官方解答｜合併區域日期

官方解答｜合併區域日期

解題目標

將兩表原始列完整合併，依 zone 分組，輸出最大 day 與所有原始列數；NULL zone 同組。

查詢步驟

先決定集合元素由哪些完整欄位組成，再按指定聯集／交集／差集或次數模型處理。集合比對用 NULL 安全相等；需要次數時不能先去重。 依序輸出欄位 zone, latest_day, row_count。最終排序：zone 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。 將兩表原始列完整合併，依 zone 分組，輸出最大 day 與所有原始列數；NULL zone 同組。

公開範例

fixture 為 {"tables":[{"name":"left_samples","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":[[11,"west",null,4,0,2,"beta"],[21,"north",7,null,null,2,"a"],[51,null,5,1,0,8,"ab"],[41,"south",-2,0,1,4,null],[31,"north",3,2,1,1,"alpha"]]},{"name":"right_samples","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":[[32,"east",5,null,null,5,"a"],[42,"south",5,0,1,4,null],[22,"west",null,4,0,2,"beta"],[12,"north",7,1,1,2,"alpha"]]}]}。按題意處理並完成排序後得到 {"columns":["zone","latest_day","row_count"],"rows":[["east",5,1],["north",2,3],["south",4,2],["west",2,2],[null,8,1]]}。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 zone AS zone, latest_day AS latest_day, row_count AS row_count
FROM (
SELECT zone AS zone, MAX(day) AS latest_day, COUNT(*) AS row_count
FROM (SELECT zone,day FROM left_samples UNION ALL SELECT zone,day FROM right_samples) combined_days
GROUP BY zone
) query_result
ORDER BY CASE WHEN zone IS NULL THEN 1 ELSE 0 END ASC, zone COLLATE utf8mb4_bin ASC;

SELECT zone AS zone, latest_day AS latest_day, row_count AS row_count
FROM (
SELECT zone AS zone, MAX(day) AS latest_day, COUNT(*) AS row_count
FROM (SELECT zone,day FROM left_samples UNION ALL SELECT zone,day FROM right_samples) combined_days
GROUP BY zone
) query_result
ORDER BY CASE WHEN zone IS NULL THEN 1 ELSE 0 END ASC, zone COLLATE BINARY ASC;

SELECT zone AS zone, latest_day AS latest_day, row_count AS row_count
FROM (
SELECT zone AS zone, MAX(day) AS latest_day, COUNT(*) AS row_count
FROM (SELECT zone,day FROM left_samples UNION ALL SELECT zone,day FROM right_samples) combined_days
GROUP BY zone
) query_result
ORDER BY CASE WHEN zone IS NULL THEN 1 ELSE 0 END ASC, zone COLLATE "C" ASC;