MySQL WHERE: how to write "!=" or "not equals"?
DELETE FROM konta WHERE taken <> '';
The != operator most certainly does exist! It is an alias for the standard <> operator.
Perhaps your fields are not actually empty strings, but instead NULL?
To compare to NULL you can use IS NULL or IS NOT NULL or the null safe equals operator <=>.
You may be using old version of Mysql but surely you can use
DELETE FROM konta WHERE taken <> ''
But there are many other options available. You can try the following ones
DELETE * from konta WHERE strcmp(taken, '') <> 0;
DELETE * from konta where NOT (taken = '');