mysql select distinct一个表中的2列，distinct两个表中的1列 - mysqli select distinct 2 column in one table and distinct one column from two table

mysqli select distinct 2 column in one table and distinct one column from two table

我有3张表

表用户

id db_name db_lname

1    xxx    yyyy
2    zzz    zzzz

tbl_supplier

id db_companyname
1  xxxxxxxxx

tbl_subcontractor

id db_companyname
1   zzzzzzzzzz
2   zzzzzzzzzz

结果应该是

xxx yyy
zzz zzz
zzzzzzz
xxxxxxx

fname和lname应该在一起

<?php
  $q=mysqli_query($conn,"SELECT db_fname as fname from tbl_user
   UNION 
  SELECT db_lname as lname from tbl_user
  UNION 
  SELECT db_CompanyName as scn from tbl_supplier 
  UNION 
  SELECT db_CompanyName as sucn from tbl_subcontractor
  ")or die(mysqli_error($conn));
  echo'<select name="txt_transferredto" class="form-control inpu-md">';
  while($row=mysqli_fetch_array($q)){
     $fname=$row['fname'];
     //$lname=$row['lname'];
     $companyname=$row['scn'];
     $subcompanyname=$row['sucn'];
     $name=$fname.' '.$lname;
  if($fname!=""){
   echo"<option value='$fname'>";echo $fname;echo"</option>";}
else if($subcompanyname!=""){      
    echo"<option value='$subcompanyname'>";echo $subcompanyname;echo"</option>";}
else if($companyname!=""){      
    echo"<option value='$companyname'>";echo $companyname;echo"</option>";}
  }
  echo'</select>';
  ?>

上面的代码给出了结果，但我不能一起打印我有这个问题

( ! ) Notice: Undefined index: db_CompanyName in C:'wamp'www'order'projectmanagment'addactivities.php on line 147 Call Stack #TimeMemoryFunctionLocation 10.0026298808{main}(  )..'addactivities.php:0    ( ! ) Notice: Undefined index: db_CompanyName in C:'wamp'www'order'projectmanagment'addactivities.php on line 148 Call Stack #TimeMemoryFunctionLocation 10.0026298808{main}(  )..'addactivities.php:0    ( ! ) Notice: Undefined variable: lname in C:'wamp'www'order'projectmanagment'addactivities.php on line 149 Call Stack #TimeMemoryFunctionLocation 10.0026298808{main}(  )..'addactivities.php:0

第一个表使用concat

 SELECT concat(db_fname,' ' , db_lname)  from tbl_user
 UNION 
 SELECT db_CompanyName from tbl_supplier 
 UNION 
 SELECT db_CompanyName from tbl_subcontractor