</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部。而事實上它有可能在子查詢內部再嵌套子查詢。 子查詢必須出現在圓括號之間。 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果最多為一行。 優化子查詢 -- 創建數據表 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','聯想','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','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','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獨顯 DVD 鍵鼠 Linux ）','臺式機','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服務器','服務器/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 專業級臺式電腦','服務器/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機架式服務器 2583i14','服務器/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('商務雙肩背包','筆記本配件','索尼','99',DEFAULT,DEFAULT); -- 求所有電腦產品的平均價格,并且保留兩位小數，AVG,MAX,MIN、COUNT、SUM為聚合函數 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; -- 使用子查詢來實現 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; -- 將分組結果寫入到tdb_goods_cates數據表 INSERT tdb_goods_cates (cate_name) SELECT goods_cate FROM tdb_goods GROUP BY goods_cate; -- 通過tdb_goods_cates數據表來更新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來創建數據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數據表來更新tdb_goods數據表(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的數據表結構 DESC tdb_goods; -- 通過ALTER TABLE語句修改數據表結構 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'),('網卡'); INSERT tdb_goods_brands(brand_name) VALUES('海爾'),('清華同方'),('神舟'); -- 在tdb_goods數據表寫入任意記錄 INSERT tdb_goods(goods_name,cate_id,brand_id,goods_price) VALUES(' LaserJet Pro P1606dn 黑白激光打印機','12','4','1849'); -- 查詢所有商品的詳細信息(通過內連接實現) 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信息(通過左外連接實現) 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信息(通過右外連接實現) 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; -- 無限分類的數據表設計 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',3); INSERT tdb_goods_types(type_name,parent_id) VALUES('電風扇',4); INSERT tdb_goods_types(type_name,parent_id) VALUES('飲水機',4); INSERT tdb_goods_types(type_name,parent_id) VALUES('電腦整機',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',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量更新到tdb_goods_types數據表 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; -- 復制編號為12,20的兩條記錄 SELECT * FROM tdb_goods WHERE goods_id IN (19,20); -- INSERT ... SELECT實現復制 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); -- 查找重復記錄 SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name HAVING count(goods_name) >= 2; -- 刪除重復記錄 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;