Android-工作总结-LX-2018-08-20-判断数据库表字段是否为空

问题的因素：

调试了一上午，我要判断数据库表的name字段是否为空，使用了TextUtils.isEmpty(nameStr)；来判断name字段是否为空，明明数据库是没有值，却一直显示有值，然后还去质疑TextUtils.isEmpty(nameStr)；，最后才发现是我存入数据的时候有问题 不能 name + ""

TextUtils.isEmpty(nameStr)没有问题， 是存入的时候 name + ""    会导致数据库表字段是null

以下代码的解释：

注意：⚠️ 由于在真实开发的项目中代码非常庞大/非常复杂，也不能暴露公司的代码，所以以下代码都是简单的测试代码：

问题的示范：

package liudeli.my_work_summary;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.os.Bundle;
import android.text.TextUtils;
import android.util.Log;

public class MainActivity02 extends Activity {

    private final String TAG = MainActivity02.class.getSimpleName();
    private final String TABLE_NAME = "MainActivity02_TABLE";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main02);

        MySQLiteOpenHelper helper = new MySQLiteOpenHelper(this, "MainActivity02_DBName", null, 1);

        // 模拟插入数据
        String name = null;
        String body = null;

        SQLiteDatabase db = helper.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put("body", body);
        values.put("name", name + ""); /** 注意：⚠️ 我这里是  name+""  */
        db.insert(TABLE_NAME, null, values);
        // 不能在这里关闭db.close();，因为以下代码还需要db

        // 判断数据库表里面的name是否为空
        Cursor cursor = db.query(TABLE_NAME, new String[]{"body","name"}, null, null, null, null,null);
        if (cursor.moveToFirst()) {
           String nameStr = cursor.getString(cursor.getColumnIndex("name"));
           if (TextUtils.isEmpty(nameStr)) {
               new AlertDialog.Builder(this)
                       .setTitle("提示")
                       .setMessage("name是空的")
                       .setPositiveButton("我知道了", null)
                       .show();
           } else {
               new AlertDialog.Builder(this)
                       .setTitle("提示")
                       .setMessage("name是有值的")
                       .setPositiveButton("我指定了", null)
                       .show();
           }
        }

        // 关闭db
        db.close();
    }

    class MySQLiteOpenHelper extends SQLiteOpenHelper {

        public MySQLiteOpenHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
            super(context, name, factory, version);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            String createTableSQL =  "CREATE TABLE IF NOT EXISTS " + TABLE_NAME + " (ID INTEGER PRIMARY KEY, "
                    + "body VARCHAR,"
                    + "name  VARCHAR);";
            try {
                db.execSQL(createTableSQL);
            } catch (Exception e) {
                e.printStackTrace();
                Log.e(TAG, TAG + "创表异常：" + e.toString());
            }

        }

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

---

解决方案一：在判断的时候以null作为空的标记来判断，不推荐此方法

package liudeli.my_work_summary;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.os.Bundle;
import android.text.TextUtils;
import android.util.Log;

public class MainActivity02 extends Activity {

    private final String TAG = MainActivity02.class.getSimpleName();
    private final String TABLE_NAME = "MainActivity02_TABLE";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main02);

        MySQLiteOpenHelper helper = new MySQLiteOpenHelper(this, "MainActivity02_DBName", null, 1);

        // 模拟插入数据
        String name = null;
        String body = null;

        SQLiteDatabase db = helper.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put("body", body);
        values.put("name", name + ""); /** 注意：⚠️ 我这里是  name+""  */
        db.insert(TABLE_NAME, null, values);
        // 不能在这里关闭db.close();，因为以下代码还需要db

        // 判断数据库表里面的name是否为空
        Cursor cursor = db.query(TABLE_NAME, new String[]{"body","name"}, null, null, null, null,null);
        if (cursor.moveToFirst()) {
           String nameStr = cursor.getString(cursor.getColumnIndex("name"));
           // if (TextUtils.isEmpty(nameStr)) {
           if ("null".equals(nameStr)) { /** 注意：⚠️ 这种解决方案会被所有安卓程序员鄙视 */
               new AlertDialog.Builder(this)
                       .setTitle("提示")
                       .setMessage("name是空的")
                       .setPositiveButton("我知道了", null)
                       .show();
           } else {
               new AlertDialog.Builder(this)
                       .setTitle("提示")
                       .setMessage("name是有值的")
                       .setPositiveButton("我指定了", null)
                       .show();
           }
        }

        // 关闭db
        db.close();
    }

    class MySQLiteOpenHelper extends SQLiteOpenHelper {

        public MySQLiteOpenHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
            super(context, name, factory, version);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            String createTableSQL =  "CREATE TABLE IF NOT EXISTS " + TABLE_NAME + " (ID INTEGER PRIMARY KEY, "
                    + "body VARCHAR,"
                    + "name  VARCHAR);";
            try {
                db.execSQL(createTableSQL);
            } catch (Exception e) {
                e.printStackTrace();
                Log.e(TAG, TAG + "创表异常：" + e.toString());
            }

        }

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

结果：

---

解决方案二：存入数据的时候，千万不能加入 name + ""

package liudeli.my_work_summary;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.os.Bundle;
import android.text.TextUtils;
import android.util.Log;

public class MainActivity02 extends Activity {

    private final String TAG = MainActivity02.class.getSimpleName();
    private final String TABLE_NAME = "MainActivity02_TABLE";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main02);

        MySQLiteOpenHelper helper = new MySQLiteOpenHelper(this, "MainActivity02_DBName", null, 1);

        // 模拟插入数据
        String name = null;
        String body = null;

        SQLiteDatabase db = helper.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put("body", body);
        values.put("name", name); /** 注意：⚠️ 这种解决方案才是正确的  不能加 + ""  */
        db.insert(TABLE_NAME, null, values);
        // 不能在这里关闭db.close();，因为以下代码还需要db

        // 判断数据库表里面的name是否为空
        Cursor cursor = db.query(TABLE_NAME, new String[]{"body","name"}, null, null, null, null,null);
        if (cursor.moveToFirst()) {
           String nameStr = cursor.getString(cursor.getColumnIndex("name"));
           if (TextUtils.isEmpty(nameStr)) {
               new AlertDialog.Builder(this)
                       .setTitle("提示")
                       .setMessage("name是空的")
                       .setPositiveButton("我知道了", null)
                       .show();
           } else {
               new AlertDialog.Builder(this)
                       .setTitle("提示")
                       .setMessage("name是有值的")
                       .setPositiveButton("我指定了", null)
                       .show();
           }
        }

        // 关闭db
        db.close();
    }

    class MySQLiteOpenHelper extends SQLiteOpenHelper {

        public MySQLiteOpenHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
            super(context, name, factory, version);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            String createTableSQL =  "CREATE TABLE IF NOT EXISTS " + TABLE_NAME + " (ID INTEGER PRIMARY KEY, "
                    + "body VARCHAR,"
                    + "name  VARCHAR);";
            try {
                db.execSQL(createTableSQL);
            } catch (Exception e) {
                e.printStackTrace();
                Log.e(TAG, TAG + "创表异常：" + e.toString());
            }

        }

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

结果：