MySQL: Join distinct rows of two tables in a certain order?

Question

I have a list of inventory units and sale transactions that I want to, (1) join by unit SKU, and (2) associate ONE transaction to ONE inventory unit in first-in-first-out order by date. I'm having trouble with the second part.

The best I can come up with is:

SELECT `units`.`unit_date`, `units`.`unit_id`, `trans`.`tran_date`, `trans`.`tran_id`, `units`.`unit_sku` FROM `units`
    INNER JOIN `trans`
        ON `trans`.`unit_sku` = `units`.`unit_sku`
GROUP BY `trans`.`tran_id`, `trans`.`unit_sku`
ORDER BY `units`.`unit_date` asc, `trans`.`tran_date` asc
;

units table:

unit_date  | unit_id | unit_sku
2015-06-01 | 1       | U1KLM
2015-06-02 | 2       | U1KLM
2015-06-03 | 3       | U2QRS
2015-06-04 | 4       | U2QRS
2015-06-05 | 5       | U1KLM

trans table:

tran_date  | tran_id | unit_sku
2015-06-11 | A       | U2QRS
2015-06-12 | B       | U1KLM
2015-06-13 | C       | U1KLM
2015-06-14 | D       | U2QRS
2015-06-15 | E       | U1KLM

The desired result is one tran_id to be joined to one unit_id of the unit_sku by earliest-to-latest order of unit_date:

unit_date  | unit_id | tran_date  | tran_id | unit_sku
2015-06-01 | 1       | 2015-06-12 | B       | U1KLM
2015-06-02 | 2       | 2015-06-13 | C       | U1KLM
2015-06-03 | 3       | 2015-06-11 | A       | U2QRS
2015-06-04 | 4       | 2015-06-14 | D       | U2QRS
2015-06-05 | 5       | 2015-06-15 | E       | U1KLM

The query (undesired) result joins tran_id only to the unit_id of the earliest occurrence of unit_sku:

unit_date  | unit_id | tran_date  | tran_id | unit_sku
2015-06-01 | 1       | 2015-06-12 | B       | U1KLM
2015-06-01 | 1       | 2015-06-13 | C       | U1KLM
2015-06-01 | 1       | 2015-06-15 | E       | U1KLM
2015-06-03 | 3       | 2015-06-11 | A       | U2QRS
2015-06-03 | 3       | 2015-06-14 | D       | U2QRS

Any ideas on how to do get the desired result? In this setup, only unit_date and tran_date are sortable; the rest are randomly generated.

Repro script:

DROP TEMPORARY TABLE IF EXISTS `units`;
DROP TEMPORARY TABLE IF EXISTS `trans`;
CREATE TEMPORARY TABLE `units` (`unit_date` date, `unit_id` char(1) , `unit_sku` char(5), PRIMARY KEY(`unit_id`));
CREATE TEMPORARY TABLE `trans` (`tran_date` date, `tran_id` char(1) , `unit_sku` char(5), PRIMARY KEY(`tran_id`));

INSERT INTO `units` (`unit_date`, `unit_id`, `unit_sku`) VALUES
    ('2015-06-01', '1', 'U1KLM')
    , ('2015-06-02', '2', 'U1KLM')
    , ('2015-06-03', '3', 'U2QRS')
    , ('2015-06-04', '4', 'U2QRS')
    , ('2015-06-05', '5', 'U1KLM')
;

INSERT INTO `trans` (`tran_date`, `tran_id`, `unit_sku`) VALUES
    ('2015-06-11', 'A', 'U2QRS')
    , ('2015-06-12', 'B', 'U1KLM')
    , ('2015-06-13', 'C', 'U1KLM')
    , ('2015-06-14', 'D', 'U2QRS')
    , ('2015-06-15', 'E', 'U1KLM')
;

SELECT `units`.`unit_date`, `units`.`unit_id`, `trans`.`tran_date`, `trans`.`tran_id`, `units`.`unit_sku` FROM `units`
    INNER JOIN `trans`
        ON `trans`.`unit_sku` = `units`.`unit_sku`
GROUP BY `trans`.`tran_id`, `trans`.`unit_sku`
ORDER BY `units`.`unit_date` asc, `trans`.`tran_date` asc
;

Answer 1

I believe this is what you're looking for: (This is assuming that 1 to 1 relationship)

SET @UNITRN := 0;
SET @TRANSRN :=0;
SELECT A.`unit_date`, A.`unit_id`, B.`tran_date`, B.`tran_id`, A.`unit_sku` FROM (SELECT @UNITRN := @UNITRN + 1 AS ROWNUM, UNIT_DATE, UNIT_ID, UNIT_SKU FROM UNITS ORDER BY UNIT_SKU, UNIT_DATE ASC) A
JOIN (SELECT @TRANSRN := @TRANSRN + 1 AS ROWNUM, TRAN_DATE, TRAN_ID, UNIT_SKU FROM TRANS ORDER BY UNIT_SKU, TRAN_DATE ASC) B
ON A.UNIT_SKU = B.UNIT_SKU
AND A.ROWNUM = B.ROWNUM
ORDER BY A.UNIT_DATE ASC