Question

我必须在MySQL中进行查询。我必须找到所有机场，最多可以在4个步骤内到达。这是我的问题：

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

结果：

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

结果错误2点（我用星星标记）。问题是，我检查，“到”机场a1是否等于“从”机场a2。但是如果“到”机场是NULL，我得到错误的结果。

有人可以帮我解决问题吗？谢谢

PS：我不允许以递归方式执行此操作。

Answer 1

如果要删除带NULL的行，请在查询末尾添加：

AND fp4.to IS NOT NULL

在NULL值上加入MySQL

1 个答案: