/pre>pre name="code" class="sql">1.子查詢是指在另一個查詢語句中的SELECT子句。 
例句： 
SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2); 
其中，SELECT * FROM t1 ...稱為Outer Query[外查詢](或者Outer Statement), 
SELECT column1 FROM t2 稱為Sub Query[子查詢]。 
所以，我們說子查詢是嵌套在外查詢內(nèi)部。而事實(shí)上它有可能在子查詢內(nèi)部再嵌套子查詢。 
子查詢必須出現(xiàn)在圓括號之間。 
行級子查詢 
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); 
行級子查詢的返回結(jié)果最多為一行。 
優(yōu)化子查詢 
-- 創(chuàng)建數(shù)據(jù)表 
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 
); 
-- 寫入記錄 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('R510VC 15.6英寸筆記本','筆記本','華碩','3399',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Y400N 14.0英寸筆記本電腦','筆記本','聯(lián)想','4899',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('G150TH 15.6英寸游戲本','游戲本','雷神','8499',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X550CC 15.6英寸筆記本','筆記本','華碩','2799',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X240(20ALA0EYCD) 12.5英寸超極本','超級本','聯(lián)想','4999',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('U330P 13.3英寸超極本','超級本','聯(lián)想','4299',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('SVP13226SCB 13.3英寸觸控超極本','超級本','索尼','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英寸平板電腦','平板電腦','蘋果','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英寸平板電腦 （16G WiFi版）','平板電腦','蘋果','3388',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES(' iPad mini ME279CH/A 配備 Retina 顯示屏 7.9英寸平板電腦 （16G WiFi版）','平板電腦','蘋果','2788',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('IdeaCentre C340 20英寸一體電腦 ','臺式機(jī)','聯(lián)想','3499',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Vostro 3800-R1206 臺式電腦','臺式機(jī)','戴爾','2899',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iMac ME086CH/A 21.5英寸一體電腦','臺式機(jī)','蘋果','9188',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('AT7-7414LP 臺式電腦 （i5-3450四核 4G 500G 2G獨(dú)顯 DVD 鍵鼠 Linux ）','臺式機(jī)','宏碁','3699',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Z220SFF F4F06PA工作站','服務(wù)器/工作站','惠普','4288',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('PowerEdge T110 II服務(wù)器','服務(wù)器/工作站','戴爾','5388',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Mac Pro MD878CH/A 專業(yè)級臺式電腦','服務(wù)器/工作站','蘋果','28888',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES(' HMZ-T3W 頭戴顯示設(shè)備','筆記本配件','索尼','6999',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('商務(wù)雙肩背包','筆記本配件','索尼','99',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X3250 M4機(jī)架式服務(wù)器 2583i14','服務(wù)器/工作站','IBM','6888',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('玄龍精英版 筆記本散熱器','筆記本配件','九州風(fēng)神','',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES(' HMZ-T3W 頭戴顯示設(shè)備','筆記本配件','索尼','6999',DEFAULT,DEFAULT); 
INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('商務(wù)雙肩背包','筆記本配件','索尼','99',DEFAULT,DEFAULT); 
-- 求所有電腦產(chǎn)品的平均價格,并且保留兩位小數(shù)，AVG,MAX,MIN、COUNT、SUM為聚合函數(shù) 
SELECT ROUND(AVG(goods_price),2) AS avg_price FROM tdb_goods; 
-- 查詢所有價格大于平均價格的商品，并且按價格降序排序 
SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price > 5845.10 ORDER BY goods_price DESC; 
-- 使用子查詢來實(shí)現(xiàn) 
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; 
-- 查詢類型為“超記本”的商品價格 
SELECT goods_price FROM tdb_goods WHERE goods_cate = '超級本'; 
-- 查詢價格大于或等于"超級本"價格的商品，并且按價格降序排列 
SELECT goods_id,goods_name,goods_price FROM tdb_goods 
WHERE goods_price = ANY(SELECT goods_price FROM tdb_goods WHERE goods_cate = '超級本') 
ORDER BY goods_price DESC; 
-- = ANY 或 = SOME 等價于 IN 
SELECT goods_id,goods_name,goods_price FROM tdb_goods 
WHERE goods_price IN (SELECT goods_price FROM tdb_goods WHERE goods_cate = '超級本') 
ORDER BY goods_price DESC; 
-- 創(chuàng)建“商品分類”表 
CREATE TABLE IF NOT EXISTS tdb_goods_cates( 
cate_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT, 
cate_name VARCHAR(40) 
); 
-- 查詢tdb_goods表的所有記錄，并且按"類別"分組 
SELECT goods_cate FROM tdb_goods GROUP BY goods_cate; 
-- 將分組結(jié)果寫入到tdb_goods_cates數(shù)據(jù)表 
INSERT tdb_goods_cates (cate_name) SELECT goods_cate FROM tdb_goods GROUP BY goods_cate; 
-- 通過tdb_goods_cates數(shù)據(jù)表來更新tdb_goods表 
UPDATE tdb_goods INNER JOIN tdb_goods_cates ON goods_cate = cate_name 
SET goods_cate = cate_id ; 
-- 通過CREATE...SELECT來創(chuàng)建數(shù)據(jù)表并且同時寫入記錄 
-- 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; 
-- 通過tdb_goods_brands數(shù)據(jù)表來更新tdb_goods數(shù)據(jù)表(錯誤) 
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 
-- 正確 
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的數(shù)據(jù)表結(jié)構(gòu) 
DESC tdb_goods; 
-- 通過ALTER TABLE語句修改數(shù)據(jù)表結(jié)構(gòu) 
ALTER TABLE tdb_goods 
CHANGE goods_cate cate_id SMALLINT UNSIGNED NOT NULL, 
CHANGE brand_name brand_id SMALLINT UNSIGNED NOT NULL; 
-- 分別在tdb_goods_cates和tdb_goods_brands表插入記錄 
INSERT tdb_goods_cates(cate_name) VALUES('路由器'),('交換機(jī)'),('網(wǎng)卡'); 
INSERT tdb_goods_brands(brand_name) VALUES('海爾'),('清華同方'),('神舟'); 
-- 在tdb_goods數(shù)據(jù)表寫入任意記錄 
INSERT tdb_goods(goods_name,cate_id,brand_id,goods_price) VALUES(' LaserJet Pro P1606dn 黑白激光打印機(jī)','12','4','1849'); 
-- 查詢所有商品的詳細(xì)信息(通過內(nèi)連接實(shí)現(xiàn)) 
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; 
-- 查詢所有商品的詳細(xì)信息(通過左外連接實(shí)現(xiàn)) 
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; 
-- 查詢所有商品的詳細(xì)信息(通過右外連接實(shí)現(xiàn)) 
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; 
-- 無限分類的數(shù)據(jù)表設(shè)計 
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('家用電器',DEFAULT); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('電腦、辦公',DEFAULT); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('大家電',1); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('生活電器',1); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('平板電視',3); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('空調(diào)',3); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('電風(fēng)扇',4); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('飲水機(jī)',4); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('電腦整機(jī)',2); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('電腦配件',2); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('筆記本',9); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('超級本',9); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('游戲本',9); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('CPU',10); 
INSERT tdb_goods_types(type_name,parent_id) VALUES('主機(jī)',10); 
-- 查找所有分類及其父類 
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; 
-- 查找所有分類及其子類 
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; 
-- 查找所有分類及其子類的數(shù)目 
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; 
-- 為tdb_goods_types添加child_count字段 
ALTER TABLE tdb_goods_types ADD child_count MEDIUMINT UNSIGNED NOT NULL DEFAULT 0; 
-- 將剛才查詢到的子類數(shù)量更新到tdb_goods_types數(shù)據(jù)表 
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; 
-- 復(fù)制編號為12,20的兩條記錄 
SELECT * FROM tdb_goods WHERE goods_id IN (19,20); 
-- INSERT ... SELECT實(shí)現(xiàn)復(fù)制 
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); 
-- 查找重復(fù)記錄 
SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name HAVING count(goods_name) >= 2; 
-- 刪除重復(fù)記錄 
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;