.rawQuery(query, null);
    return data;
}

Answer 1

您需要在不打开附加数据库的情况下将一个数据库附加到另一个数据库，或者拥有一个包含两个表的数据库。

请注意，您通过给定的模式引用了附加的数据库，例如，如果使用ATTACH path_to_2nd_db AS seconddb然后您将附加数据库中的表称为seconddb.the_table_name

但是，您有两个数据库的原因似乎是在单独的帮助程序中您使用表名作为数据库名（基于您的 SyncStudentsHelper 类（缺少类名和 extends 关键字之间的空格））。

我建议你有一个 DatabaseHelper，它在同一个数据库中包含两个表。

单个数据库示例

数据库助手：-

public class DatabaseHelper extends SQLiteOpenHelper {

    public static final String DBNAME = "Students_and_Teachers";
    public static final int DBVERSION = 1;

    public static final String TEACHERS_TABLE_NAME = "teachers";
    public static final String STUDENTS_TABLE_NAME = "students";
    public static final String COL1 = "matchingfield";
    public static final String COL2 = "field1";
    public static final String COL3 = "field2";
    public static final String COL4 = "field3";
    public static final String COL5 = "field4";

    public DatabaseHelper(Context context) {
        super(context, DBNAME, null, DBVERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String create_teachers_table_sql =
                "CREATE TABLE IF NOT EXISTS " + TEACHERS_TABLE_NAME +
                        "(" +
                        COL1 + " INTEGER," +
                        COL2 + " TEXT," +
                        COL3 + " TEXT," +
                        COL4 + " TEXT," +
                        COL5 + " TEXT" +
                        ")"
                ;
        String create_students_table_sql =
                "CREATE TABLE IF NOT EXISTS " + STUDENTS_TABLE_NAME +
                        "(" +
                        COL1 + " INTEGER," +
                        COL2 + " TEXT," +
                        COL3 + " TEXT," +
                        COL4 + " TEXT," +
                        COL5 + " TEXT" +
                        ")"
                ;
        db.execSQL(create_teachers_table_sql);
        db.execSQL(create_students_table_sql);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }

    public long insertStudent(long teacherToMatch, String studentName) {
        ContentValues cv = new ContentValues();
        cv.put(COL1,teacherToMatch);
        cv.put(COL2,studentName);
        return this.getWritableDatabase().insert(STUDENTS_TABLE_NAME,null,cv);
    }

    public long insertTeacher(long teacherToBeMatched, String teacherName) {
        ContentValues cv = new ContentValues();
        cv.put(COL1,teacherToBeMatched);
        cv.put(COL2,teacherName);
        return this.getWritableDatabase().insert(TEACHERS_TABLE_NAME,null,cv);
    }

    public Cursor getInfo() {
        //String query = "SELECT * FROM students Inner Join teachers ON student.matchingfield = teachers.matchingfield" ;
        //Cursor data = db.rawQuery(query, null);
        return this.getWritableDatabase().query(
                STUDENTS_TABLE_NAME + " INNER JOIN " + TEACHERS_TABLE_NAME +
                        " ON " + STUDENTS_TABLE_NAME + "." + COL1 + " = " +
                        TEACHERS_TABLE_NAME + "." + COL1,
                null,null,null,null,null,null
        );
    }
}

使用数据库的活动，添加一些教师 (2) 和一些学生（3 匹配第一个老师，2 匹配第二个老师），然后通过查询提取它们。

public class MainActivity extends AppCompatActivity {
    DatabaseHelper mDB;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mDB = new DatabaseHelper(this);

        mDB.insertTeacher(1,"Mr Jones");
        mDB.insertTeacher(2,"Mrs Smith");

        mDB.insertStudent(1,"Tom Smith");
        mDB.insertStudent(1,"Glenda Burrows");
        mDB.insertStudent(1,"Gloria Jennings");
        mDB.insertStudent(2,"Fred Sharp");
        mDB.insertStudent(2,"Mary Collins");

        Cursor csr = mDB.getInfo();

        int row = 1;
        while (csr.moveToNext()) {
            StringBuilder sb = new StringBuilder();
            sb.append("Processing row ").append(row++);
            for (int i=0; i < csr.getColumnCount(); i++) {
                sb.append("\n\t").append(csr.getColumnName(i)).append(" = " ).append(csr.getString(i));
            }
            Log.d("CURSORINFO",sb.toString());
        }
    }
}

运行上述结果：-

2019-12-11 02:14:15.169 D/CURSORINFO: Processing row 1
        matchingfield = 1
        field1 = Tom Smith
        field2 = null
        field3 = null
        field4 = null
        matchingfield = 1
        field1 = Mr Jones
        field2 = null
        field3 = null
        field4 = null
2019-12-11 02:14:15.169 D/CURSORINFO: Processing row 2
        matchingfield = 1
        field1 = Glenda Burrows
        field2 = null
        field3 = null
        field4 = null
        matchingfield = 1
        field1 = Mr Jones
        field2 = null
        field3 = null
        field4 = null
2019-12-11 02:14:15.169 D/CURSORINFO: Processing row 3
        matchingfield = 1
        field1 = Gloria Jennings
        field2 = null
        field3 = null
        field4 = null
        matchingfield = 1
        field1 = Mr Jones
        field2 = null
        field3 = null
        field4 = null
2019-12-11 02:14:15.170 D/CURSORINFO: Processing row 4
        matchingfield = 2
        field1 = Fred Sharp
        field2 = null
        field3 = null
        field4 = null
        matchingfield = 2
        field1 = Mrs Smith
        field2 = null
        field3 = null
        field4 = null
2019-12-11 02:14:15.170 D/CURSORINFO: Processing row 5
        matchingfield = 2
        field1 = Mary Collins
        field2 = null
        field3 = null
        field4 = null
        matchingfield = 2
        field1 = Mrs Smith
        field2 = null
        field3 = null
        field4 = null

多数据库示例

SyncStudentsHelper类

public class SyncStudentsHelper extends SQLiteOpenHelper {

    public static final String TABLE_NAME = "students";
    public static final String COL1 = "matchingfield";
    public static final String COL2 = "field1";
    public static final String COL3 = "field2";
    public static final String COL4 = "field3";
    public static final String COL5 = "field4";

    public SyncStudentsHelper(Context context) {
        super(context, TABLE_NAME, null, 2);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String create_students_table_sql =
                "CREATE TABLE IF NOT EXISTS " + TABLE_NAME +
                        "(" +
                        COL1 + " INTEGER," +
                        COL2 + " TEXT," +
                        COL3 + " TEXT," +
                        COL4 + " TEXT," +
                        COL5 + " TEXT" +
                        ")"
                ;
        db.execSQL(create_students_table_sql);

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }

    public long insertStudent(long teacherToMatch, String studentName) {
        ContentValues cv = new ContentValues();
        cv.put(COL1,teacherToMatch);
        cv.put(COL2,studentName);
        return this.getWritableDatabase().insert(TABLE_NAME,null,cv);
    }
}

SyncTeachersHelper类：-

public class SyncTeachersHelper extends SQLiteOpenHelper {

    public static final String TABLE_NAME = "teachers";
    public static final String COL1 = "matchingfield";
    public static final String COL2 = "field1";
    public static final String COL3 = "field2";
    public static final String COL4 = "field3";
    public static final String COL5 = "field4";

    public SyncTeachersHelper(@Nullable Context context) {
        super(context, TABLE_NAME, null, 2);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String create_teachers_table_sql =
                "CREATE TABLE IF NOT EXISTS " + TABLE_NAME +
                        "(" +
                        COL1 + " INTEGER," +
                        COL2 + " TEXT," +
                        COL3 + " TEXT," +
                        COL4 + " TEXT," +
                        COL5 + " TEXT" +
                        ")"
                ;
        db.execSQL(create_teachers_table_sql);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }

    public long insertTeacher(long teacherToBeMatched, String teacherName) {
        ContentValues cv = new ContentValues();
        cv.put(COL1,teacherToBeMatched);
        cv.put(COL2,teacherName);
        return this.getWritableDatabase().insert(TABLE_NAME,null,cv);
    }
}

活动代码（对于单个和多个）：-

public class MainActivity extends AppCompatActivity {
    DatabaseHelper mDB;
    SyncTeachersHelper mTDB;
    SyncStudentsHelper mSDB;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mDB = new DatabaseHelper(this);

        mDB.insertTeacher(1,"Mr Jones");
        mDB.insertTeacher(2,"Mrs Smith");

        mDB.insertStudent(1,"Tom Smith");
        mDB.insertStudent(1,"Glenda Burrows");
        mDB.insertStudent(1,"Gloria Jennings");
        mDB.insertStudent(2,"Fred Sharp");
        mDB.insertStudent(2,"Mary Collins");

        Cursor csr = mDB.getInfo();

        int row = 1;
        while (csr.moveToNext()) {
            StringBuilder sb = new StringBuilder();
            sb.append("Processing row ").append(row++);
            for (int i=0; i < csr.getColumnCount(); i++) {
                sb.append("\n\t").append(csr.getColumnName(i)).append(" = " ).append(csr.getString(i));
            }
            Log.d("CURSORINFO",sb.toString());
        }

        Log.d("STAGE2","Now using dual databases");
        // USING Seperate Databases
        mTDB = new SyncTeachersHelper(this);
        mTDB.insertTeacher(1,"Mr Jones");
        mTDB.insertTeacher(2,"Mrs Smith");
        mTDB.close();

        mSDB = new SyncStudentsHelper(this);
        mSDB.insertStudent(1,"Tom Smith");
        mSDB.insertStudent(1,"Glenda Burrows");
        mSDB.insertStudent(1,"Gloria Jennings");
        mSDB.insertStudent(2,"Fred Sharp");
        mSDB.insertStudent(2,"Mary Collins");

        SQLiteDatabase studentsdb = mSDB.getWritableDatabase();
        String  attached = "attached_teacher";
        studentsdb.execSQL("ATTACH DATABASE '" + this.getDatabasePath(SyncTeachersHelper.TABLE_NAME) +"' AS " + attached);

        String fromclause = SyncStudentsHelper.TABLE_NAME + " INNER JOIN " + attached + "." + SyncTeachersHelper.TABLE_NAME +
                " ON " + SyncStudentsHelper.TABLE_NAME + "." + SyncStudentsHelper.COL1 + " = " +
                SyncTeachersHelper.TABLE_NAME + "." + SyncTeachersHelper.COL1;
        Log.d("STAGE2"," From clause for query is \n\t" + fromclause);
        csr = studentsdb.query(
        SyncStudentsHelper.TABLE_NAME + " INNER JOIN " + attached + "." + SyncTeachersHelper.TABLE_NAME +
                " ON " + SyncStudentsHelper.TABLE_NAME + "." + SyncStudentsHelper.COL1 + " = " +
                SyncTeachersHelper.TABLE_NAME + "." + SyncTeachersHelper.COL1,
                null,null,null,null,null,null
        );

        row = 1;
        while (csr.moveToNext()) {
            StringBuilder sb = new StringBuilder();
            sb.append("Processing row ").append(row++);
            for (int i=0; i < csr.getColumnCount(); i++) {
                sb.append("\n\t").append(csr.getColumnName(i)).append(" = " ).append(csr.getString(i));
            }
            Log.d("CURSORINFO",sb.toString());
        }
    }
}

请注意，查询的 FROM 子句（包括 JOIN）是：-

students INNER JOIN attached_teacher.teachers ON students.matchingfield = teachers.matchingfield

结果（对于 STAGE2）是：-

2019-12-11 02:43:59.699 D/STAGE2: Now using dual databases
2019-12-11 02:43:59.773 D/STAGE2:  From clause for query is 
        students INNER JOIN attached_teacher.teachers ON students.matchingfield = teachers.matchingfield
2019-12-11 02:43:59.774 D/CURSORINFO: Processing row 1
        matchingfield = 1
        field1 = Tom Smith
        field2 = null
        field3 = null
        field4 = null
        matchingfield = 1
        field1 = Mr Jones
        field2 = null
        field3 = null
        field4 = null
2019-12-11 02:43:59.774 D/CURSORINFO: Processing row 2
        matchingfield = 1
        field1 = Glenda Burrows
        field2 = null
        field3 = null
        field4 = null
        matchingfield = 1
        field1 = Mr Jones
        field2 = null
        field3 = null
        field4 = null
2019-12-11 02:43:59.774 D/CURSORINFO: Processing row 3
        matchingfield = 1
        field1 = Gloria Jennings
        field2 = null
        field3 = null
        field4 = null
        matchingfield = 1
        field1 = Mr Jones
        field2 = null
        field3 = null
        field4 = null
2019-12-11 02:43:59.775 D/CURSORINFO: Processing row 4
        matchingfield = 2
        field1 = Fred Sharp
        field2 = null
        field3 = null
        field4 = null
        matchingfield = 2
        field1 = Mrs Smith
        field2 = null
        field3 = null
        field4 = null
2019-12-11 02:43:59.775 D/CURSORINFO: Processing row 5
        matchingfield = 2
        field1 = Mary Collins
        field2 = null
        field3 = null
        field4 = null
        matchingfield = 2
        field1 = Mrs Smith
        field2 = null
        field3 = null
        field4 = null

Answer 2

只需将 SQL 命令应用于 SQLite：

ATTACH DATABASE path_file_name AS my_database_name;

然后稍后应用您的 SQL 命令

select * from mytable
    left join my_database_name.my_other_table
    on field_name = database_name.my_other_table.myfield_name

https://www.sqlite.org/lang_attach.html