Why I can't I drop field_data and field_revision table in phpmyadmin? I wanted to delete a filed named MyField.

So I first deleted the corresponding record for MyField in field_config table and field_config_instance table.

And now I'm trying to drop these tables by using phpmyadmin.

field_data_field_MyField
field_revision_field_MyField


But even if I drop them they are still there.
It seems that they can't be droped.

Isn't it possible to delete fields directly in DB?
@hampercm explained why directly deleting records and tables from database is not recommended. It is so true that everyone is encouraged to use admin UI or Drupal API to remove the fields and field instances.

But, I guess you know what the best practise is and wanna do that by SQL query with some reason (e.g. company policy. YES, I faced this stupid requirement before)

You are on the right track. Drupal save field and field instances config in field_config and field_config_instance. BUT, Drupal also cache these data in cache table.

So, you need to truncate the cache table.