如何在父查詢 case 語句中使用子查詢的結果

Question

我有以下返回預期結果的查詢：

SELECT
    locations.*,
    (
        SELECT
            id
        FROM
            hauls
        WHERE
            haul_type_id = 1
            AND location_id = locations.id
        ORDER BY
            created_at DESC
        LIMIT 1) AS last_delivery_id,
    (
        SELECT
            id
        FROM
            hauls
        WHERE
            haul_type_id = 2
            AND location_id = locations.id
        ORDER BY
            created_at DESC
        LIMIT 1) AS last_pickup_id
FROM
    locations

我想在父查詢的 case 語句中使用子查詢（ last_delivery_id 、 last_pickup_id ）的結果，但出現錯誤：

錯誤：未知列“last_delivery_id”

SELECT
    locations.*, 
    case when last_delivery_id = 1 then 'pending' when last_delivery_id = 2 then 'active' end as status,
    (
        SELECT
            id
        FROM
            hauls
        WHERE
            haul_type_id = 1
            AND location_id = locations.id
        ORDER BY
            created_at DESC
        LIMIT 1) AS last_delivery_id,
    (
        SELECT
            id
        FROM
            hauls
        WHERE
            haul_type_id = 2
            AND location_id = locations.id
        ORDER BY
            created_at DESC
        LIMIT 1) AS last_pickup_id
FROM
    locations

Answer 1

您可以使用子查詢或 CTE：

SELECT lh.*, . . .   -- any expressions you want
       (case when last_delivery_id = 1 then 'pending'
             when last_delivery_id = 2 then 'active'
        end) as status
FROM (SELECT l.*,
             (SELECT id
              FROM hauls h
              WHERE h.haul_type_id = 1 AND
                    h.location_id = l.id
              ORDER BY h.h.created_at DESC
              LIMIT 1
             ) AS last_delivery_id,
             (SELECT id
              FROM hauls h
              WHERE h.haul_type_id = 2 AND
                    h.location_id = l.id
              ORDER BY h.created_at DESC
              LIMIT 1
             ) AS last_pickup_id
      FROM locations l
     ) lh

請注意表別名的使用，這使得查詢更易於編寫和閱讀。 也就是說，如果您只想要字符串版本，我不確定您是否真的需要delivery_id ：

      SELECT l.*,
             (SELECT (case when h.id = 1 then 'pending'
                           when h.id = 2 then 'active'
                      end)
              FROM hauls h
              WHERE h.haul_type_id = 1 AND
                    h.location_id = l.id
              ORDER BY h.created_at DESC
              LIMIT 1
             ) AS status
      FROM locations l

Answer 2

考慮使用 CTE 和 window function 重寫內聯子查詢。這樣可以避免使用LIMIT子句，如果未以最佳方式運行，可能會出現性能問題。 這需要支持 CTE 和 window 功能的 MySQL 版本 8.0+：

WITH sub AS (
  SELECT id AS last_delivery_id
       , location_id
       , haul_type_id
       , ROW_NUMBER() OVER (PARTITION BY location_id, haul_type_id
                            ORDER BY created_at DESC) AS rn
  FROM hauls
)

SELECT l.*       -- CONSIDER EXPLICITLY SELECTING COLUMNS FOR PERFORMANCE
     , CASE
          WHEN h1.last_delivery_id = 1 THEN 'pending' 
          WHEN h2.last_delivery_id = 2 THEN 'active' 
       END AS `status`
FROM locations l
LEFT JOIN sub h1
  ON h1.location_id = l.id
 AND h1.haul_type_id = 1
 AND h1.rn = 1
LEFT JOIN sub h2
  ON h2.location_id = l.id
 AND h2.haul_type_id = 2
 AND h2.rn = 1

對於早期版本（<= MySQL 5.7），集成一個聚合查詢：

SELECT l.*       -- CONSIDER EXPLICITLY SELECTING COLUMNS FOR PERFORMANCE 
     , CASE
          WHEN h1.id = 1 THEN 'pending' 
          WHEN h2.id = 2 THEN 'active' 
       END AS `status`
FROM locations l
LEFT JOIN
    (SELECT location_id
          , MAX(CASE WHEN haul_type_id = 1 THEN created_at END) AS h1_max_date
          , MAX(CASE WHEN haul_type_id = 2 THEN created_at END) AS h2_max_date
     FROM hauls
     GROUP BY location_id
    ) h_agg
   ON l.location_id = h_agg.location_id
LEFT JOIN hauls h1
   ON  h1.location_id = h_agg.location_id
   AND h1.haul_type_id = 1
   AND h1.created_at = h_agg.h1_max_date
LEFT JOIN hauls h2
   ON  h2.location_id = h_agg.location_id
   AND h2.haul_type_id = 2
   AND h2.created_at = h_agg.h2_max_date

Answer 3

如果您正在運行 MySQL 8.0.14 或更高版本，您可以為此使用橫向連接：

select l.*, h1.*, h2.*
    case 
        when last_delivery_id = 1 then 'pending' 
        when last_delivery_id = 2 then 'active' 
    end as status
from locations l
left join lateral (
    select h.id as last_delivery_id
    from hauls h
    where h.haul_type_id = 1 and h.location_id = l.id
    order by h.created_at desc limit 1
) h1 on true
left join lateral (
    select h.id as last_pickup_id
    from hauls h
    where h.haul_type_id = 2 and h.location_id = l.id
    order by h.created_at desc limit 1
) h2 on true

橫向連接是一個強大的功能，MySQL 已經丟失了很長時間。 例如，上面的語句可以很容易地修改為從每個子查詢返回更多列。

Answer 4

因為 select 部分中的所有列都一次執行並且 last_delivery_id 列尚未創建並且您嘗試使用它，sql 服務器返回錯誤並且為了解決它您可以使用 CTE，視圖和驅動表。