官方解答｜測站配對數

官方解答｜測站配對數

解題目標

先以 zone 相等做內部連接，NULL zone 不匹配。輸出每個有匹配的測站之配對筆數。

查詢步驟

建立每個符合 ON 條件的配對；一對多需保留所有配對。再執行指定篩選或分組，不能用 DISTINCT 隱藏連接重複。 依序輸出欄位 station_id, match_count。最終排序：station_id 升序，NULL 最後。

正確性

各階段分別實現指定篩列、數值或分組規則。每個保留項目只貢獻題意要求的次數；未配對、缺值與平手依明文規則處理。最後固定別名及排序，因而得到要求的列數、欄位順序與內容。

NULL 與邊界

NULL 表示缺值，不等於 0 或空字串。WHERE／HAVING 只保留真值，假或未知皆排除；一般等號不匹配 NULL。算術操作數缺值時結果為 NULL，除非題目指定備援規則。COUNT(欄位)、SUM、AVG、MIN、MAX 忽略該欄 NULL；COUNT(*) 計算列數。未要求分組或去重時保留所有重複紀錄。空結果仍須具有指定欄名。 先以 zone 相等做內部連接，NULL zone 不匹配。輸出每個有匹配的測站之配對筆數。

公開範例

fixture 為 {"tables":[{"name":"readings","columns":[{"name":"id","type":"INTEGER","nullable":false},{"name":"zone","type":"TEXT","nullable":true},{"name":"reading","type":"INTEGER","nullable":true},{"name":"weight","type":"INTEGER","nullable":true},{"name":"flag","type":"INTEGER","nullable":true},{"name":"day","type":"INTEGER","nullable":false},{"name":"label","type":"TEXT","nullable":true}],"rows":[[21,"north",7,null,null,2,"a"],[41,"south",-2,0,1,4,null],[11,"west",null,4,0,2,"beta"],[31,"north",3,2,1,1,"alpha"]]},{"name":"stations","columns":[{"name":"station_id","type":"INTEGER","nullable":false},{"name":"zone","type":"TEXT","nullable":true},{"name":"target","type":"INTEGER","nullable":true},{"name":"enabled","type":"INTEGER","nullable":true}],"rows":[[9,"north",5,1],[7,"south",null,1],[12,"east",2,null],[3,"north",7,0]]}]}。按題意處理並完成排序後得到 {"columns":["station_id","match_count"],"rows":[[3,2],[7,1],[9,2]]}。即使沒有結果列，也必須保留指定欄名。

複雜度

設所有輸入列合計為 n、輸出／中間配對數為 k。沒有索引時，多表或相關子查詢的保守參考成本可達 O(n²)，再以比較排序處理 k 列需 O(k log(k+1))，暫存資料最多與輸入及中間結果同階。最佳化器可能使用雜湊、索引或重寫降低成本；CTE 的寫法本身不保證實體暫存。

三種方言

下方分別提供完整 SQLite、PostgreSQL、MySQL 查詢。文字排序明寫二進位／字元碼排序；本題 ASCII 域中 SQLite LENGTH 對應其他引擎 CHAR_LENGTH。SQL NULL 輸出 JSON null，不輸出文字 "NULL"。

SELECT station_id AS station_id, match_count AS match_count
FROM (
SELECT s.station_id AS station_id, COUNT(*) AS match_count
FROM readings r JOIN stations s ON r.zone=s.zone
GROUP BY s.station_id
) query_result
ORDER BY CASE WHEN station_id IS NULL THEN 1 ELSE 0 END ASC, station_id ASC;

SELECT station_id AS station_id, match_count AS match_count
FROM (
SELECT s.station_id AS station_id, COUNT(*) AS match_count
FROM readings r JOIN stations s ON r.zone=s.zone
GROUP BY s.station_id
) query_result
ORDER BY CASE WHEN station_id IS NULL THEN 1 ELSE 0 END ASC, station_id ASC;

SELECT station_id AS station_id, match_count AS match_count
FROM (
SELECT s.station_id AS station_id, COUNT(*) AS match_count
FROM readings r JOIN stations s ON r.zone=s.zone
GROUP BY s.station_id
) query_result
ORDER BY CASE WHEN station_id IS NULL THEN 1 ELSE 0 END ASC, station_id ASC;