如果不存在则创建表

Question

创建新表时遇到一点问题。当我使用 CREATE TABLE 命令时，我的新表按原样形成，但是当我退出 Activity 时，应用程序崩溃并且我在 logcat 中得到一个 TABLE ALREADY EXISTS。如果我使用 CREATE TABLE IF NOT EXISTS，则不会形成新表，而只是将我的新数据行添加到以前的表中，它不会崩溃。这有什么问题？我想添加表而不给我已经存在的表，我希望它添加而不将行添加到不同的表。

SqliteHelper 类:

public class MySQLiteHelper extends SQLiteOpenHelper {


public static final String TABLE_NAME = MainActivity.NameName;

public static final String COLUMN_ID = "_id";
public static final String COLUMN_COMMENT = "comment";
public static final String COLUMN_LAT = "lat";
public static final String COLUMN_LONG = "long";
public static final String COLUMN_RADI = "radi";
private static final String DATABASE_NAME = "spraylogs.db";
private static final int DATABASE_VERSION = 1;



public static final String NEWTABLE = "CREATE TABLE  "
         + TABLE_COMMENTS + "(" + COLUMN_ID
          + " integer primary key autoincrement, " + COLUMN_COMMENT
          + " text not null," + COLUMN_LAT+ ","  + COLUMN_LONG + "," + COLUMN_RADI +  ");";

public static final String SaveIt = "CREATE TABLE IF NOT EXISTS "
         + TABLE_COMMENTS + "(" + COLUMN_ID
          + " integer primary key autoincrement, " + COLUMN_COMMENT
          + " text not null," + COLUMN_LAT+ ","  + COLUMN_LONG + "," + COLUMN_RADI +  ");";



 MySQLiteHelper(Context context) {
  super(context, context.getExternalFilesDir(null).getAbsolutePath() + "/" + DATABASE_NAME, null, DATABASE_VERSION);
}


@Override
public void onCreate(SQLiteDatabase database) {
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
Log.w(MySQLiteHelper.class.getName(),
    "Upgrading database from version " + oldVersion + " to "
        + newVersion + ", which will destroy all old data");
db.execSQL("DROP TABLE IF EXISTS " + NEWTABLE);

onCreate(db);
 } 

} 

Answer 1

它应该是这样工作的。如果表已经存在，CREATE TABLE 将抛出异常。如果表不存在，CREATE TABLE IF NOT EXISTS 将创建表，如果存在则忽略该命令。如果您希望它删除旧表，请在 CREATE TABLE 之前使用 DELETE TABLE IF EXISTS。如果要更改架构，请使用 ALTER TABLE，而不是 CREATE TABLE。