Joins in MySQL on NULL values

Question

I have to do a query in MySQL. I have to find all airports, which can be reached within maximum 4 stepovers. This is my query:

SELECT fp0.from AS start, fp1.from AS stop1, 
  fp2.from AS stop2, fp3.from AS stop3, 
  fp4.from AS stop4,fp4.to AS end 
FROM flightplan AS fp0 
LEFT JOIN flightplan AS fp1 ON( fp0.to=fp1.from ) 
LEFT JOIN flightplan AS fp2 ON( fp1.to=fp2.from ) 
LEFT JOIN flightplan AS fp3 ON( fp2.to=fp3.from ) 
LEFT JOIN flightplan AS fp4 ON( fp3.to=fp4.from ) 
WHERE fp0.from=6626

The result:

6626,"9895","4887","12836","10304","9915"   
6626,"9895","4887","12836","10304","4595"    
6626,"9895","4887","12836","10304","2685"    
6626,"9895","4887","12836","164","12081"    
6626,"9895","4887","12836","1652","8686"    
6626,"9895","4887","12836","1298","6682"    
6626,"9895","4887","12836","5965","10953"    
6626,"9895","4887","12836","5965","7212"    
6626,"9895","13070",**NULL**,NULL,NULL    
6626,"9895","13070","4859","12820","6908"    
6626,"9895",**NULL**,NULL,NULL,NULL

The result is wrong at 2 points ( I marked them with stars ). The problem is, that I check, if the "to" airport a1 equals the "from" airport a2. But if the "to" airport is NULL, I get the wrong result.

Can somebody help me with my problem? Thank you

PS: I am not allowed to do this recursively.

Answer 1

If you want to eliminate the rows with NULL, add this at the end of your query:

AND fp4.to IS NOT NULL