create table student( sid varchar(50),
sname varchar(50),
sage varchar(50),
ssex varchar(50)
);
insert into student( sid,sname,sage,ssex ) values('1','zhaolei','1990-01-01','nan');
insert into student values('2','qiandian','1990-12-21','nan');
insert into student values('3','sunfeng','1990-05-20','nan');
insert into student values('4','liyun','1990-08-06','nan');
insert into student values('5','zhoumei','1991-12-01','nv'),
('6','wulan','1992-03-01','nv'),
('7','zhenzu','1989-07-01','nv'),
('8','wangju','1990-01-20','nv');
select * from student;
select sid,sname,sage,ssex from student;
select sid,sname,ssex from student;
=============================================================================================
1、修改表名
语法格式:alter table 旧表名 rename [to] 新表名;
注释:修改后example1表就不存在了,只存在名为user的新表,但是其内容是一致的,只是换了个名称.
alter table student rename to sww;
select * from student;
select * from sww;
alter table sww rename to student;
=====================================================================================================
2、修改字段名
语法格式:alter table 表名 change 旧属性名 新属性名 新数据类型;
注释:新数据类型指修改后的数据类型,如不需要修改,则将新数据类型设置成与原来一样
alter table student change sname sww varchar(50);
select * from student;
alter table student change sww sname varchar(50);
==============================================================================================
3、修改字段的数据类型
语法格式:alter table 表名 modify 属性名 数据类型;
注释:表名指所要修改数据类型的字段的表的名称;
属性名指:所要修改数据类型字段的名称;
数据类型指:修改后的新的数据类型
=========================================================================================================
4、修改字段的排列位置
语法格式:alter table 表名 modify 属性名1 数据类型 first|after 属性名2;
alter table student modify sage varchar(50) first;
select * from student;
alter table student modify sage varchar(50) after sname;
======================================================================================
5、增加字段
语法格式:alter table 表名 add 属性名1 数据类型 [完整性约束条件] [first | after 属性名2];
完整性约束条件:是可选参数,用来设置新增字段的完整性约束条件
first:是可选参数,其作用是将新增字段设置为表的第一个字的
after:是可选参数,其作用是将新增字段添加到“属性名2”所指的字段后
如果执行的SQL语句中没有“first”或者“after 属性名2”参数指定新增字段的位置,则新增字段默认为表的最后一个字段
alter table student add saddress varchar(50) after ssex;
select * from student;
==============================================================================================================
6、删除字段
删除字段是删除表中已经定义好的表中的某个字段,删除后其字段所属的数据都会被删除
语法格式:alter table 表名 drop 属性名;
alter table student drop saddress;
select * from student;
==========================================================================================================
select * from student;
select * from student where sid in('1','3','5','7');
select * from student where sid not in('1','3','5','7');
select * from student where sid > 4;
select * from student where sid != 4;
select * from student where sid between 4 and 6;
select * from student where sid not between 4 and 6;
select * from student where sname like '%n%';
select * from student where sname not like '%n%';
=====================================================================================================
select * from student;
select * from student order by sid desc;
select count(*) from student;
select sum(sid) from student;
select avg(sid) from student;
select max(sid) from student;
select min(sid) from student;
select count(*) from student group by ssex;
select sid as a, sname as b,sage as c, ssex as d from student;
select * from student limit 3;
select * from student limit 1,2;
====================================================================================
增加一个字段,插入数据,并更改数据
alter table student add saddress varchar(50) after ssex;
select * from student;
update student set saddress = 'jiangsu' where sid = '1';
update student set saddress = 'jiangsu' where sid = '2';
update student set saddress = 'shanghai' where sid = '3';
update student set saddress = 'shanghai' where sid = '4';
update student set saddress = 'shanghai' where sid = '5';
update student set saddress = 'beijing' where sid = '6';
update student set saddress = 'beijing' where sid = '7';
update student set saddress = 'anhui' where sid = '8';
insert into student ( sid,sname,sage,ssex ) values ('9','shenweiwei','1989-10-18','nan');
update student set saddress = 'jiangsu' where sid = '9';
一、将查询结果插入到表中
insert语句可以将一个表中查询到的数据插入到另外一个表中
语法格式:
insert into 表名1 (属性列表1) select 属性列表2 from 表名2 where 条件表达式;
表名1说明记录插入到哪个表中;
表名2表示记录是从哪个表中查询出来的;
属性列表1参数表示为哪些字段赋值;
属性列表2表示从表中查询出哪些字段的数据;
条件表达式参数设置了select语句的查询条件;
注意:使用这种方法时,必须保证属性列表1和属性列表2中的字段个数是一样的,而且每个对应字段的数据类型必须是一样的。
create table student2( sid varchar(50),
sname varchar(50),
sage varchar(50),
ssex varchar(50),
saddress varchar(50)
);
select * from student2;
delete from student2;
insert into student2 ( sid,sname,sage,ssex,saddress ) values ('11','fenglili2','1988-09-18','nv','jiangsu');
insert into student2 ( sid,sname,sage,ssex,saddress ) values ('12','fenglili3','1988-08-18','nv','jiangsu');
insert into student2 ( sid,sname,sage,ssex,saddress ) values ('13','fenglili4','1988-07-18','nv','shanghai');
insert into student ( sid,sname,sage,ssex,saddress ) values ('11','fenglili2','1988-09-18','nv','jiangsu');
insert into student ( sid,sname,sage,ssex,saddress ) values ('12','fenglili3','1988-08-18','nv','jiangsu');
insert into student ( sid,sname,sage,ssex,saddress ) values ('13','fenglili4','1988-07-18','nv','shanghai');
delete from student where sid in('11','12','13');
select * from student;
insert into student(sid,sname,sage,ssex) select sid,sname,sage,ssex from student2 where sid = '11';
select * from student;
insert into student(sid,sname,sage,ssex,saddress) select sid,sname,sage,ssex,saddress from student2 where sid in ('12','13');
select * from student;
update student set saddress = 'jiangsu' where saddress is null;
select * from student;
================================================================================================================================
一、基本查询语句
select的基本语法格式如下:
select 属性列表 from 表名和视图列表
[ where 条件表达式1 ]
[ group by 属性名1 [ having 条件表达式2 ] ]
[ order by 属性名2 [ asc | desc ] ]
属性列表参数表示需要查询的字段名;
表名和视图列表参数表示从此处指定的表或者视图中查询数据,表和视图可以有多个;
条件表达式1参数指定查询条件;
属性名1参数指按照该字段的数据进行分组;
条件表达式2参数满足该表达式的数据才能输出;
属性名2参数指按照该字段中的数据进行排序;排序方式由asc和desc这两个参数指出;
asc参数表示升序,这是默认参数,desc表示降序;(升序表示从小到大)
对记录没有指定是asc或者desc,默认情况下是asc;
如果有where子句,就按照“条件表达式1”指定的条件进行查询;如果没有where子句,就查询所有记录;
如果有group by子句,就按照“属性名1”指定的字段进行分组,如果group by后面带having关键字,那么只有
满足“条件表达式2”中知道的条件才能输出。group by子句通常和count()、sum()等聚合函数一起使用;
如果有order by子句,就按照“属性名2”指定的字段进行排序,排序方式由asc和desc两个参数指出;默认情况下是asc;
查询结果不重复:distinct 关键字
select * from student;
select distinct ssex from student;
select distinct saddress from student;
分组查询:
select * from student group by ssex;
select * from student group by saddress;
group by 关键字与group_concat()函数一起使用,每个分组中指定字段值都显示出来:
select ssex,GROUP_CONCAT(ssex) from student group by ssex;
select ssex,GROUP_CONCAT(sname) from student group by ssex;
select saddress,GROUP_CONCAT(sname) from student group by saddress;
group by 关键字与集合函数一起使用:
select ssex,count(ssex) from student group by ssex;
select saddress,count(saddress) from student group by saddress;
group by 关键字与"having 条件表达式"一起使用,可以限制输出结果,只有满足条件表达式的结果才会显示:
select ssex,count(ssex) from student group by ssex;
select ssex,count(ssex) from student group by ssex having count(ssex) >6;
select saddress,count(saddress) from student group by saddress;
select saddress,count(saddress) from student group by saddress having count(saddress) >3;
注意:“having 表达式” 与 "where 表达式"都是用来限制显示的,但是两者起作用的地方不一样;
"where 表达式"用于表或者视图,是表和视图的查询条件;
“having 表达式”作用于分组后的记录,用于选择满足条件的分组。
group by 关键字与 with rollup 一起使用,会在所有记录的最后加上一条记录,这条记录是上面所有记录的总和。
select ssex,count(ssex) from student group by ssex;
select ssex,count(ssex) from student group by ssex with rollup;
select saddress,count(saddress) from student group by saddress;
select saddress,count(saddress) from student group by saddress with rollup;
select * from student;
select * from student limit 4;
select * from student limit 1,5;
=======================================================================================================
create table score ( xh int(50),
km varchar(50),
cj int(50)
);
select * from score;
insert into score values(1,'shuxue',80);
insert into score values(1,'yuwen',70);
insert into score values(1,'yingyu',40);
insert into score values(2,'shuxue',40);
insert into score values(2,'yuwen',60);
insert into score values(2,'yingyu',50);
insert into score values(3,'shuxue',60);
insert into score values(3,'yuwen',20);
insert into score values(3,'yingyu',90);
insert into score values(4,'shuxue',50);
insert into score values(4,'yuwen',60);
insert into score values(4,'yingyu',70);
select xh,sum(cj) from score where xh = 1; 查询此同学的总成绩;
select xh,sum(cj) from score where xh = 4;
select xh,sum(cj) from score group by xh; 查询每一个同学的各科总和成绩;
select km,max(cj) from score group by km; 查询各个科目的最高成绩;
select km,avg(cj) from score group by km; 查询每一科目的平均成绩;
select km,max(cj) from score group by km; 查询每一科目的最高成绩;
select km,min(cj) from score group by km; 查询每一科目的最低成绩;
=======================================================================================================================
create table employee ( num int(50),
d_id int(50),
name varchar(50),
age int(50),
sex varchar(50),
homeadd varchar(50)
);
insert into employee values(1,1001,'zhangsan',26,'nan','beijing');
insert into employee values(2,1001,'lisi',24,'nv','hunan');
insert into employee values(3,1002,'wangwu',25,'nan','jiangsu');
insert into employee values(4,1004,'aric',15,'nan','yingguo');
select * from employee;
create table department ( d_id int(50),
d_name varchar(50),
functione varchar(50),
address varchar(50)
);
insert into department values(1001,'keyanbu','yanfachanpin','3lou5hao');
insert into department values(1002,'shengchanbu','shengchanchanp','5louyiceng');
insert into department values(1003,'xiaoshoubu','cehuaxiaoshou','1louxiaoshoudating');
select * from department;
select num,name,employee.d_id,age,sex,d_name,functione from employee,department where employee.d_id = department.d_id;
内连接查询:可以查询两个或者两个以上的表,当两个表中存在表示相同意义的字段时,可以通过该字段来连接这两个表;
当该字段的值相等时,就查询出该记录。
======================================================================================================================
外连接查询:可以查询两个或者两个以上的表,外连接查询也需要通过指定字段来进行连。
当该字段取值相等时,可以查询出该记。
而且该字段取值不相等的记录也可以查询出来。
外连接包括:左连接、右连接
语法:
select 属性列表 from 表名1 left | right join 表名2 on 表名1.属性名1 = 表名2.属性名2;
属性列表表示要查询的字段的名称,这些字段可以来自不同的表;
Left表示左连接查询;
rigth表示右连接查询;
on后面接的是连接的条件;
1、左连接查询
进行左连接查询时,可以查询出表名1所指的表中的所有记录。而表名2所指的表中,只能查询出匹配的记录
select * from employee;
select * from department;
select num,name,employee.d_id,age,sex,d_name,functione from employee left join department on employee.d_id = department.d_id;
2、右连接查询
进行右连接查询时,可以查询出表名2所指的表中的所有记录。而表名1所指的表中,只能查询出匹配的记录
select * from employee;
select * from department;
select num,name,employee.d_id,age,sex,d_name,functione from employee right join department on employee.d_id = department.d_id;
复合条件查询
在连接查询时,通过增加其他的限制条件,可以使查询结果更加准确
select * from employee;
select * from department;
select num,name,employee.d_id,age,sex,d_name,functione from employee,department where employee.d_id = department.d_id;
select num,name,employee.d_id,age,sex,d_name,functione from employee,department where employee.d_id = department.d_id and age > 24;
select num,name,employee.d_id,age,sex,d_name,functione from employee,department where employee.d_id = department.d_id order by age asc;
select 语句先按照内连接的方式从employee和department表中查询出数据。然后查询结果按照age字段从小到大的顺序进行排序。
=================================================================================================================================================
子查询
子查询是将一个查询语句嵌套在另外一个查询语句中,内层查询语句的查询结果,可以作为外来层查询语句提供查询条件。
1、带in关键词的子查询
select * from employee;
select * from department;
select * from employee where d_id in (select d_id from department );
select * from employee where d_id not in (select d_id from department );
2、带exists关键字的子查询
exists关键字表示存在,使用exists关键字时,内层查询语句不用返回查询的记录。而是返回一个真假值。
如果内层查询语句查询到满足条件的记录,就返回一个真值(true);否则,返回一个假值(false);
当返回值为true时,外层查询语句将进行查询;而返回false时,外层查询语句不进行查询或者查询不出任何记录。
select * from employee;
select * from department;
select * from employee where exists (select d_name from department where d_id = 1003);
select * from employee where exists (select d_name from department where d_id = 1004);
exists关键字可以与其他查询条件一起使用。条件表达式与exists关键字之间用and或者or来连接。
select * from employee where age > 24 and exists (select d_name from department where d_id = 1003);
select * from employee where age > 24 and exists (select d_name from department where d_id = 1004);
not exists与exists相反。
select * from employee where age > 24 and not exists (select d_name from department where d_id = 1003);
select * from employee where age > 24 and not exists (select d_name from department where d_id = 1004);
3、带比较运算符的子查询
create table schoarship ( levela int(50),
score int(50)
);
insert into schoarship(levela,score) values(1,90);
insert into schoarship values(2,80);
insert into schoarship values(3,70);
select * from schoarship;
create table computer_stu ( id int(50),
name varchar(50),
score int(50)
);
insert into computer_stu(id,name,score) values (1001,'lily',85);
insert into computer_stu(id,name,score) values (1002,'tom',91),
(1003,'jim',87),
(1004,'aric',77),
(1005,'lucy',65),
(1006,'andy',99),
(1007,'ada',85),
(1008,'jeck',70);
select * from computer_stu;
select id,name,score from computer_stu where score >= (select score from schoarship where levela = 1);
/* 查询获得一等奖学金的学生有哪些,第一个表为奖学金等级和最低分数*/
select d_id,d_name from department where d_id != (select d_id from employee where age = 24);
/*只有生产部和销售部没有年龄等于24岁的员工*/
4、带any关键字的子查询
any关键字表示满足其中任何一个条件。使用any关键字时,只要满足内查询语句返回的结果中的任何一个,就可以通过该条件来执行外层查询语句
select * from computer_stu where score >= any ( select score from schoarship );
/*结果显示,有7个人获得奖学金,只有id为1005的人没有,因为分数为65,不高于奖学金指定最低分数的任何一个*/
5、带all关键字的子查询
all关键字表示满足所有条件。使用all关键字时,只有满足内层查询语句返回的所有结果,才可以执行外层查询语句。
select * from computer_stu where score >= all ( select score from schoarship );
/*结果显示,只有两个人获得奖学金。因为这两个人的分数比所有奖学金要求的分数都高*/
============================================================================================================================
合并查询结果
合并查询结果 是将多个select语句的查询结果合并到一起
union关键字,数据库会将所有的查询结果合并到一起,然后除掉相同的记录;
union all关键字,只是简单的合并到一起
select d_id from department;
select d_id from employee;
select d_id from department union select d_id from employee;
select d_id from department union all select d_id from employee;
=====================================================================================================================
为表和字段取别名
1、语法:表名 表的别名
select * from department d where d.d_id = 1001;
2、语法:属性名 [as] 别名
select d_id as department_id,d_name department_name from department;
数据库中,可以同时为表和字段取别名
select d_id as department_id,d_name department_name,d.functione,d.address from department d where d.d_id = 1001;
=================================================================================================================================