Junior year final review
The following questions need to be written proficiently
Data is structured, with connections between data and oriented to the entire system; data has high sharing, low redundancy, and is easy to expand; data independence is high.
MySQL, SQL Server, Oracle, Hadoop, etc.
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Early daysThere are three popular database models, namely
In today's Internet, there are mainly two most commonly used database models, namely
DDL (Data Definition Language) Data Definition Language
Used to define database objects: databases, tables, columns, etc. Keywords: create, drop, alter, etc.
DQL (Data Query Language) Data Query Language
Used to query records (data) in tables in the database . Keywords: select, etc.
DML (Data Manipulation Language)Data Manipulation Language
is used to add, delete, and modify data in tables in the database. Keywords: insert, delete, update, etc.
| Name | Function |
|---|---|
| Return the average value of a certain field | |
| Return the number of rows in a certain field | |
| Return the maximum value of a certain field | |
| Return the minimum value of a certain field | |
| The sum of the returned fields |
limit m,n;
SELECT DISTINCT <字段名> FROM <表名>;
[NOT] LIKE '字符串
immutable, while the length of varchar is variable, that is, define a char[10] and varchar[10] , if what is saved is 'csdn', then the length occupied by char is still 10, except for the character 'csdn', followed by followed by six spaces , and varchar immediately changes the length to 4. When fetching data, use trim() to remove extra spaces for char type, but varchar is not needed.
Furthermore, the storage method of char is that each English character (ASCII) occupies 1 byte, and each Chinese character occupies two bytes; while the storage method of varchar is that each English character occupies 2 bytes. Bytes, Chinese characters also occupy 2 bytes. New view, delete view statement?New view:create view 视图名字 as select 语句;
drop view 视图名字;
Focus on: database.md, data Notes on definition language (DDL).md
The last big question is 30 points
Test points: database building, table building, queryReference: All homework exercises Example:
-- 创建create table stuInfo( stuId int primary key auto_increment, stuName varchar(10) not null, stuAge int not null, stuSex int not null check(stuSex = 0 or stuSex = 1), time datetime)create table courseInf( courseId int primary key auto_increment, courseName varchar(10) not null, courseMarks int not null)create table scoreInfo( scoreId int primary key auto_increment, stuId int, courseId int, score int not null, constraint scoreInfo_stuInfo_fk foreign key(stuId) references stuInfo(stuId), constraint scoreInfo_courseInf_fk foreign key(courseId) references courseInf(courseId))-- 插入insert into stuInfo()values(default,'Tom',19,1,null),(default,'Jack',20,0,null),(default,'Rose',21,1,null),(default,'Lulu',19,1,null),(default,'Lili',21,0,null),(default,'adc',20,1,'2007-01-07 01:11:36.590');insert into courseInf()values(default,'JavaBase',4),(default,'HTML',2),(default,'JavaScript',2),(default,'SqlBase',2);insert into scoreInfo()values(default,1,1,80),(default,1,2,85),(default,1,4,50),(default,2,1,75),(default,2,3,45),(default,2,4,75),(default,3,1,45),(default,4,1,95),(default,4,2,75),(default,4,3,90),(default,4,4,45);-- 查询出每个学生所选修的课程的数量和所选修的课程的考试的平均分select count(courseId),avg(score)from scoreInfogroup by stuId;-- 修改UPDATE table_name SET field1=new-value1, field2=new-value2[WHERE Clause]-- 删除DELETE FROM table_name [WHERE Clause]
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