SQL: Basic select 586. Customer Placing the Largest Number of Orders

SQL/Leetcode/Basic select/586. Customer Placing the Largest Number of Orders/gpt/Customer_Placing_the_Largest_Number_of_Orders_mysql.ipynb

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+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "eeeadbe0",
+   "metadata": {},
+   "source": [
+    "# MySQL 8.0.40\n",
+    "\n",
+    "## 0) 前提\n",
+    "\n",
+    "* エンジン: **MySQL 8**\n",
+    "* 並び順: 任意（`ORDER BY` を付けない）\n",
+    "* `NOT IN` は NULL 罠のため回避\n",
+    "* 判定は **ID 基準**、表示は仕様どおりの列名と順序\n",
+    "\n",
+    "## 1) 問題\n",
+    "\n",
+    "* `Orders` から **最も多く注文を行った顧客の `customer_number`** を返す\n",
+    "  （テストでは最大が一意。Follow up: 同数最大が複数いても全件返す）\n",
+    "\n",
+    "* 入力テーブル例:\n",
+    "\n",
+    "  ```markdown\n",
+    "  Table: Orders\n",
+    "  +-----------------+----------+\n",
+    "  | Column Name     | Type     |\n",
+    "  +-----------------+----------+\n",
+    "  | order_number    | int      | -- PK\n",
+    "  | customer_number | int      |\n",
+    "  +-----------------+----------+\n",
+    "  ```\n",
+    "\n",
+    "* 出力仕様:\n",
+    "\n",
+    "  ```markdown\n",
+    "  +-----------------+\n",
+    "  | customer_number |\n",
+    "  +-----------------+\n",
+    "  ```\n",
+    "\n",
+    "## 2) 最適解（単一クエリ）\n",
+    "\n",
+    "> **ウィンドウ関数＋事前集計**で 1 クエリ。`OVER` 内の `ORDER BY` は順位付けのためで、最終 `SELECT` に `ORDER BY` は不要。\n",
+    "\n",
+    "```sql\n",
+    "WITH cnt AS (\n",
+    "  SELECT\n",
+    "    customer_number,\n",
+    "    COUNT(*) AS order_cnt\n",
+    "  FROM Orders\n",
+    "  GROUP BY customer_number\n",
+    "),\n",
+    "win AS (\n",
+    "  SELECT\n",
+    "    customer_number,\n",
+    "    DENSE_RANK() OVER (ORDER BY order_cnt DESC) AS rnk\n",
+    "  FROM cnt\n",
+    ")\n",
+    "SELECT\n",
+    "  customer_number\n",
+    "FROM win\n",
+    "WHERE rnk = 1;\n",
+    "\n",
+    "Runtime 429 ms\n",
+    "Beats 75.41%\n",
+    "\n",
+    "```\n",
+    "\n",
+    "* これで **一意最大**も**同数最大が複数**も対応（Follow up 充足）\n",
+    "\n",
+    "## 3) 代替解\n",
+    "\n",
+    "> **最大値をサブクエリで求めて一致フィルタ**。ウィンドウが重い環境や互換用に。\n",
+    "\n",
+    "```sql\n",
+    "WITH cnt AS (\n",
+    "  SELECT customer_number, COUNT(*) AS order_cnt\n",
+    "  FROM Orders\n",
+    "  GROUP BY customer_number\n",
+    "),\n",
+    "mx AS (\n",
+    "  SELECT MAX(order_cnt) AS max_cnt FROM cnt\n",
+    ")\n",
+    "SELECT c.customer_number\n",
+    "FROM cnt AS c\n",
+    "JOIN mx ON c.order_cnt = mx.max_cnt;\n",
+    "\n",
+    "Runtime 469 ms\n",
+    "Beats 41.83%\n",
+    "\n",
+    "```\n",
+    "\n",
+    "※ `NOT IN` は未使用。`ORDER BY ... LIMIT 1` でも実現できるが、本要件では**結果順は任意**かつ **最大同率全件**を自然に返せる上記方式が安全。\n",
+    "\n",
+    "## 4) 要点解説\n",
+    "\n",
+    "* **方針**: まず `customer_number` 単位で件数を集計 → 上位判定（`DENSE_RANK` か `MAX` 照合） → 仕様列のみ投影。\n",
+    "* **NULL / 重複**:\n",
+    "\n",
+    "  * `customer_number` が NULL の行が存在するなら集計前に `WHERE customer_number IS NOT NULL` を入れる（問題文では想定外だが堅牢性の観点）。\n",
+    "  * `order_number` は PK のため重複はなし。\n",
+    "* **安定性**: 出力順は問わないため **最終 `ORDER BY` は不要**。上位選別はウィンドウ内で完結。\n",
+    "\n",
+    "## 5) 計算量（概算）\n",
+    "\n",
+    "* `GROUP BY` 集計: **O(N)**～**O(N log N)**（ヒープ/ソート次第）\n",
+    "* ウィンドウ `DENSE_RANK`（代替は `MAX` 照合）: 集計後のユニーク顧客数を M として **O(M log M)**（内部ソート含む）\n",
+    "  ※ 代替解（`MAX` 照合）は **O(M)** でやや軽量。\n",
+    "\n",
+    "## 6) 図解（Mermaid 超保守版）\n",
+    "\n",
+    "```mermaid\n",
+    "flowchart TD\n",
+    "  A[Orders] --> B[customer_number ごとに COUNT]\n",
+    "  B --> C[順位付け DENSE_RANK または MAX 照合]\n",
+    "  C --> D[rnk=1 または order_cnt=max_cnt を抽出]\n",
+    "  D --> E[出力 customer_number]\n",
+    "```\n"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "4392213e",
+   "metadata": {},
+   "source": [
+    "## 結論（用途別ベスト）\n",
+    "\n",
+    "### 1) 最大が一意（本問題の前提）なら最短クエリが最速になりやすい\n",
+    "\n",
+    "```sql\n",
+    "SELECT customer_number\n",
+    "FROM Orders\n",
+    "GROUP BY customer_number\n",
+    "ORDER BY COUNT(*) DESC\n",
+    "LIMIT 1;\n",
+    "\n",
+    "Runtime 470 ms\n",
+    "Beats 41.18%\n",
+    "\n",
+    "```\n",
+    "\n",
+    "* 余計な CTE・結合・ウィンドウ不要。\n",
+    "* `GROUP BY` 後に件数降順で **先頭 1 件だけ**返すので、実装や実行計画的にもシンプル。\n",
+    "\n",
+    "### 2) 同数最大をすべて返したい（Follow-up 汎用）\n",
+    "\n",
+    "**ウィンドウ関数なし**で 1 クエリ：\n",
+    "\n",
+    "```sql\n",
+    "SELECT customer_number\n",
+    "FROM Orders\n",
+    "GROUP BY customer_number\n",
+    "HAVING COUNT(*) = (\n",
+    "  SELECT COUNT(*) AS mx\n",
+    "  FROM Orders\n",
+    "  GROUP BY customer_number\n",
+    "  ORDER BY mx DESC\n",
+    "  LIMIT 1\n",
+    ");\n",
+    "\n",
+    "Runtime 433 ms\n",
+    "Beats 72.43%\n",
+    "\n",
+    "```\n",
+    "\n",
+    "* 内側サブクエリで「最大件数」だけを 1 行取得 → 外側で一致フィルタ。\n",
+    "* あなたの `cnt→mx→JOIN` 版よりも結合が無いぶん軽くなることが多いです。\n",
+    "\n",
+    "### 3) ウィンドウ関数派（可読性重視）なら CTE をやめて 1 段で\n",
+    "\n",
+    "```sql\n",
+    "SELECT customer_number\n",
+    "FROM (\n",
+    "  SELECT\n",
+    "    customer_number,\n",
+    "    DENSE_RANK() OVER (ORDER BY COUNT(*) DESC) AS rnk\n",
+    "  FROM Orders\n",
+    "  GROUP BY customer_number\n",
+    ") t\n",
+    "WHERE rnk = 1;\n",
+    "\n",
+    "Runtime 461 ms\n",
+    "Beats 48.24%\n",
+    "\n",
+    "```\n",
+    "\n",
+    "* `COUNT(*)` を直接 `ORDER BY` に使い、そのまま `DENSE_RANK`。\n",
+    "* 中間 `cnt` CTE のマテリアライズを避けられる分だけ有利になる場合があります。\n",
+    "\n",
+    "---\n",
+    "\n",
+    "## 実務チューニングのヒント\n",
+    "\n",
+    "1. **インデックス**\n",
+    "\n",
+    "```sql\n",
+    "CREATE INDEX idx_orders_customer ON Orders(customer_number);\n",
+    "```\n",
+    "\n",
+    "* `GROUP BY customer_number` の集約が大幅にラクになります（全表スキャン回避/ソート削減）。\n",
+    "\n",
+    "2. **CTE は必要最小限に**\n",
+    "   MySQL 8 では CTE がマテリアライズされるケースがあり、単回参照の中間表は**派生表**に畳んだ方が速いことが多いです（上の「3）」がそれ）。\n",
+    "\n",
+    "3. **LeetCode の Runtime はノイズ大**\n",
+    "   実環境では **EXPLAIN** で実行計画を確認し、`rows` 見積もり・ファイルソート有無・テンポラリ使用などをチェックしてく さい。\n",
+    "\n",
+    "---\n",
+    "\n",
+    "## まとめ\n",
+    "\n",
+    "* **一意最大前提**なら ⇒ `GROUP BY ... ORDER BY COUNT(*) DESC LIMIT 1` が最有力。\n",
+    "* **同率最大も返す**なら ⇒ `HAVING COUNT(*) = (SELECT ... LIMIT 1)` がシンプル＆速いことが多い。\n",
+    "* **ウィンドウ採用**なら ⇒ 中間 CTE 省略の 1 段構成に。\n",
+    "* **物理対策** ⇒ `customer_number` にインデックス。\n"
+   ]
+  }
+ ],
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