php - sql 語句寫成這個樣子有效能問題嗎？ ？如何改進？ ？

Question

多重表格聯合查詢 子查詢 條件查詢 排序

SELECT
  ac.*,
  c.car_number,
  c.car_name,
  cs.car_source,
  cb.brand_name,
  cm.car_model,
  ct.car_type,
  c.number_plate,
  (
  SELECT
    COUNT(ar.auction_record_id)
  FROM
    csp_auction_record AS ar
  WHERE
    ac.auction_car_id = ar.auction_car_id
) AS auction_count
FROM
  csp_auction_car AS ac
INNER JOIN
  csp_car AS c
ON
  ac.car_id = c.car_id
INNER JOIN
  csp_car_brand AS cb
ON
  c.car_brand_id = cb.car_brand_id
INNER JOIN
  csp_car_source AS cs
ON
  c.car_source_id = cs.car_source_id
INNER JOIN
  csp_car_type AS ct
ON
  c.car_type_id = ct.car_type_id
INNER JOIN
  csp_car_model AS cm
ON
  c.car_model_id = cm.car_model_id
WHERE
  ac.auction_s_time <= CURRENT_TIMESTAMP AND ac.auction_e_time >= CURRENT_TIMESTAMP
ORDER BY
  ac.auction_car_id
DESC
LIMIT 0, 10

相關資料表：

csp_car_brand，車輛品牌表：

#csp_car_type，車輛類型表：

#csp_car_source，車輛來源表：

#csp_car_model，車輛型號表（所屬品牌的特定型號）：

##csp_car，車輛表（這張表格欄位太多，只列出部分）

#csp_auction_record，拍賣紀錄表：

#請問這 sql 語句有效能問題嗎？ ？該如何優化？ ？是用 php 來拆成一條簡單sql語句，然後將結果組合，還是怎樣？ ？

Answer 1

既然你這麼問了，就表示你知道自己的sql有問題。問題在於csp_auction_car表有多少筆記錄，csp_auction_record表就被掃描了多少次。想想就覺得可怕，但我確實看到很多人喜歡這麼寫。還有，where條件盡量寫在子查詢裡面，ac表的資料量查出來會減少很多，跟後面的關聯會更快。
可以改成下面這樣，csp_auction_record表只掃描1次

SELECT ac.*,
       c.car_number,
       c.car_name,
       cs.car_source,
       cb.brand_name,
       cm.car_model,
       ct.car_type,
       c.number_plate,
       IFNULL(ar.count, 0) AS auction_count
  FROM (select *
          from csp_auction_car
         WHERE auction_s_time <= CURRENT_TIMESTAMP
           AND auction_e_time >= CURRENT_TIMESTAMP) AS ac
 INNER JOIN csp_car AS c
    ON ac.car_id = c.car_id
 INNER JOIN csp_car_brand AS cb
    ON c.car_brand_id = cb.car_brand_id
 INNER JOIN csp_car_source AS cs
    ON c.car_source_id = cs.car_source_id
 INNER JOIN csp_car_type AS ct
    ON c.car_type_id = ct.car_type_id
 INNER JOIN csp_car_model AS cm
    ON c.car_model_id = cm.car_model_id
  left outer join (select auction_car_id, count(*) count
                     from csp_auction_record
                    group by auction_car_id) ar
    on ac.auction_car_id = ar.auction_car_id
 ORDER BY ac.auction_car_id DESC LIMIT 0, 10