Reducing the complexity of the code

I'm stuck right now with some really weird classes, that have the logic mixed up. Here is the example of the code that generates a query to the database:

if(realTraffic.getPvkp() != null) {
   //Admission point
   if(BeanUtils.isGuidEntity(realTraffic.getPvkp())) {
      findParameters +=
         " and (" + staticTableName() + ".guidPvkp = '" + realTraffic.getPvkp().getGuid()
         + "' or (" + staticTableName() + ".guidPvkpOut = '" + realTraffic.getPvkp().getGuid()
         + "' and " + staticTableName() + ".requestType = " + RequestBean.TRANSIT_TYPE
         + ")";
      if (companyType == CompanyBean.PP_TYPE && !realTraffic.isSkipOther()) {
         // TODO - add non-formed
         findParameters += " or (" + staticTableName() + ".guidPvkpOut is null "
         + " and " + staticTableName() + ".requestType = " + RequestBean.TRANSIT_TYPE
         + ")";
      }
      findParameters += ") ";
   } else {
     // Territorial department
      if(BeanUtils.isGuidEntity(realTraffic.getPvkp().getTerritorialDepartment())) {
         findParameters +=
            " and (Pvkp.guidTerritorialDepartment = '" + realTraffic.getPvkp().getTerritorialDepartment().getGuid()
            + "' or Pvkp.guidFtsDepartment = '" + realTraffic.getPvkp().getTerritorialDepartment().getGuid()
            + "' ) ";
      }
   }
}

This is just a part of a huge set of complex checks I have in method. The question is - how to deal with such code - it has lots of nested if's and checks. What are the common approaches in order to make this code simpler and more elegant?

UPD: I understand how to avoid such code, when writing a new project, but what to do with the existing legacy code?

Solution

A good guide to handle such things is in the book from Uncle Bob, called "Clean Code". In your case I'd say:

put the string concatenations into a method (and use StringBuilder)
convert an else { if (condition) } to an else if (condition)
consider to put the companyType == CompanyBean.PP_TYPE && !realTraffic.isSkipOther() in a separate method, since it appears to be some kind of business logic, which might be clearer for the reader if being put in a method called if (isCompanySkippedOver(companyType, realTraffic)
consider to invert if(realTraffic.getPvkp() != null) to
```
if(realTraffic.getPvkp() == null) {return;}
```

to reduce block indentation.