MySQL 常用查詢範例精選：最大值、分組統計、子查詢替代方案與外鍵實戰

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);

3.1 單欄最大值

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

3.2 擁有最大值的完整列（最貴商品）

ANSI SQL 子查詢寫法（MySQL 早期版本不支援）：

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

MySQL 相容寫法（兩步驟）：

1. 先查最大值：SELECT MAX(price) FROM shop; -- 19.95
2. 再查明細：SELECT article, dealer, price FROM shop WHERE price = 19.95;

技巧寫法（ORDER BY + LIMIT）：

SELECT article, dealer, price
FROM shop
ORDER BY price DESC
LIMIT 1;

注意： 若有多筆相同最高價，LIMIT 1 僅回傳其中一筆。

3.3 分組最大值（每種商品最高價）

SELECT article, MAX(price) AS price
FROM shop
GROUP BY article;
-- 結果：0001→3.99, 0002→10.99, 0003→1.69, 0004→19.95

3.4 分組最大值對應的完整列（每種商品最貴的經銷商）

標準做法（臨時表）：

CREATE TEMPORARY TABLE tmp (
  article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
  price   DOUBLE(16,2) DEFAULT '0.00' 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;

單一查詢技巧（MAX-CONCAT Trick，效能較差但單指令）：

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;

原理：將價格左補零至固定寬度 6 位與經銷商名串聯，MAX 取字典序最大者（即最高價），再拆解還原。

3.5 外鍵關聯查詢（無需資料庫層面強制外鍵）

CREATE TABLE persons (
  id   SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
  name CHAR(60) NOT NULL,
  PRIMARY KEY (id)
);

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

-- 插入測試資料
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 擁有的非白色襯衫
SELECT s.* FROM persons p, shirts s
WHERE p.name LIKE 'Lilliana%' AND s.owner = p.id AND s.color <> 'white';

關鍵點： MySQL 不強制檢查外鍵存在性、不支援級聯刪除，但應用層正確使用 LAST_INSERT_ID() 維護關聯即可正常運作。