SQLiteDataBase db;

Cursor cursor = db.query(参数);

if(cursor.moveToFirst){
	do{
		val param = cursor.getString(cursor.getColumnIndex("param"));
	}while(cursor.moveToNext)
}
cursor.close()

首先需要通过query方法查找所需要的数据，query方法会返回一个Cursor对象，需要通过moveToFirst方法来判断是否数据为空，然后通过do…while循环遍历Cursor对象，其中需要用到cursor的getColumnIndex方法来得到具体参数的下标，再通过诸如getString、getInt等方法来通过下标获得数据，最后记得要关闭cursor

Cursor的原理：

//AbstrctCursor的源码
@Override
public final boolean moveToPosition(int position) {
    // Make sure position isn't past the end of the cursor
    final int count = getCount();
    if (position >= count) {
        mPos = count;
        return false;
    }

    // Make sure position isn't before the beginning of the cursor
    if (position < 0) {
        mPos = -1;
        return false;
    }

    // Check for no-op moves, and skip the rest of the work for them
    if (position == mPos) {
        return true;
    }

    boolean result = onMove(mPos, position);
    if (result == false) {
        mPos = -1;
    } else {
        mPos = position;
    }

    return result;
}
	@Override
    public final boolean moveToFirst() {
        return moveToPosition(0);
    }

    @Override
    public final boolean moveToLast() {
        return moveToPosition(getCount() - 1);
    }

    @Override
    public final boolean moveToNext() {
        return moveToPosition(mPos + 1);
    }

    @Override
    public final boolean moveToPrevious() {
        return moveToPosition(mPos - 1);
    }

显而易见，moveToFirst和moveToNext等方法的本质都是调用的moveToPosition方法

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cursor的意思就是光标，

一开始cursor所在的位置是-1，如果表中有数据，并且调用了moveToFirst或者moveToNext，cursor就会移动到第一个，也就是下标为0的位置，然后继续调用moveToNext的话，就实现了遍历数据，直到结束