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+# 💎 水晶男孩推廣部：SQL 三日修煉手冊
+
+> **任務清單 (SOP)**
+> * style(file): 檔案命名為 .md
+> * fix(db): 確保最後不帶多餘逗號
+> * perf(save): 隨手 Ctrl + S 存檔
+
+---
+
+## 📅 DAY 1：連連看基礎 (JOIN)
+**邏輯**：用 album_id 把「歌曲」與「專輯」黏起來。
+### 1. 傳統連表 (WHERE)
+```sql
+SELECT songs.title, albums.title 
+FROM songs, albums 
+WHERE songs.album_id = albums.id 
+ORDER BY songs.id;
+```
+
+### 2. 標準內連線 (INNER JOIN)
+```sql
+SELECT songs.title, albums.title 
+FROM songs
+INNER JOIN albums ON songs.album_id = albums.id 
+ORDER BY songs.id;
+```
+### 3. 左外連線 (LEFT JOIN)
+```sql
+SELECT songs.title, albums.title 
+FROM songs
+LEFT JOIN albums ON songs.album_id = albums.id;
+```
+### 4. 右外連線 (RIGHT JOIN)
+```
+SELECT songs.title, albums.title 
+FROM songs
+RIGHT JOIN albums ON songs.album_id = albums.id;
+```
+## 📅 DAY 2：資料美化與彙整
+**邏輯**：處理空值與統計數量。
+
+### 1. 處理空值 (IFNULL)
+```sql
+SELECT 
+    IFNULL(songs.title, '未命名') AS '歌名',
+    IFNULL(albums.title, '未知') AS '專輯'
+FROM songs
+RIGHT JOIN albums ON songs.album_id = albums.id;
+```
+### 2. 分組統計 (COUNT)
+```sql
+SELECT 
+    albums.title AS '專輯名稱',
+    COUNT(songs.id) AS '歌曲總數'
+FROM albums
+LEFT JOIN songs ON albums.id = songs.album_id
+GROUP BY albums.id;
+```
+## 📅 DAY 3：魔王級子查詢 (Subquery)
+
+### 1. 巢狀條件查詢
+```sql
+SELECT * FROM songs 
+WHERE album_id = (SELECT id FROM albums WHERE title = 'Road Fighter');
+```
+-- 在每首歌後面顯示目前所有歌曲 album_id 的平均值（對比用）
+
+### 2.多表關聯查詢
+```sql
+-- 邏輯：同時連三張表（歌曲、專輯、成員）
+SELECT 
+    songs.title AS '歌名',
+    albums.title AS '專輯名稱',
+    albums.release_date AS '發行日期'
+FROM 
+    songs, 
+    albums
+WHERE 
+    songs.album_id = albums.id AND 
+    albums.id = 1; -- 這裡的 1 代表第一張專輯（對應老師的 dept=1）
+
+```
+### 3. 計算式子查詢
+```sql
+SELECT 
+    title, 
+    album_id, 
+    (SELECT AVG(album_id) FROM songs) AS '全庫平均編號'
+FROM 
+    songs;
+```
+
+
+## 🛠 努比的開發 SOP 筆記
+- **feat(db)**: 修改語法後先按 `Ctrl + S` 存檔。
+- **style(code)**: 關鍵字大寫，欄位名小寫。
+- **fix(db)**: 確保最後一個欄位沒有多餘的逗號。
+- **perf(note)**: 執行成功後，截圖貼到你的筆記網站。
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