MySQL之實(shí)用篇 MySQL之牛刀小試

子查詢是指出現(xiàn)在其他SQL語(yǔ)句內(nèi)的SELECT子句. 例如: SELECT * FROM t1 WHERE column1 = (SELECT column2 FROM t2) 其中 SELECT * FRIN t1 稱為outerQuery SELECT column2 FROM t2 稱為subQuery 注意:子查詢指嵌套在查詢內(nèi)部,且必須始終出現(xiàn)在圓括號(hào)內(nèi). 子查詢可以包含多個(gè)關(guān)鍵字或者條件,如 DISTINCT , GROUP BY, ORDER BY,函數(shù)等. 子查詢的外層查詢可以是SELECT , INSERT, UPDATE,SET 或DO. 子查詢可以返回標(biāo)量,一行,一列或子查詢.

###下面是我創(chuàng)建的goods表結(jié)構(gòu)以及添加的一些數(shù)據(jù)

###子查詢 ######查詢商品價(jià)格的平均值

SELECT AVG(goods_price) FROM goods 復(fù)制代碼

######查詢商品價(jià)格的平均值并對(duì)值進(jìn)行四舍五入取兩位小數(shù) SELECT ROUND(AVG(goods_price),2) FROM goods

######子查詢 查詢出價(jià)格大于平均值得商品 SELECT goods_id,goods_name,goods_price FROM goods WHERE goods_price > ANY (SELECT goods_price FROM goods WHERE goods_cate = '超級(jí)本') ORDER BY goods_price DESC SELECT goods_id,goods_name,goods_price FROM goods WHERE goods_price NOT IN (SELECT goods_price FROM goods WHERE goods_cate = '超級(jí)本') ORDER BY goods_price DESC

• AVG,MAX,MIN、COUNT、SUM為聚合函數(shù)(平均值,最大值,最小值,行數(shù),求和)
• ROUND 是四舍五入 2代表保留兩位小數(shù)
• 用ANY,SOME或ALL修飾比較運(yùn)算符 (ANY,SOME 代表的滿足任意一個(gè)就可以,ALL代表必須全部滿足)
• [NOT] IN ,[NOT] EXISTS

多表插入

INSERT [INTO] table_name[(column_name,....)] SELECT ...

再創(chuàng)建一個(gè)新的表

CREATE TABLE IF NOT EXISTS goods_cates(cate_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT, cate_name TEXT NOT NULL) 復(fù)制代碼

######查詢商品的分類 SELECT goods_cate FROM goods GROUP BY goods_cate ######插入所有商品的分類到goods_cate分類的表中 INSERT INTO goods_cates(cate_name) SELECT goods_cate FROM goods GROUP BY goods_cate ###多表更新 ######UPDATE table_references SET column_name1 = {expr1 | DEFAULT} [, column_name2 = {expr2 | DEFAULT}] ...... [WHERE where_condition]

for example:

UPDATE goods INNER JOIN goods_cates ON goods_cate = cate_name SET goods_cate = cate_id 復(fù)制代碼

CREATE ...SELECT

######創(chuàng)建數(shù)據(jù)表的同時(shí)將查詢結(jié)果寫入到數(shù)據(jù)表 CREATE TABLE [IF NOT EXISTS] table_name [(create_definition,....)] select_statement for example:

CREATE TABLE goods_brands (brand_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT, brand_name TEXT NOT NULL) SELECT brand_name FROM goods GROUP BY brand_name 復(fù)制代碼

######再進(jìn)行剛剛的多表更新 UPDATE goods INNER JOIN goods_brands ON goods.brand_name = goods_brands.brand_name SET goods.brand_name = goods_brands.brand_id ###連接 ######MySQL在SELECT 語(yǔ)句,多表更新,多表刪除語(yǔ)句中支持JOIN 操作 #####語(yǔ)法結(jié)構(gòu) table_reference {[INNER | CROSS] JOIN | {LEFT | RIGHT} [OUTER] JOIN} table_reference ON conditional_expr

A表 + 連接類型{INNER JOIN | LEFT JOIN | RIGHT JOIN} + B表 ON + 連接的條件

#####數(shù)據(jù)表參照 table_reference

table_name [[AS] alias] | table_subquery [AS] alias 復(fù)制代碼

• 數(shù)據(jù)表可以使用table_name AS alias_name 或 table_name alias_name 賦予別名
• table_subquery可以作為子查詢使用在FROM子句中,這樣的子查詢必須為其賦予別名 #####連接類型
• INNER JOIN (內(nèi)連接) 在MySQL中,JOIN , CROSS JOIN 和 INNER JOIN 是等價(jià)的
• LEFT [OUTER] JOIN (左外連接)
• RIGHT [OUTER] JOIN (右外連接) ######注意:使用ON關(guān)鍵字來設(shè)定連接條件,也可以使用WHERE來代替通常使用ON關(guān)鍵字來設(shè)定連接條件,使用WHERE關(guān)鍵字進(jìn)行結(jié)果集記錄的過濾 #####內(nèi)連接

for example:

SELECT goods_id, goods_name, cate_name FROM goods INNER JOIN goods_cates ON goods_cate = cate_id 復(fù)制代碼

###外鏈接

• 如果使用內(nèi)連接查找的記錄在連接數(shù)據(jù)表中不存在,并且在WHERE子句中嘗試一下操作: column_name IS NULL 時(shí),如果column_name被定義為NOT NULL, MySQL將在找到符合連接條件的記錄后停止搜索更多的行
• A LEFT JOIN B join_condition
• 數(shù)據(jù)表B的結(jié)果集依賴數(shù)據(jù)表A
• 數(shù)據(jù)表A的結(jié)果集根據(jù)左連接條件依賴所有數(shù)據(jù)表(B表除外)
• 左外連接條件決定如何縮減數(shù)據(jù)表B(在沒有指定WHERE條件的情況下)
• 如果數(shù)據(jù)表A的魔調(diào)記錄符合WHERE條件,但是在數(shù)據(jù)表B不存在連接條件的記錄,將會(huì)生成一個(gè)所有列為空的額外的B行 #####左外連接

for example:

SELECT goods_id, goods_name, cate_name FROM goods LEFT JOIN goods_cates ON goods_cate = cate_id 復(fù)制代碼

#####右外連接

for example:

SELECT goods_id, goods_name, cate_name FROM goods RIGHT 復(fù)制代碼

######多表連接 ######我們先把goods表的兩個(gè)字段名稱以及類型修改一下 ALTER TABLE goods CHANGE brand_name brand_id SMALLINT NOT NULL UNSIGNED ALTER TABLE goods CHANGE goods_cate cate_id SMALLINT NOT NULL UNSIGNED

for example :

SELECT goods_id,goods_name, goods_price, cate_name ,brand_name FROM goods AS good INNER JOIN goods_cates AS cate ON good.cate_id = cate.cate_id INNER JOIN goods_brands AS brand ON good.brand_id = brand.brand_id 復(fù)制代碼

###無(wú)限分類的數(shù)據(jù)表設(shè)計(jì) CREATE TABLE goods_types ( type_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMEN type_name TEXT NOT NULL, parent_id SMALLINT UNSIGNED NOT NULL DEFAULT 0 )

• INSERT goods_types(type_name,parent_id) VALUES('家用電器',DEFAULT);

• INSERT goods_types(type_name,parent_id) VALUES('電腦、辦公',DEFAULT);

• INSERT goods_types(type_name,parent_id) VALUES('大家電',1);

• INSERT goods_types(type_name,parent_id) VALUES('生活電器',1);

• INSERT goods_types(type_name,parent_id) VALUES('平板電視',3);

• INSERT goods_types(type_name,parent_id) VALUES('空調(diào)',3);

• INSERT goods_types(type_name,parent_id) VALUES('電風(fēng)扇',4);

• INSERT goods_types(type_name,parent_id) VALUES('飲水機(jī)',4);

• INSERT goods_types(type_name,parent_id) VALUES('電腦整機(jī)',2);

• INSERT goods_types(type_name,parent_id) VALUES('電腦配件',2);

• INSERT goods_types(type_name,parent_id) VALUES('筆記本',9);

• INSERT goods_types(type_name,parent_id) VALUES('超級(jí)本',9);

• INSERT goods_types(type_name,parent_id) VALUES('游戲本',9);

• INSERT goods_types(type_name,parent_id) VALUES('CPU',10);

• INSERT goods_types(type_name,parent_id) VALUES('主機(jī)',10);

######查詢父類下邊的子類 SELECT s.type_id,s.type_name,p.type_name FROM goods_types AS s LEFT JOIN goods_types AS p ON s.parent_id = p.type_id ######查父類下的父類的子類 SELECT p.type_id,p.type_name,s.type_name FROM goods_types AS p LEFT JOIN goods_types AS s ON s.parent_id = p.type_id GROUP BY p.type_name ORDER BY p.type_id

SELECT p.type_id,p.type_name,s.type_name child_count FROM goods_types AS p LEFT JOIN goods_types AS s ON s.parent_id = p.type_id 復(fù)制代碼

###多表刪除 DELETE table_name [.] [table_name[.]]..... FROM table_references [WHERE where_condition]

for example:

SELECT goods_id,goods_name FROM goods GROUP BY goods_name HAVING count(goods_name) >= 2;DELETE t1 FROM goods AS t1 LEFT JOIN (SELECT goods_id,goods_name FROM 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; 復(fù)制代碼

轉(zhuǎn)載于:https://juejin.im/post/5a31ea556fb9a045204c3b7e

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