MySQL 經典查詢案例集：從最大值、分組到外鍵關聯實戰

範例資料準備

CREATE TABLE shop (
  article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
  dealer  CHAR(20) DEFAULT '' NOT NULL,
  price   DOUBLE(16,2) DEFAULT '0.00' NOT NULL,
  PRIMARY KEY (article, dealer)
);
INSERT INTO shop VALUES
(1,'A',3.45),(1,'B',3.99),(2,'A',10.99),
(3,'B',1.45),(3,'C',1.69),(3,'D',1.25),
(4,'D',19.95);

案例 1：單列最大值

SELECT MAX(article) AS article FROM shop;
-- 結果：4

案例 2：擁有最大值的完整行（最貴商品）

MySQL 早期不支援子查詢，兩種替代方案：

-- 方案 A：兩步驟
SELECT MAX(price) FROM shop;  -- 得到 19.95
SELECT article, dealer, price FROM shop WHERE price = 19.95;

-- 方案 B：排序 + LIMIT (單行)
SELECT article, dealer, price FROM shop ORDER BY price DESC LIMIT 1;
-- 注意：若多行並列最大值，僅返回其中一行

案例 3：分組最大值（各商品最高價）

SELECT article, MAX(price) AS price FROM shop GROUP BY article;
-- 結果：1→3.99, 2→10.99, 3→1.69, 4→19.95

案例 4：分組最大值對應的完整行（各商品最貴經銷商）

標準 SQL 子查詢寫法：

SELECT article, dealer, price FROM shop s1
WHERE price = (SELECT MAX(s2.price) FROM shop s2 WHERE s1.article = s2.article);

MySQL 早期無子查詢，臨時表方案：

CREATE TEMPORARY TABLE tmp (
  article INT(4) UNSIGNED ZEROFILL NOT NULL,
  price   DOUBLE(16,2) NOT NULL
);
LOCK TABLES shop READ;
INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article;
SELECT shop.article, shop.dealer, shop.price
FROM shop, tmp
WHERE shop.article = tmp.article AND shop.price = tmp.price;
UNLOCK TABLES;
DROP TABLE tmp;

單查詢「MAX-CONCAT 技巧」（效能較差，僅供參考）：

SELECT article,
  SUBSTRING(MAX(CONCAT(LPAD(price,6,'0'), dealer)), 7) AS dealer,
  0.00 + LEFT(MAX(CONCAT(LPAD(price,6,'0'), dealer)), 6) AS price
FROM shop GROUP BY article;

案例 5：外鍵關聯查詢（人員-襯衫）

CREATE TABLE persons (
  id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
  name CHAR(60) NOT NULL
);
CREATE TABLE shirts (
  id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
  style ENUM('t-shirt','polo','dress') NOT NULL,
  color ENUM('red','blue','orange','white','black') NOT NULL,
  owner SMALLINT UNSIGNED NOT NULL REFERENCES persons(id)
);

INSERT INTO persons VALUES (NULL,'Antonio Paz');
INSERT INTO shirts VALUES (NULL,'polo','blue',LAST_INSERT_ID()),
                         (NULL,'dress','white',LAST_INSERT_ID()),
                         (NULL,'t-shirt','blue',LAST_INSERT_ID());
INSERT INTO persons VALUES (NULL,'Lilliana Angelovska');
INSERT INTO shirts VALUES (NULL,'dress','orange',LAST_INSERT_ID()),
                         (NULL,'polo','red',LAST_INSERT_ID()),
                         (NULL,'dress','blue',LAST_INSERT_ID()),
                         (NULL,'t-shirt','white',LAST_INSERT_ID());

-- 查詢 Lilliana 的非白色襯衫
SELECT s.* FROM persons p, shirts s
WHERE p.name LIKE 'Lilliana%' AND s.owner = p.id AND s.color <> 'white';

結果：橙色連衣裙、紅色馬球衫、藍色連衣裙（3 行）。

核心啟示

• MySQL 早期版本以臨時表、連接技巧彌補子查詢缺失；5.0+ 完整支援子查詢，請優先使用標準寫法。
• `LAST_INSERT_ID()` 確保主外鍵關聯插入原子性。
• `ENUM` 型別節省空間並隱含檢查約束。
• 外鍵 `REFERENCES` 子句在 MyISAM 中僅作文檔，InnoDB 才強制執行。