title: export tables from database to mdf file
body: <p>i am python beginner. i have Database that has 5 tables, i use python to export all those tables to mdf file using asammdf.</p>
  <pre><code>path_data = os.path.join('..\\Data', 'db_2021-05-06_11-49-42_day.db')
  con = sqlite3.connect(path_data)
  cursor = con.cursor()
  cursor.execute('SELECT name FROM sqlite_master WHERE type=&quot;table&quot;;')
  tables = cursor.fetchall()
  print(tables)
  
  sigs = []
  for equipment in tables:
      df = pd.read_sql_query('select * from' + equipment , con)
      df_small = df[0:1000]
      df_time = (df_small['ts']-df_small.iloc[0]['ts'])
      df_time_int = df_time.astype('int64')
      for signal in df_small.columns.to_list():
          test_signal = Signal(samples=df_small[signal],  timestamps=df_time,
                          name=signal,
                          unit='A')
          sigs.append(test_signal)
  mdf.append(sigs)
  mdf.save('test_complete.mf4', overwrite=True)
  </code></pre>
  <p>and i find this error:</p>
  <pre><code>TypeError                                 Traceback (most recent call last)
  c:\Users\Muhammad Lazaroni\SW_Workspace\MeasurementMerger\Scripts\merger.py in 
        37 # table_list= list[tables]
        38 for equipment in tables:
  ----&gt; 39     df = pd.read_sql_query('select * from' + equipment , con)
        40     df_small = df[0:1000]
        41     df_time = (df_small['ts']-df_small.iloc[0]['ts'])
  
  TypeError: can only concatenate str (not &quot;tuple&quot;) to str
  </code></pre>