陣列遍歷forEach,for in,for of和for優缺點
阿新 • • 發佈：2020-09-08
目錄
一：介紹
多表連線查詢
複合條件查詢
子查詢
準備表
#建表 create table department( id int, name varchar(20) ); create table employee( id int primary key auto_increment, name varchar(20), sex enum('male','female') not null default 'male', age int, dep_id int ); #插入資料 insert into department values (200,'技術'), (201,'人力資源'), (202,'銷售'), (203,'運營'); insert into employee(name,sex,age,dep_id) values ('egon','male',18,200), ('alex','female',48,201), ('wupeiqi','male',38,201), ('yuanhao','female',28,202), ('liwenzhou','male',18,200), ('jingliyang','female',18,204) ; #查看錶結構和資料 mysql> desc department; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ mysql> desc employee; +--------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | YES | | NULL | | | sex | enum('male','female') | NO | | male | | | age | int(11) | YES | | NULL | | | dep_id | int(11) | YES | | NULL | | +--------+-----------------------+------+-----+---------+----------------+ mysql> select * from department; +------+--------------+ | id | name | +------+--------------+ | 200 | 技術 | | 201 | 人力資源 | | 202 | 銷售 | | 203 | 運營 | +------+--------------+ mysql> select * from employee; +----+------------+--------+------+--------+ | id | name | sex | age | dep_id | +----+------------+--------+------+--------+ | 1 | egon | male | 18 | 200 | | 2 | alex | female | 48 | 201 | | 3 | wupeiqi | male | 38 | 201 | | 4 | yuanhao | female | 28 | 202 | | 5 | liwenzhou | male | 18 | 200 | | 6 | jingliyang | female | 18 | 204 | +----+------------+--------+------+--------+ 表department與employee
二：多表聯合查詢
一：多表聯合語法
SELECT 欄位列表
FROM 表1 INNER|LEFT|RIGHT JOIN 表2
ON 表1.欄位 = 表2.欄位;
二：交叉連線：不適用於任何匹配條件。生成笛卡爾積
mysql> select * from employee,department; +----+------------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+------------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技術 | | 1 | egon | male | 18 | 200 | 201 | 人力資源 | | 1 | egon | male | 18 | 200 | 202 | 銷售 | | 1 | egon | male | 18 | 200 | 203 | 運營 | | 2 | alex | female | 48 | 201 | 200 | 技術 | | 2 | alex | female | 48 | 201 | 201 | 人力資源 | | 2 | alex | female | 48 | 201 | 202 | 銷售 | | 2 | alex | female | 48 | 201 | 203 | 運營 | | 3 | wupeiqi | male | 38 | 201 | 200 | 技術 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力資源 | | 3 | wupeiqi | male | 38 | 201 | 202 | 銷售 | | 3 | wupeiqi | male | 38 | 201 | 203 | 運營 | | 4 | yuanhao | female | 28 | 202 | 200 | 技術 | | 4 | yuanhao | female | 28 | 202 | 201 | 人力資源 | | 4 | yuanhao | female | 28 | 202 | 202 | 銷售 | | 4 | yuanhao | female | 28 | 202 | 203 | 運營 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技術 | | 5 | liwenzhou | male | 18 | 200 | 201 | 人力資源 | | 5 | liwenzhou | male | 18 | 200 | 202 | 銷售 | | 5 | liwenzhou | male | 18 | 200 | 203 | 運營 | | 6 | jingliyang | female | 18 | 204 | 200 | 技術 | | 6 | jingliyang | female | 18 | 204 | 201 | 人力資源 | | 6 | jingliyang | female | 18 | 204 | 202 | 銷售 | | 6 | jingliyang | female | 18 | 204 | 203 | 運營 | +----+------------+--------+------+--------+------+--------------+
三：內連線，只連線匹配的行
#找兩張表共有的部分，相當於利用條件從笛卡爾積結果中篩選出了正確的結果 #department沒有204這個部門，因而employee表中關於204這條員工資訊沒有匹配出來 mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id; +----+-----------+------+--------+--------------+ | id | name | age | sex | name | +----+-----------+------+--------+--------------+ | 1 | egon | 18 | male | 技術 | | 2 | alex | 48 | female | 人力資源 | | 3 | wupeiqi | 38 | male | 人力資源 | | 4 | yuanhao | 28 | female | 銷售 | | 5 | liwenzhou | 18 | male | 技術 | +----+-----------+------+--------+--------------+ #上述sql等同於 mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;
四：外連線之左連線：優先顯示左表全部記錄
#以左表為準，即找出所有員工資訊，當然包括沒有部門的員工
#本質就是：在內連線的基礎上增加左邊有右邊沒有的結果
mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;
+----+------------+--------------+
| id | name | depart_name |
+----+------------+--------------+
| 1 | egon | 技術 |
| 5 | liwenzhou | 技術 |
| 2 | alex | 人力資源 |
| 3 | wupeiqi | 人力資源 |
| 4 | yuanhao | 銷售 |
| 6 | jingliyang | NULL |
+----+------------+--------------+
五：外連線之右連線，優先顯示右表全部記錄
#以右表為準，即找出所有部門資訊，包括沒有員工的部門
#本質就是：在內連線的基礎上增加右邊有左邊沒有的結果
mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;
+------+-----------+--------------+
| id | name | depart_name |
+------+-----------+--------------+
| 1 | egon | 技術 |
| 2 | alex | 人力資源 |
| 3 | wupeiqi | 人力資源 |
| 4 | yuanhao | 銷售 |
| 5 | liwenzhou | 技術 |
| NULL | NULL | 運營 |
+------+-----------+--------------+
六：全外連線：顯示左右兩個表全部記錄
全外連線：在內連線的基礎上增加左邊有右邊沒有的和右邊有左邊沒有的結果
#注意：mysql不支援全外連線 full JOIN
#強調：mysql可以使用此種方式間接實現全外連線
select * from employee left join department on employee.dep_id = department.id
union
select * from employee right join department on employee.dep_id = department.id
;
#檢視結果
+------+------------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+------+------------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技術 |
| 5 | liwenzhou | male | 18 | 200 | 200 | 技術 |
| 2 | alex | female | 48 | 201 | 201 | 人力資源 |
| 3 | wupeiqi | male | 38 | 201 | 201 | 人力資源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 銷售 |
| 6 | jingliyang | female | 18 | 204 | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | 203 | 運營 |
+------+------------+--------+------+--------+------+--------------+
#注意 union與union all的區別：union會去掉相同的紀錄
三：符合條件連線查詢
#示例1：以內連線的方式查詢employee和department表，並且employee表中的age欄位值必須大於25,即找出年齡大於25歲的員工以及員工所在的部門
select employee.name,department.name from employee inner join department
on employee.dep_id = department.id
where age > 25;
#示例2：以內連線的方式查詢employee和department表，並且以age欄位的升序方式顯示
select employee.id,employee.name,employee.age,department.name from employee,department
where employee.dep_id = department.id
and age > 25
order by age asc;
四：子查詢
1：子查詢是將一個查詢語句巢狀在另一個查詢語句中。
2：內層查詢語句的查詢結果，可以為外層查詢語句提供查詢條件。
3：子查詢中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等關鍵字
4：還可以包含比較運算子：= 、 !=、> 、<等
一：帶IN關鍵字的子查詢
#查詢平均年齡在25歲以上的部門名
select id,name from department
where id in
(select dep_id from employee group by dep_id having avg(age) > 25);
#檢視技術部員工姓名
select name from employee
where dep_id in
(select id from department where name='技術');
#檢視不足1人的部門名(子查詢得到的是有人的部門id)
select name from department where id not in (select distinct dep_id from employee);
注意問題
not in 無法處理null的值，即子查詢中如果存在null的值，not in將無法處理，如下
mysql> select * from emp;
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
| 7 | xxx | male | 19 | NULL |
+----+------------+--------+------+--------+
7 rows in set (0.00 sec)
mysql> select * from dep;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技術 |
| 201 | 人力資源 |
| 202 | 銷售 |
| 203 | 運營 |
+------+--------------+
4 rows in set (0.00 sec)
# 子查詢中存在null
mysql> select * from dep where id not in (select distinct dep_id from emp);
Empty set (0.00 sec)
# 解決方案如下
mysql> select * from dep where id not in (select distinct dep_id from emp where dep_id is not null);
+------+--------+
| id | name |
+------+--------+
| 203 | 運營 |
+------+--------+
1 row in set (0.00 sec)
mysql>
！！！注意not in
二：帶ANY關鍵字的子查詢
#在 SQL 中 ANY 和 SOME 是同義詞，SOME 的用法和功能和 ANY 一模一樣。
# ANY 和 IN 運算子不同之處1
ANY 必須和其他的比較運算子共同使用，而且ANY必須將比較運算子放在 ANY 關鍵字之前，所比較的值需要匹配子查詢中的任意一個值，這也就是 ANY 在英文中所表示的意義
例如：使用 IN 和使用 ANY運算子得到的結果是一致的
select * from employee where salary = any (
select max(salary) from employee group by depart_id);
select * from employee where salary in (
select max(salary) from employee group by depart_id);
結論：也就是說“=ANY”等價於 IN 運算子，而“<>ANY”則等價於 NOT IN 運算子
# ANY和 IN 運算子不同之處2
ANY 運算子不能與固定的集合相匹配，比如下面的 SQL 語句是錯誤的
SELECT
*
FROM
T_Book
WHERE
FYearPublished < ANY (2001, 2003, 2005)
三： 帶ALL關鍵字的子查詢
# all同any類似，只不過all表示的是所有，any表示任一
查詢出那些薪資比所有部門的平均薪資都高的員工=》薪資在所有部門平均線以上的狗幣資本家
select * from employee where salary > all (
select avg(salary) from employee group by depart_id);
查詢出那些薪資比所有部門的平均薪資都低的員工=》薪資在所有部門平均線以下的無產階級勞苦大眾
select * from employee where salary < all (
select avg(salary) from employee group by depart_id);
查詢出那些薪資比任意一個部門的平均薪資低的員工=》薪資在任一部門平均線以下的員工select * from employee where salary < any ( select avg(salary) from employee group by depart_id);
查詢出那些薪資比任意一個部門的平均薪資高的員工=》薪資在任一部門平均線以上的員工
select * from employee where salary > any (
select avg(salary) from employee group by depart_id);
四：帶比較運算的子查詢
#比較運算子：=、!=、>、>=、<、<=、<>
#查詢大於所有人平均年齡的員工名與年齡
mysql> select name,age from emp where age > (select avg(age) from emp);
+---------+------+
| name | age |
+---------+------+
| alex | 48 |
| wupeiqi | 38 |
+---------+------+
2 rows in set (0.00 sec)
#查詢大於部門內平均年齡的員工名、年齡
select t1.name,t1.age from emp t1
inner join
(select dep_id,avg(age) avg_age from emp group by dep_id) t2
on t1.dep_id = t2.dep_id
where t1.age > t2.avg_age;
五：帶EXISTS關鍵字的子查詢
EXISTS關字鍵字表示存在。在使用EXISTS關鍵字時，內層查詢語句不返回查詢的記錄。
而是返回一個真假值。True或False
當返回True時，外層查詢語句將進行查詢；當返回值為False時，外層查詢語句不進行查詢
#department表中存在dept_id=203，Ture
mysql> select * from employee
-> where exists
-> (select id from department where id=200);
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+
#department表中存在dept_id=205，False
mysql> select * from employee
-> where exists
-> (select id from department where id=204);
Empty set (0.00 sec)
一：in與exists
！！！！！！當in和exists在查詢效率上比較時，in查詢的效率快於exists的查詢效率！！！！！！
==============================exists==============================
# exists
exists後面一般都是子查詢，後面的子查詢被稱做相關子查詢（即與主語句相關），當子查詢返回行數時，exists條件返回true，
否則返回false，exists是不返回列表的值的，exists只在乎括號裡的資料能不能查找出來，是否存在這樣的記錄。
# 例
查詢出那些班級裡有學生的班級
select * from class where exists (select * from stu where stu.cid=class.id)
# exists的執行原理為：
1、依次執行外部查詢：即select * from class
2、然後為外部查詢返回的每一行分別執行一次子查詢：即(select * from stu where stu.cid=class.cid)
3、子查詢如果返回行，則exists條件成立，條件成立則輸出外部查詢取出的那條記錄
==============================in==============================
# in
in後跟的都是子查詢，in()後面的子查詢 是返回結果集的
# 例
查詢和所有女生年齡相同的男生
select * from stu where sex='男' and age in(select age from stu where sex='女')
# in的執行原理為：
in()的執行次序和exists()不一樣,in()的子查詢會先產生結果集,
然後主查詢再去結果集裡去找符合要求的欄位列表去.符合要求的輸出,反之則不輸出.
二:not in與 not exists
!!!!!!not exists查詢的效率遠遠高與not in查詢的效率。!!!!!!
==============================not in==============================
not in()子查詢的執行順序是：
為了證明not in成立，即找不到，需要一條一條地查詢表，符合要求才返回子查詢的結果集，不符合的就繼續查詢下一條記錄，直到把表中的記錄查詢完，只能查詢全部記錄才能證明，並沒有用到索引。
==============================not exists==============================
not exists：
如果主查詢表中記錄少，子查詢表中記錄多，並有索引。
例如:查詢那些班級中沒有學生的班級
select * from class
where not exists
(select * from student where student.cid = class.cid)
not exists的執行順序是：
在表中查詢，是根據索引查詢的，如果存在就返回true，如果不存在就返回false，不會每條記錄都去查詢。
資料準備
create database db13;
use db13
create table student(
id int primary key auto_increment,
name varchar(16)
);
create table course(
id int primary key auto_increment,
name varchar(16),
comment varchar(20)
);
create table student2course(
id int primary key auto_increment,
sid int,
cid int,
foreign key(sid) references student(id),
foreign key(cid) references course(id)
);
insert into student(name) values
("egon"),
("lili"),
("jack"),
("tom");
insert into course(name,comment) values
("資料庫","資料倉庫"),
("數學","根本學不會"),
("英語","鳥語花香");
insert into student2course(sid,cid) values
(1,1),
(1,2),
(1,3),
(2,1),
(2,2),
(3,2);
示例
# 1、查詢選修了所有課程的學生id、name:（即該學生根本就不存在一門他沒有選的課程。）
select * from student s where not exists
(select * from course c where not exists
(select * from student2course sc where sc.sid=s.id and sc.cid=c.id));
select s.name from student as s
inner join student2course as sc
on s.id=sc.sid
group by s.name
having count(sc.id) = (select count(id) from course);
# 2、查詢沒有選擇所有課程的學生，即沒有全選的學生。（存在這樣的一個學生，他至少有一門課沒有選）
select * from student s where exists
(select * from course c where not exists
(select * from student2course sc where sc.sid=s.id and sc.cid=c.id));
# 3、查詢一門課也沒有選的學生。（不存這樣的一個學生，他至少選修一門課程）
select * from student s where not exists
(select * from course c where exists
(select * from student2course sc where sc.sid=s.id and sc.cid=c.id));
# 4、查詢至少選修了一門課程的學生。
select * from student s where exists
(select * from course c where exists
(select * from student2course sc where sc.sid=s.id and sc.cid=c.id));
練習 :查詢每個部門最新入職的那位員工
company.employee
員工id id int
姓名 emp_name varchar
性別 sex enum
年齡 age int
入職日期 hire_date date
崗位 post varchar
職位描述 post_comment varchar
薪水 salary double
辦公室 office int
部門編號 depart_id int
#建立表
create table employee(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, #一個部門一個屋子
depart_id int
);
#查看錶結構
mysql> desc employee;
+--------------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| sex | enum('male','female') | NO | | male | |
| age | int(3) unsigned | NO | | 28 | |
| hire_date | date | NO | | NULL | |
| post | varchar(50) | YES | | NULL | |
| post_comment | varchar(100) | YES | | NULL | |
| salary | double(15,2) | YES | | NULL | |
| office | int(11) | YES | | NULL | |
| depart_id | int(11) | YES | | NULL | |
+--------------+-----------------------+------+-----+---------+----------------+
#插入記錄
#三個部門：教學，銷售，運營
insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'20170301','老男孩駐沙河辦事處外交大使',7300.33,401,1), #以下是教學部
('alex','male',78,'20150302','teacher',1000000.31,401,1),
('wupeiqi','male',81,'20130305','teacher',8300,401,1),
('yuanhao','male',73,'20140701','teacher',3500,401,1),
('liwenzhou','male',28,'20121101','teacher',2100,401,1),
('jingliyang','female',18,'20110211','teacher',9000,401,1),
('jinxin','male',18,'19000301','teacher',30000,401,1),
('成龍','male',48,'20101111','teacher',10000,401,1),
('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是銷售部門
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),
('張野','male',28,'20160311','operation',10000.13,403,3), #以下是運營部門
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬銀','female',18,'20130311','operation',19000,403,3),
('程咬銅','male',18,'20150411','operation',18000,403,3),
('程咬鐵','female',18,'20140512','operation',17000,403,3)
;
#ps：如果在windows系統中，插入中文字元，select的結果為空白，可以將所有字元編碼統一設定成gbk
準備表和記錄
答案
SELECT
*
FROM
emp AS t1
INNER JOIN (
SELECT
post,
max(hire_date) max_date
FROM
emp
GROUP BY
post
) AS t2 ON t1.post = t2.post
WHERE
t1.hire_date = t2.max_date;
答案一（連結串列）
答案二
mysql> select (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post;
+---------------------------------------------------------------------------------------+
| (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) |
+---------------------------------------------------------------------------------------+
| 張野 |
| 格格 |
| alex |
| egon |
+---------------------------------------------------------------------------------------+
rows in set (0.00 sec)
mysql> select (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post;
+-------------------------------------------------------------------------------------+
| (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) |
+-------------------------------------------------------------------------------------+
| 14 |
| 13 |
| 2 |
| 1 |
+-------------------------------------------------------------------------------------+
rows in set (0.00 sec)
#正確答案
mysql> select t3.name,t3.post,t3.hire_date from emp as t3 where id in (select (select id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post);
+--------+-----------------------------------------+------------+
| name | post | hire_date |
+--------+-----------------------------------------+------------+
| egon | 老男孩駐沙河辦事處外交大使 | 2017-03-01 |
| alex | teacher | 2015-03-02 |
| 格格 | sale | 2017-01-27 |
| 張野 | operation | 2016-03-11 |
+--------+-----------------------------------------+------------+
rows in set (0.00 sec)
答案二（子查詢）
答案一為正確答案，答案二中的limit 1有問題（每個部門可能有>1個為同一時間入職的新員工），我只是想用該例子來說明可以在select後使用子查詢
可以基於上述方法解決：比如某網站在全國各個市都有站點，每個站點一條資料，想取每個省下最新的那一條市的網站質量資訊