Include grouped column with median calculating theory - mysql

Question

Thanks to this post, I was able to calculate the median for a corresponding vendor in the invoices table.
This was the query used:
SELECT AVG(middle_values) AS 'median'
FROM (
SELECT t1.invoice_total AS 'middle_values'
FROM
(
SELECT #row:=#row+1 as `row`, iv.invoice_total
FROM invoices AS iv, (SELECT #row:=0) AS r
WHERE iv.vendor_id = 97
ORDER BY iv.invoice_total
) AS t1,
(
SELECT COUNT(*) as 'count'
FROM invoices iv
WHERE iv.vendor_id = 97
) AS t2
-- the following condition will return 1 record for odd number sets, or 2 records for even number sets.
WHERE t1.row >= t2.count/2 and t1.row <= ((t2.count/2) +1)) AS t3;
Instead of this just outputting one column in the resultbox, I'd like it to display two columns: vendor_id, median_invoice.
CREATE TABLE IF NOT EXISTS `invoices` (
`invoice_id` int(11) NOT NULL AUTO_INCREMENT,
`vendor_id` int(11) NOT NULL,
`invoice_number` varchar(50) NOT NULL,
`invoice_date` date NOT NULL,
`invoice_total` decimal(9,2) NOT NULL,
`payment_total` decimal(9,2) NOT NULL DEFAULT '0.00',
`credit_total` decimal(9,2) NOT NULL DEFAULT '0.00',
`terms_id` int(11) NOT NULL,
`invoice_due_date` date NOT NULL,
`payment_date` date DEFAULT NULL,
PRIMARY KEY (`invoice_id`),
KEY `invoices_fk_vendors` (`vendor_id`),
KEY `invoices_fk_terms` (`terms_id`),
KEY `invoices_invoice_date_ix` (`invoice_date`),
CONSTRAINT `invoices_fk_terms` FOREIGN KEY (`terms_id`) REFERENCES `terms` (`terms_id`),
CONSTRAINT `invoices_fk_vendors` FOREIGN KEY (`vendor_id`) REFERENCES `vendors` (`vendor_id`)
) ENGINE=InnoDB AUTO_INCREMENT=119 DEFAULT CHARSET=latin1;
Insert statements:
INSERT INTO `invoices` VALUES (118, 97, '456792', '2011-08-03', 565.60, 0.00, 0.00, 2, '2011-09-02', NULL);
INSERT INTO `invoices` VALUES (117, 97, '456791', '2011-08-03', 4390.00, 0.00, 0.00, 2, '2011-09-02', NULL);
INSERT INTO `invoices` VALUES (116, 97, '456701', '2011-08-02', 270.50, 0.00, 0.00, 2, '2011-09-01', NULL);
INSERT INTO `invoices` VALUES (115, 97, '456789', '2011-08-01', 8344.50, 0.00, 0.00, 2, '2011-08-31', NULL);
INSERT INTO `invoices` VALUES (114, 123, '963253249', '2011-08-02', 127.75, 127.75, 0.00, 3, '2011-09-01', '2011-09-04');
INSERT INTO `invoices` VALUES (113, 37, '547480102', '2011-08-01', 224.00, 0.00, 0.00, 3, '2011-08-31', NULL);
INSERT INTO `invoices` VALUES (112, 110, '0-2436', '2011-07-31', 10976.06, 0.00, 0.00, 3, '2011-08-30', NULL);
INSERT INTO `invoices` VALUES (111, 123, '263253257', '2011-07-30', 22.57, 22.57, 0.00, 3, '2011-08-29', '2011-09-03');
Doing the following was no good:
SELECT t1.vendor_id, AVG(middle_values) AS 'median'
FROM (
SELECT vendor_id, t1.invoice_total AS 'middle_values'
FROM
(
SELECT #row:=#row+1 as `row`, iv.invoice_total
FROM invoices AS iv, (SELECT #row:=0) AS r
WHERE iv.vendor_id = 97
ORDER BY iv.invoice_total
) AS t1,
(
SELECT COUNT(*) as 'count'
FROM invoices iv
WHERE iv.vendor_id = 97
) AS t2, invoices
-- the following condition will return 1 record for odd number sets, or 2 records for even number sets.
WHERE t1.row >= t2.count/2 and t1.row <= ((t2.count/2) +1)) AS t3;

Answer 1

In order to use vendor_id in the parent query you need to return it (select it) in each nested subquery:
SELECT t3.vendor_id, AVG(middle_values) AS 'median'
FROM (
SELECT t1.invoice_total AS 'middle_values', t1.vendor_id
FROM
(
SELECT #row:=#row+1 as `row`, iv.invoice_total, iv.vendor_id
FROM invoices AS iv, (SELECT #row:=0) AS r
WHERE iv.vendor_id = 97
ORDER BY iv.invoice_total
) AS t1,
(
SELECT COUNT(*) as 'count'
FROM invoices iv
WHERE iv.vendor_id = 97
) AS t2
-- the following condition will return 1 record for odd number sets, or 2 records for even number sets.
WHERE t1.row >= t2.count/2 and t1.row <= ((t2.count/2) +1)) AS t3