如何使用Order BY正确使用Union？

Question

The code I have below joins 5 tables and then is suppose to sort by date_timed_added. The query worked perfectly if i only join 4 tables. For some reason after the 4th table, its giving me issues. The issue is that it sorts and displays the 5th table first and then the rest follow. How can i fix it so that it sorts date_time_added properly by querying all the other tables?

//$sid is a variable that is drawn from DB

$sql = "select `client_visit`.`visit_id`, `client_visit`.
`why_visit`, `client_visit`.`date_time_added`, `client_visit`.
`just_date`, `client_visit`.`type` from `client_visit` where 
`client_visit`.`system_id` = '$sid' and `client_visit`.
`added_by` = '$sid' 
UNION
select `client_notes`.`note_id`, `client_notes`.`note_name`, 
`client_notes`.`date_time_added`, `client_notes`.`just_date`
, `client_notes`.`type` from `client_notes` where `client_notes`.
`added_by` = '$sid'
UNION
select `client_conditions`.`med_id`, `client_conditions`.`med_name`,  
`client_conditions`.`date_time_added`, `client_conditions`.`just_date`,    
`client_conditions`.`type` from `client_conditions` where 
`client_conditions`.`system_id` = '$sid' and `client_conditions`.
`added_by` = '$sid'
UNION
select `client_stats`.`stat_test_id`, `client_stats`.`stat_why`,  
`client_stats`.`date_time_added`, `client_stats`.`just_date`, 
`client_stats`.`type`
from `client_stats` where `client_stats`.`system_id` = '$sid' 
and `client_stats`.`added_by` = '$sid'
UNION
select `client_documents`.`doc_id`, `client_documents`.`doc_name`,  
`client_documents`.`date_time_added`, `client_documents`.`just_date`, 
`client_documents`.`type` from `client_documents` where `client_documents`.
`system_id` = '$sid' and `client_documents`.`added_by` = '$sid'
 ORDER BY `date_time_added` DESC LIMIT $startrow, 20";

 $query = mysql_query($sql) or die ("Error: ".mysql_error());

 $result = mysql_query($sql);

 if ($result == "")
 {
 echo "";
 }
 echo "";


 $rows = mysql_num_rows($result);

 if($rows == 0)
 {

 }
 elseif($rows > 0)
 {
 while($row = mysql_fetch_array($query))
 {

 //Just using these two variables i can display the same row info 
 //for all the other tables

 $stuffid = htmlspecialchars($row['visit_id']);
 $title = htmlspecialchars($row['why_visit');

 }

 }

 }

Answer 1

As per the MySQL docs: http://dev.mysql.com/doc/refman/5.0/en/union.html

If you want to order the ENTIRE result set, the ORDER BY clause must be placed on the LAST query in the UNION, with each query being bracketed.

(SELECT ...)
UNION
(SELECT ...)
ORDER BY ...