检索单个列或从sqlite的

Question

public class DatabaseHelper extends SQLiteOpenHelper { 
    public static final String DATABASE_NAME = "Student.db"; 
    public static final String TABLE_NAME = "student_table"; 
    public static final String COL_1 = "ID"; 
    public static final String COL_2 = "NAME"; 
    public static final String COL_3 = "SURNAME"; 
    public static final String COL_4 = "MARKS"; 
    public static final String COL_5 = "DATETIMEZ"; 

public Cursor NAME_MARKS() { 
     SQLiteDatabase db = this.getWritableDatabase(); 
Cursor res = db.rawQuery("select * from " + TABLE_NAME, null); 

在Android Studio中两列//不起作用

Cursor res = db.rawQuery("select COL_2,COL_3 from " + TABLE_NAME, null);{} 

Answer 1

col_2和col_3是变量，而不是在数据库中列的名称。如果你想使用它们，你需要使用某种字符串操作/连接。例如：

Cursor res = 
    db.rawQuery("select " + COL_2 + ", " + COL_3 + " from " + TABLE_NAME, null);