在同一列上具有联合/联接和两个别名的MySQL

Question

I'm having problem on getting the quantity value when I want to separate the quantity of group_status 1 and 2. Whether I set the quantity by using the Alias and Union,its only display the first query. Fyi, I did tried using the double left join and set the table to 'a' and 'b', its also not working. Any helps or solution are welcome. Thanks!

Below are the table and the query result that I need.

$a //start date $b //end date

Query :

"SELECT items.item_name, requesters.item_version, requesters.quantity AS 'approved'
          FROM requesters
          JOIN items ON items.item_id=requesters.item_id 
          WHERE requesters.requested_date >= '$a' AND requesters.requested_date <='$b'
          AND group_status =1
UNION ALL

SELECT items.item_name, requesters.item_version, requesters.quantity AS notapproved
          FROM requesters
          JOIN items ON items.item_id=requesters.item_id 
          WHERE requesters.requested_date >= '$a' AND requesters.requested_date <='$b'
          AND group_status =2";

Requesters:

 ------ --------- ------------- --------------- ------------- -------------
| id   |  item_id| item_version| requested_date|     quantity| group_status|
 ------ --------- ------------- --------------- ------------- -------------
|    1 |   2     |        2013 |    2016-01-01 |         100 |           1 |
|    2 |   2     |        2013 |    2016-01-12 |         200 |           2 |
|    3 |   3     |        2007 |    2016-02-04 |         300 |           2 |
|    4 |   3     |        2010 |    2016-03-25 |         400 |           1 |
 ------  -------- ------------- --------------- ------------- -------------

Items:

 ------ -------- ------------- 
| id   | item_id|    item_name| 
 ------ -------- ------------- 
|    1 |    1   |    Ms Office| 
|    1 |    2   | Ms Visio Pro|      
|    2 |    3   | Ms Visio Std|           
 ------ -------- -------------

Status:

 ------ ---------- ------------- 
| id   | status_id|  status_name| 
 ------ ---------- ------------- 
|    1 |      1   | Approved    |      
|    2 |      2   | Not Approved|           
 ------ ---------- -------------

Example of query result:

Date: 2016-01-01 to 2016-01-31

 ------------- -------------- ------------- ------------- -------------
|   item_name |  item_version|    Approved | Not Approved|        Total|
 ------------- -------------- ------------- ------------- -------------
| Ms Visio Pro|         2013 |         100 |         200 |         300 |
| Ms Visio Std|         2007 |           0 |         300 |         300 |
| Ms Visio Std|         2010 |         400 |           0 |         400 |
 ------------  -------------- ------------- ------------- -------------

Answer 1

With a UNION and UNION ALL set operators, the column names for the resultset are specified in the first SELECT . The column names and aliases in subsquent queries are ignored. (It's not possible to specify two different aliases for one column in the resultset. Each column gets assigned a single name.)

But for your result, you don't necessarily need a UNION ALL operation.

It looks like you want the SUM of the quantity for each status, on the same line. You can use conditional aggregation. For example:

SELECT i.item_name
     , r.item_version
     , SUM(IF(r.group_status=1,r.quantity,0)) AS `Approved`
     , SUM(IF(r.group_status=2,r.quantity,0)) AS `Not Approved`
     , SUM(r.quantity)                        AS `Total`
  FROM requesters r
  JOIN items i
    ON i.item_id = r.item_id
 WHERE r.group_status IN (1,2)
   AND r.requested_date >= ?
   AND r.requested_date <= ?
 GROUP
    BY i.item_name
     , r.item_version

If you also want to return rows for items that don't have any related rows in requesters (with group_status 1 or 2), you could use an outer join, and move the predicates in the ON clause... for example:

SELECT i.item_name
     , r.item_version
     , IFNULL(SUM(IF(r.group_status=1,r.quantity,0)),0) AS `Approved`
     , IFNULL(SUM(IF(r.group_status=2,r.quantity,0)),0) AS `Not Approved`
     , IFNULL(SUM(r.quantity),0)                        AS `Total`
  FROM items i
  LEFT
  JOIN requesters r
    ON r.item_id = i.item_id
   AND r.group_status IN (1,2)
   AND r.requested_date >= ?
   AND r.requested_date <= ?
 GROUP
    BY i.item_name
     , r.item_version

---

FOLLOWUP

NOTE: The join predicate r.item_id = i.item_id follows the same pattern as the original OP query.

Demonstration:

create table requesters  (id int, item_id int, item_version int
  , requested_date date, quantity int, group_status int)
;
insert into requesters (id, item_id, item_version
  , requested_date, quantity, group_status) values
 ('1','2','2013','2016-01-01','100','1')
,('2','2','2013','2016-01-12','200','2')
,('3','3','2007','2016-02-04','300','2')
,('4','3','2010','2016-03-25','400','1')
;
create table items (id int, item_id int, item_name varchar(12))
;
insert into items (id, item_id, item_name) values
 ('1','1','Ms Office')
,('1','2','Ms Visio Pro')
,('2','3','Ms Visio Std')
;

First query returns three rows:

item_name     item_version  Approved  Not Approved   Total
------------  ------------  --------  ------------  ------
Ms Visio Pro          2013       100           200     300
Ms Visio Std          2007         0           300     300
Ms Visio Std          2010       400             0     400  

Second query returns four rows (includes "zeros" row for Ms Office):

item_name     item_version  Approved  Not Approved   Total
------------  ------------  --------  ------------  ------
Ms Office           (NULL)         0             0       0
Ms Visio Pro          2013       100           200     300
Ms Visio Std          2007         0           300     300
Ms Visio Std          2010       400             0     400