En mulighed ville være en forespørgsel, der arbejder med brugervariabler for at generere en rangordning baseret på dato. Og vælg kun de tre sidste datoer.
Forespørgsel
SELECT
*
FROM (
SELECT
*
, CASE
WHEN @supplier = supplier
THEN @rank := @rank + 1
ELSE @rank := 1
END
AS rank
, @supplier := supplier
FROM
Expenses
CROSS JOIN (
SELECT
@supplier := NULL
, @rank := 0
)
AS
init_user_params
WHERE
product_id = 1
ORDER BY
supplier ASC
, DATE DESC
)
AS Expenses_ranked
WHERE
Expenses_ranked.rank <= 3
Resultat
id product_id cost quantity supplier date @supplier := NULL @rank := 0 rank @supplier := supplier
------ ---------- ------ -------- -------- ---------- ----------------- ---------- ------ -----------------------
4 1 3.98 2 1 2017-09-22 (NULL) 0 1 1
3 1 2.50 1 1 2017-09-20 (NULL) 0 2 1
1 1 2.99 1 1 2017-09-05 (NULL) 0 3 1
6 1 8.00 2 2 2017-09-27 (NULL) 0 1 2
5 1 4.00 1 2 2017-09-25 (NULL) 0 2 2
2 1 3.00 2 2 2017-09-10 (NULL) 0 3 2
Brug af disse resultater til at generere en gennemsnitsliste pr. leverandør.
Forespørgsel
SELECT
Expenses_ranked.supplier
, AVG(Expenses_ranked.cost / Expenses_ranked.quantity) AS AVG
FROM (
SELECT
*
, CASE
WHEN @supplier = supplier
THEN @rank := @rank + 1
ELSE @rank := 1
END
AS rank
, @supplier := supplier
FROM
Expenses
CROSS JOIN (
SELECT
@supplier := NULL
, @rank := 0
)
AS
init_user_params
WHERE
product_id = 1
ORDER BY
supplier ASC
, DATE DESC
)
AS Expenses_ranked
WHERE
Expenses_ranked.rank <= 3
GROUP BY
Expenses_ranked.supplier
Resultat
supplier avg
-------- --------------
1 2.4933333333
2 3.1666666667
Nu kan vi bruge en simpel ORDER BY [] ASC LIMIT 1 at få den billigste leverandør
Forespørgsel
SELECT
Expenses_ranked_avg.supplier AS cheapest_supplier
FROM (
SELECT
Expenses_ranked.supplier
, AVG(Expenses_ranked.cost / Expenses_ranked.quantity) AS AVG
FROM (
SELECT
*
, CASE
WHEN @supplier = supplier
THEN @rank := @rank + 1
ELSE @rank := 1
END
AS rank
, @supplier := supplier
FROM
Expenses
CROSS JOIN (
SELECT
@supplier := NULL
, @rank := 0
)
AS
init_user_params
WHERE
product_id = 1
ORDER BY
supplier ASC
, DATE DESC
)
AS
Expenses_ranked
WHERE
Expenses_ranked.rank <= 3
GROUP BY
Expenses_ranked.supplier
)
AS Expenses_ranked_avg
ORDER BY
Expenses_ranked_avg.avg ASC
LIMIT 1
Resultat
cheapest_supplier
-------------------
1
Flere optimale forespørgsler.
Hvad er også muligt at erklære brugervariablerne i where-sætningen. Gør det direkte muligt at filtrere rangeringen fra.
Forespørgsel
SELECT
*
FROM
Expenses
WHERE
(
CASE
WHEN @supplier = supplier
THEN @rank := @rank + 1
ELSE @rank := 1
END
)
AND
(@supplier := supplier )
AND
@rank <= 3
AND
product_id = 1
ORDER BY
supplier ASC
, DATE ASC
Resultat
id product_id cost quantity supplier date
------ ---------- ------ -------- -------- ------------
1 1 2.99 1 1 2017-09-05
3 1 2.50 1 1 2017-09-20
4 1 3.98 2 1 2017-09-22
2 1 3.00 2 2 2017-09-10
5 1 4.00 1 2 2017-09-25
6 1 8.00 2 2 2017-09-27
Nu er det nemt at bruge dette resultatsæt til at finde den billigste leverandør.
Forespørgsel
SELECT
Expenses_ranked.supplier AS cheapest_supplier
FROM (
SELECT
*
FROM
Expenses
WHERE
(
CASE
WHEN @supplier = supplier
THEN @rank := @rank + 1
ELSE @rank := 1
END
) IS NOT NULL
AND
(@supplier := supplier ) IS NOT NULL
AND
@rank <= 3
AND
product_id = 1
ORDER BY
supplier ASC
, DATE ASC
)
AS Expenses_ranked
GROUP BY
Expenses_ranked.supplier
ORDER BY
AVG(Expenses_ranked.cost / Expenses_ranked.quantity) ASC
LIMIT 1
Resultat
cheapest_supplier
-------------------
1