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Consulta MySQL para calcular os preços médios mais recentes usando dados de várias tabelas


Uma opção seria uma consulta que trabalhasse com variáveis ​​do usuário para gerar um ranking com base na data. E selecione apenas as três últimas datas.

Consulta
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

Resultado
    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

Usando esses resultados para gerar uma lista média por fornecedor.

Consulta
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 

Resultado
supplier  avg           
--------  --------------
       1  2.4933333333  
       2  3.1666666667  

Agora podemos usar um simples ORDER BY [] ASC LIMIT 1 para obter o fornecedor mais barato

Consulta
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

Resultado
cheapest_supplier  
-------------------
                  1

Mais consultas ideais.

O que também é possível declarar as variáveis ​​do usuário dentro do comando where.Tornando diretamente possível filtrar o ranking.

Consulta
  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  

Resultado
    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 

Agora é fácil usar este conjunto de resultados para encontrar o fornecedor mais barato.

Consulta
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  

Resultado
cheapest_supplier  
-------------------
                  1