python 3 mysql 单表查询

1、准备表

company.employee
员工id id int
姓名 emp_name varchar
性别 sex enum
年龄 age int
入职日期 hire_date date
岗位 post varchar
职位描述 post_comment varchar
薪水 salary double
办公室 office int
部门编号 depart_id int #创建表
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, #一个部门一个屋子
depart_id int
); #查看表结构
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 | |
+--------------+-----------------------+------+-----+---------+----------------+ #插入记录
#三个部门:教学,销售,运营
insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部
('alex','male',78,'','teacher',1000000.31,401,1),
('wupeiqi','male',81,'','teacher',8300,401,1),
('yuanhao','male',73,'','teacher',3500,401,1),
('liwenzhou','male',28,'','teacher',2100,401,1),
('jingliyang','female',18,'','teacher',9000,401,1),
('jinxin','male',18,'','teacher',30000,401,1),
('成龙','male',48,'','teacher',10000,401,1), ('歪歪','female',48,'','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'','sale',2000.35,402,2),
('丁丁','female',18,'','sale',1000.37,402,2),
('星星','female',18,'','sale',3000.29,402,2),
('格格','female',28,'','sale',4000.33,402,2), ('张野','male',28,'','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'','operation',20000,403,3),
('程咬银','female',18,'','operation',19000,403,3),
('程咬铜','male',18,'','operation',18000,403,3),
('程咬铁','female',18,'','operation',17000,403,3)
;

二 、查询语法

SELECT 字段1,字段2... FROM 表名
WHERE 条件
GROUP BY field
HAVING 筛选
ORDER BY field
LIMIT 限制条数

三、关键字的执行优先级(重点)

重点中的重点:关键字的执行优先级
from
where
group by
having
select
distinct
order by
limit

1.找到表:from

2.拿着where指定的约束条件,去文件/表中取出一条条记录

3.将取出的一条条记录进行分组group by,如果没有group by,则整体作为一组

4.按照select后的字段得到一张新的虚拟表,如果有聚合函数,则将组内数据进行聚合

5.将4的结果过滤:having,如果有聚合函数也是先执行聚合再having过滤

6.查出结果:select

7.去重

8.将结果按条件排序:order by

9.限制结果的显示条数

四、简单查询

#简单查询
SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id
FROM employee; SELECT * FROM employee; SELECT name,salary FROM employee; #避免重复DISTINCT
SELECT DISTINCT post FROM employee; #通过四则运算查询
SELECT name, salary*12 FROM employee;
SELECT name, salary*12 AS Annual_salary FROM employee;
SELECT name, salary*12 Annual_salary FROM employee; #定义显示格式
CONCAT() 函数用于连接字符串
SELECT CONCAT('姓名: ',name,' 年薪: ', salary*12) AS Annual_salary
FROM employee; CONCAT_WS() 第一个参数为分隔符
SELECT CONCAT_WS(':',name,salary*12) AS Annual_salary
FROM employee;
1 查出所有员工的名字,薪资,格式为
<名字:egon> <薪资:3000>
2 查出所有的岗位(去掉重复)
3 查出所有员工名字,以及他们的年薪,年薪的字段名为annual_year select concat('<名字:',name,'> ','<薪资:',salary,'>') from employee;
select distinct depart_id from employee;
select name,salary*12 annual_salary from employee;

练习

五 、WHERE约束

强调:where是一种约束条件,mysql会拿着where指定的条件去表中取数据,而having则是在取出数据后进行过滤
where字句中可以使用:
1. 比较运算符:> < >= <= <> !=
2. between 80 and 100 值在10到20之间
3. in(80,90,100) 值是10或20或30
4. like 'egon%'
pattern可以是%或_,
%表示任意多字符
_表示一个字符
5. 逻辑运算符:在多个条件直接可以使用逻辑运算符 and or not
#1:单条件查询
SELECT name FROM employee
WHERE post='sale'; #2:多条件查询
SELECT name,salary FROM employee
WHERE post='teacher' AND salary>10000; #3:关键字BETWEEN AND
SELECT name,salary FROM employee
WHERE salary BETWEEN 10000 AND 20000; SELECT name,salary FROM employee
WHERE salary NOT BETWEEN 10000 AND 20000; #4:关键字IS NULL(判断某个字段是否为NULL不能用等号,需要用IS)
SELECT name,post_comment FROM employee
WHERE post_comment IS NULL; SELECT name,post_comment FROM employee
WHERE post_comment IS NOT NULL; SELECT name,post_comment FROM employee
WHERE post_comment=''; 注意''是空字符串,不是null
ps:
执行
update employee set post_comment='' where id=2;
再用上条查看,就会有结果了 #5:关键字IN集合查询
SELECT name,salary FROM employee
WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ; SELECT name,salary FROM employee
WHERE salary IN (3000,3500,4000,9000) ; SELECT name,salary FROM employee
WHERE salary NOT IN (3000,3500,4000,9000) ; #6:关键字LIKE模糊查询
通配符’%’
SELECT * FROM employee
WHERE name LIKE 'eg%'; 通配符’_’
SELECT * FROM employee
WHERE name LIKE 'al__';
1. 查看岗位是teacher的员工姓名、年龄
2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
3. 查看岗位是teacher且薪资在9000-1000范围内的员工姓名、年龄、薪资
4. 查看岗位描述不为NULL的员工信息
5. 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资
6. 查看岗位是teacher且薪资不是10000或9000或30000的员工姓名、年龄、薪资
7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪 select name,age from employee where post = 'teacher';
select name,age from employee where post='teacher' and age > 30;
select name,age,salary from employee where post='teacher' and salary between 9000 and 10000;
select * from employee where post_comment is not null;
select name,age,salary from employee where post='teacher' and salary in (10000,9000,30000);
select name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000);
select name,salary*12 from employee where post='teacher' and name like 'jin%';

练习

六 、分组查询:GROUP BY

大前提:可以按照任意字段分组,但分完组后,只能查看分组的那个字段,要想取的组内的其他字段信息,需要借助函数

单独使用GROUP BY关键字分组
SELECT post FROM employee GROUP BY post;
注意:我们按照post字段分组,那么select查询的字段只能是post,想要获取组内的其他相关信息,需要借助函数 GROUP BY关键字和GROUP_CONCAT()函数一起使用
SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post;#按照岗位分组,并查看组内成员名
SELECT post,GROUP_CONCAT(name) as emp_members FROM employee GROUP BY post; GROUP BY与聚合函数一起使用
select post,count(id) as count from employee group by post;#按照岗位分组,并查看每个组有多少人

强调:

如果我们用unique的字段作为分组的依据,则每一条记录自成一组,这种分组没有意义

多条记录之间的某个字段值相同,该字段通常用来作为分组的依据

参考链接:http://www.ywnds.com/?p=8184
#分组查询的常见问题:
mysql> select id,count from tt group by id;
ERROR 1055 (42000): Expression #2 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'test.tt.count' which is
not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by #查看MySQL 5.7默认的sql_mode如下:
mysql> select @@global.sql_mode;
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION #去掉ONLY_FULL_GROUP_BY模式,如下操作:
mysql> set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION'; #!!!注意
ONLY_FULL_GROUP_BY的语义就是确定select target list中的所有列的值都是明确语义,简单的说来,在ONLY_FULL_GROUP_BY模式下,target list中的值要么是来自于聚集函数的结果,要么是来自于group by list中的表达式的值。

MySQL 5.7默认ONLY_FULL_GROUP_BY语义介绍

1. 查询岗位名以及岗位包含的所有员工名字
2. 查询岗位名以及各岗位内包含的员工个数
3. 查询公司内男员工和女员工的个数
4. 查询岗位名以及各岗位的平均薪资
5. 查询岗位名以及各岗位的最高薪资
6. 查询岗位名以及各岗位的最低薪资
7. 查询男员工与男员工的平均薪资,女员工与女员工的平均薪资
#题1:分组
mysql> select post,group_concat(name) from employee group by post;
+-----------------------------------------+---------------------------------------------------------+
| post | group_concat(name) |
+-----------------------------------------+---------------------------------------------------------+
| operation | 张野,程咬金,程咬银,程咬铜,程咬铁 |
| sale | 歪歪,丫丫,丁丁,星星,格格 |
| teacher | alex,wupeiqi,yuanhao,liwenzhou,jingliyang,jinxin,成龙 |
| 老男孩驻沙河办事处外交大使 | egon |
+-----------------------------------------+---------------------------------------------------------+ #题目2:
mysql> select post,count(id) from employee group by post;
+-----------------------------------------+-----------+
| post | count(id) |
+-----------------------------------------+-----------+
| operation | 5 |
| sale | 5 |
| teacher | 7 |
| 老男孩驻沙河办事处外交大使 | 1 |
+-----------------------------------------+-----------+ #题目3:
mysql> select sex,count(id) from employee group by sex;
+--------+-----------+
| sex | count(id) |
+--------+-----------+
| male | 10 |
| female | 8 |
+--------+-----------+ #题目4:
mysql> select post,avg(salary) from employee group by post;
+-----------------------------------------+---------------+
| post | avg(salary) |
+-----------------------------------------+---------------+
| operation | 16800.026000 |
| sale | 2600.294000 |
| teacher | 151842.901429 |
| 老男孩驻沙河办事处外交大使 | 7300.330000 |
+-----------------------------------------+---------------+ #题目5
mysql> select post,max(salary) from employee group by post;
+-----------------------------------------+-------------+
| post | max(salary) |
+-----------------------------------------+-------------+
| operation | 20000.00 |
| sale | 4000.33 |
| teacher | 1000000.31 |
| 老男孩驻沙河办事处外交大使 | 7300.33 |
+-----------------------------------------+-------------+ #题目6
mysql> select post,min(salary) from employee group by post;
+-----------------------------------------+-------------+
| post | min(salary) |
+-----------------------------------------+-------------+
| operation | 10000.13 |
| sale | 1000.37 |
| teacher | 2100.00 |
| 老男孩驻沙河办事处外交大使 | 7300.33 |
+-----------------------------------------+-------------+ #题目七
mysql> select sex,avg(salary) from employee group by sex;
+--------+---------------+
| sex | avg(salary) |
+--------+---------------+
| male | 110920.077000 |
| female | 7250.183750 |
+--------+---------------+

练习

七、 使用聚合函数查询

先from找到表

再用where的条件约束去表中取出记录

然后进行分组group by,没有分组则默认一组

然后进行聚合

最后select出结果

示例:
SELECT COUNT(*) FROM employee;
SELECT COUNT(*) FROM employee WHERE depart_id=1;
SELECT MAX(salary) FROM employee;
SELECT MIN(salary) FROM employee;
SELECT AVG(salary) FROM employee;
SELECT SUM(salary) FROM employee;
SELECT SUM(salary) FROM employee WHERE depart_id=3;

八 、HAVING过滤

HAVING与WHERE在语法上是一样的

select * from employee where salary > 10000;

select * from employee having salary > 10000;
#!!!执行优先级从高到低:where > group by > 聚合函数 > having
#1. Where 是一个约束声明,使用Where约束来自数据库的数据,Where是在结果返回之前起作用的(先找到表,按照where的约束条件,从表(文件)中取出数据),Where中不能使用聚合函数。 #2. Having是一个过滤声明,是在查询返回结果集以后对查询结果进行的过滤操作(先找到表,按照where的约束条件,从表(文件)中取出数据,然后group by分组,如果没有group by则所有记录整体为一组,然后执行聚合函数,然后使用having对聚合的结果进行过滤),在Having中可以使用聚合函数。 #3. having可以放到group by之后,而where只能放到group by之前 #4. 在查询过程中聚合语句(sum,min,max,avg,count)要比having子句优先执行。而where子句在查询过程中执行优先级高于聚合语句。
验证不同之处

HAVING与WHERE不一样的地方在于

#验证之前再次强调:执行优先级从高到低:where > group by > 聚合函数 > having
select count(id) from employee where salary > 10000; #正确,分析:where先执行,后执行聚合count(id),然后select出结果
select count(id) from employee having salary > 10000;#错误,分析:先执行聚合count(id),后执行having过滤,无法对id进行salary>10000的过滤 #以上两条sql的顺序是
1:找到表employee--->用where过滤---->没有分组则默认一组执行聚合count(id)--->select执行查看组内id数目
2:找到表employee--->没有分组则默认一组执行聚合count(id)---->having 基于上一步聚合的结果(此时只有count(id)字段了)进行salary>10000的过滤,很明显,根本无法获取到salary字段

验证不同之处

select post,group_concat(name) from employee group by post having salary > 10000;#错误,分组后无法直接取到salary字段
select post,group_concat(name) from employee group by post having avg(salary) > 10000;

其他需要注意的问题

1. 查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数
3. 查询各岗位平均薪资大于10000的岗位名、平均工资
4. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资 #题1:
mysql> select post,group_concat(name),count(id) from employee group by post having count(id) < 2;
+-----------------------------------------+--------------------+-----------+
| post | group_concat(name) | count(id) |
+-----------------------------------------+--------------------+-----------+
| 老男孩驻沙河办事处外交大使 | egon | 1 |
+-----------------------------------------+--------------------+-----------+ #题目2:
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;
+-----------+---------------+
| post | avg(salary) |
+-----------+---------------+
| operation | 16800.026000 |
| teacher | 151842.901429 |
+-----------+---------------+ #题目3:
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000;
+-----------+--------------+
| post | avg(salary) |
+-----------+--------------+
| operation | 16800.026000 |
+-----------+--------------+

练习

九 、查询排序:ORDER BY

按单列排序
SELECT * FROM employee ORDER BY salary;
SELECT * FROM employee ORDER BY salary ASC;
SELECT * FROM employee ORDER BY salary DESC; 按多列排序:先按照age排序,如果年纪相同,则按照薪资排序
SELECT * from employee
ORDER BY age,
salary DESC;
1. 查询所有员工信息,先按照age升序排序,如果age相同则按照hire_date降序排序
2. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列
3. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列 #题目1
mysql> select * from employee ORDER BY age asc,hire_date desc; #题目2
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc;
+-----------+---------------+
| post | avg(salary) |
+-----------+---------------+
| operation | 16800.026000 |
| teacher | 151842.901429 |
+-----------+---------------+ #题目3
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc;
+-----------+---------------+
| post | avg(salary) |
+-----------+---------------+
| teacher | 151842.901429 |
| operation | 16800.026000 |
+-----------+---------------+

练习

十 、限制查询的记录数:LIMIT

示例:
SELECT * FROM employee ORDER BY salary DESC
LIMIT 3; #默认初始位置为0 SELECT * FROM employee ORDER BY salary DESC
LIMIT 0,5; #从第0开始,即先查询出第一条,然后包含这一条在内往后查5条 SELECT * FROM employee ORDER BY salary DESC
LIMIT 5,5; #从第5开始,即先查询出第6条,然后包含这一条在内往后查5条
1. 分页显示,每页5条
mysql> select * from employee limit 0,5;
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 |
| 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 |
| 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
5 rows in set (0.00 sec) mysql> select * from employee limit 5,5;
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
| 8 | 成龙 | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 |
| 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 |
| 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
5 rows in set (0.00 sec) mysql> select * from employee limit 10,5;
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 |
| 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 |
| 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 |
| 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 |
| 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
5 rows in set (0.00 sec)

练习

十一 、使用正则表达式查询

SELECT * FROM employee WHERE name REGEXP '^ale';

SELECT * FROM employee WHERE name REGEXP 'on$';

SELECT * FROM employee WHERE name REGEXP 'm{2}';

小结:对字符串匹配的方式
WHERE name = 'egon';
WHERE name LIKE 'yua%';
WHERE name REGEXP 'on$';
查看所有员工中名字是jin开头,n或者g结果的员工信息
select * from employee where name regexp '^jin.*[gn]$';

练习

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