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Non ci dilunghiamo, passiamo direttamente all'uso delle sottoquery nel database MySQL.
Il codice è descritto come segue:
</pre><pre name="code" class="sql">1. Una sottoquery è una clausola SELECT in un'altra query.
Esempio:
SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);
Tra cui, SELECT * FROM t1 ...si chiama Outer Query[外查询](o Outer Statement),
SELECT column1 FROM t2 si chiama Sub Query[子查询].
Quindi, diciamo che una sottoquery è嵌套在内查询内部。In effetti, potrebbe esserci una sottoquery ulteriore all'interno di una sottoquery.
La sottoquery deve apparire tra parentesi tonde.
Sottoquery a livello di riga
SELECT * FROM t1 WHERE (col1,col2) = (SELECT col3, col4 FROM t2 WHERE id = 10);
SELECT * FROM t1 WHERE ROW(col1,col2) = (SELECT col3, col4 FROM t2 WHERE id = 10);
Il risultato della sottoquery a livello di riga può essere al massimo una riga.
Ottimizzazione delle sottoquery
-- Creazione della tabella dei dati
CREATE TABLE IF NOT EXISTS tdb_goods,
goods_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
goods_name VARCHAR(150) NOT NULL,
goods_cate VARCHAR(40) NOT NULL,
brand_name VARCHAR(40) NOT NULL,
goods_price DECIMAL(15,3) UNSIGNED NOT NULL DEFAULT 0,
is_show BOOLEAN NOT NULL DEFAULT 1,
is_saleoff BOOLEAN NOT NULL DEFAULT 0
);
-- Inserimento dei record
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('R510VC 15.6 pollici laptop','laptop','Asus','3399',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Y400N 14.0 pollici laptop','laptop','Lenovo','4899',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('G150TH 15.6 pollici laptop per gioco','laptop per gioco','Lei Shen','8499',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X550CC 15.6 pollici notebook','notebook','ASUS','2799',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X240(20ALA0EYCD) 12.5 pollici ultra-thin laptop','ultra-thin laptop','Lenovo','4999',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('U330P 13.3 pollici ultra-thin laptop','ultra-thin laptop','Lenovo','4299',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('SVP13226SCB 13.3 pollici touch ultra-thin laptop','ultra-thin laptop','Sony','7999',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iPad mini MD531CH/A 7.9 pollici tablet computer','tablet computer','Apple','1998',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iPad Air MD788CH/A 9.7 pollici tablet computer (16G WiFi version)','tablet computer','Apple','3388',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iPad mini ME279CH/A with Retina Display 7.9-inch Tablet Computer (16GB WiFi Version)','Tablet Computer','Apple','2788',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('IdeaCentre C340 20-inch All-in-One Computer ','Desktop Computer','Lenovo','3499',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Vostro 3800-R1206 Desktop Computer','Desktop Computer','Dell','2899',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iMac ME086CH/A 21.5-inch All-in-One Computer','Desktop Computer','Apple','9188',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('AT7-7414LP Desktop Computer (i5-3450 Quad-Core 4G 500G 2GB Dedicated Graphics DVD Mouse Linux)','Desktop Computer','Acer','3699',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Z220SFF F4F06PA Workstation','Server/Workstation','HP','4288',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('PowerEdge T110 II服务器','服务器/工作站','戴尔','5388',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Mac Pro MD878CH/A 专业级台式电脑','服务器/工作站','苹果','28888',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('HMZ-T3W 头戴显示设备','笔记本配件','索尼','6999',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('商务双肩背包','笔记本配件','索尼','99',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X3250 M4机架式服务器 2583i14','服务器/工作站','IBM','6888',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('玄龙精英版 笔记本散热器','笔记本配件','九州风神','',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('HMZ-T3W 头戴显示设备','笔记本配件','索尼','6999',DEFAULT,DEFAULT);
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('商务双肩背包','笔记本配件','索尼','99',DEFAULT,DEFAULT);
-- Calculate the average price of all computer products and keep two decimal places, AVG, MAX, MIN, COUNT, SUM are aggregate functions
SELECT ROUND(AVG(goods_price),2) AS avg_price FROM tdb_goods;
-- Query all products with prices greater than the average price and sort by price in descending order
SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price > 5845.10 ORDER BY goods_price DESC;
-- Implement using a subquery
SELECT goods_id,goods_name,goods_price FROM tdb_goods
WHERE goods_price > (SELECT ROUND(AVG(goods_price),2) AS avg_price FROM tdb_goods)
ORDER BY goods_price DESC;
-- Query the price of products of type "Superbook"
SELECT goods_price FROM tdb_goods WHERE goods_cate = 'Superbook';
-- Query products with prices greater than or equal to the price of "Superbook" and sort by price in descending order
SELECT goods_id,goods_name,goods_price FROM tdb_goods
WHERE goods_price = ANY(SELECT goods_price FROM tdb_goods WHERE goods_cate = 'Superbook')
ORDER BY goods_price DESC;
-- = ANY or = SOME is equivalent to IN
SELECT goods_id,goods_name,goods_price FROM tdb_goods
WHERE goods_price IN (SELECT goods_price FROM tdb_goods WHERE goods_cate = 'Superbook')
ORDER BY goods_price DESC;
-- Create the "Product Category" table
CREATE TABLE IF NOT EXISTS tdb_goods_cates(
cate_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
cate_name VARCHAR(40)
);
-- Query all records from the tdb_goods table and group by "category"
SELECT goods_cate FROM tdb_goods GROUP BY goods_cate;
-- 将分组结果写入到tdb_goods_cates数据表
INSERT tdb_goods_cates (cate_name) SELECT goods_cate FROM tdb_goods GROUP BY goods_cate;
-- 通过tdb_goods_cates数据表来更新tdb_goods表
UPDATE tdb_goods INNER JOIN tdb_goods_cates ON goods_cate = cate_name
SET goods_cate = cate_id ;
-- 通过CREATE...SELECT来创建数据表并且同时写入记录
-- SELECT brand_name FROM tdb_goods GROUP BY brand_name;
CREATE TABLE tdb_goods_brands (
brand_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
brand_name VARCHAR(40) NOT NULL
) SELECT brand_name FROM tdb_goods GROUP BY brand_name;
-- 通过tdb_goods_brands数据表来更新tdb_goods数据表(错误)
UPDATE tdb_goods INNER JOIN tdb_goods_brands ON brand_name = brand_name
SET brand_name = brand_id;
-- Column 'brand_name' in field list is ambigous
-- 正确
UPDATE tdb_goods AS g INNER JOIN tdb_goods_brands AS b ON g.brand_name = b.brand_name
SET g.brand_name = b.brand_id;
-- 查看tdb_goods的数据表结构
DESC tdb_goods;
-- 通过ALTER TABLE语句修改数据表结构
ALTER TABLE tdb_goods
CHANGE goods_cate cate_id SMALLINT UNSIGNED NOT NULL,
CHANGE brand_name brand_id SMALLINT UNSIGNED NOT NULL;
-- Inserimento di record nelle tabelle tdb_goods_cates e tdb_goods_brands
INSERT tdb_goods_cates(cate_name) VALUES('Router'),('Switch'),('Network card');
INSERT tdb_goods_brands(brand_name) VALUES('Haier'),('Tianjin University of Technology'),('Hasee');
-- Inserimento di una qualsiasi registrazione nella tabella tdb_goods
INSERT tdb_goods(goods_name,cate_id,brand_id,goods_price) VALUES(' LaserJet Pro P1606dn Stampante laser a colori monocromatica','12','4','1849');
-- Ricerca di dettagli di tutti i prodotti (realizzata tramite un join interno)
SELECT goods_id,goods_name,cate_name,brand_name,goods_price FROM tdb_goods AS g
INNER JOIN tdb_goods_cates AS c ON g.cate_id = c.cate_id
INNER JOIN tdb_goods_brands AS b ON g.brand_id = b.brand_id\G;
-- Ricerca di dettagli di tutti i prodotti (realizzata tramite un join esterno a sinistra)
SELECT goods_id,goods_name,cate_name,brand_name,goods_price FROM tdb_goods AS g
LEFT JOIN tdb_goods_cates AS c ON g.cate_id = c.cate_id
LEFT JOIN tdb_goods_brands AS b ON g.brand_id = b.brand_id\G;
-- Ricerca di dettagli di tutti i prodotti (realizzata tramite un join esterno a destra)
SELECT goods_id,goods_name,cate_name,brand_name,goods_price FROM tdb_goods AS g
RIGHT JOIN tdb_goods_cates AS c ON g.cate_id = c.cate_id
RIGHT JOIN tdb_goods_brands AS b ON g.brand_id = b.brand_id\G;
-- Progettazione di tabelle di dati a classificazione infinita
CREATE TABLE tdb_goods_types(
type_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
type_name VARCHAR(20) NOT NULL,
parent_id SMALLINT UNSIGNED NOT NULL DEFAULT 0
);
INSERT tdb_goods_types(type_name,parent_id) VALUES('家用电器',DEFAULT);
INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑、办公',DEFAULT);
INSERT tdb_goods_types(type_name,parent_id) VALUES('大家电',1);
INSERT tdb_goods_types(type_name,parent_id) VALUES('生活电器',1);
INSERT tdb_goods_types(type_name,parent_id) VALUES('平板电视',3);
INSERT tdb_goods_types(type_name,parent_id) VALUES('空调',3);
INSERT tdb_goods_types(type_name,parent_id) VALUES('电风扇',4);
INSERT tdb_goods_types(type_name,parent_id) VALUES('饮水机',4);
INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑整机',2);
INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑配件',2);
INSERT tdb_goods_types(type_name,parent_id) VALUES('笔记本电脑',9);
INSERT tdb_goods_types(type_name,parent_id) VALUES('超级本',9);
INSERT tdb_goods_types(type_name,parent_id) VALUES('游戏本',9);
INSERT INTO tdb_goods_types(type_name,parent_id) VALUES('CPU',10);
INSERT INTO tdb_goods_types(type_name,parent_id) VALUES('主机',10);
-- Trova tutte le categorie e i loro tipi genitori
SELECT s.type_id,s.type_name,p.type_name FROM tdb_goods_types AS s LEFT JOIN tdb_goods_types AS p ON s.parent_id = p.type_id;
-- Trova tutte le categorie e i loro sottotipi
SELECT p.type_id,p.type_name,s.type_name FROM tdb_goods_types AS p LEFT JOIN tdb_goods_types AS s ON s.parent_id = p.type_id;
-- Trova il numero di sottocategorie per tutte le categorie
SELECT p.type_id,p.type_name,count(s.type_name) AS children_count FROM tdb_goods_types AS p LEFT JOIN tdb_goods_types AS s ON s.parent_id = p.type_id GROUP BY p.type_name ORDER BY p.type_id;
-- Aggiungi il campo child_count a tdb_goods_types
ALTER TABLE tdb_goods_types ADD child_count MEDIUMINT UNSIGNED NOT NULL DEFAULT 0;
-- Aggiorna il numero di sotto tipi trovato nella tabella tdb_goods_types
UPDATE tdb_goods_types AS t1 INNER JOIN ( SELECT p.type_id,p.type_name,count(s.type_name) AS children_count FROM tdb_goods_types AS p
LEFT JOIN tdb_goods_types AS s ON s.parent_id = p.type_id
GROUP BY p.type_name
ORDER BY p.type_id ) AS t2
ON t1.type_id = t2.type_id
SET t1.child_count = t2.children_count;
-- Copia dei record con numeri di identificazione 12,20
SELECT * FROM tdb_goods WHERE goods_id IN (19,20);
-- Implementazione di INSERT ... SELECT per la copia
INSERT tdb_goods(goods_name,cate_id,brand_id) SELECT goods_name,cate_id,brand_id FROM tdb_goods WHERE goods_id IN (19,20);
-- Ricerca dei record duplicati
SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name HAVING count(goods_name) >= 2;
-- Eliminazione dei record duplicati
DELETE t1 FROM tdb_goods AS t1 LEFT JOIN (SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name HAVING count(goods_name) >= 2 ) AS t2 ON t1.goods_name = t2.goods_name WHERE t1.goods_id > t2.goods_id;
Bene, vi ho presentato tutto ciò che c'è da sapere sull'uso delle sottoquery in MySQL, spero che sia stato utile!