[MySQL database] a collection of basic MySQL operations - the basis of seeing (adding, deleting, modifying, and querying)

1、examdb There are two tables in the library, as shown in the following figure ：

2、 Operate on these two tables , Including building tables

user ExamDB;
#mysql If the self increasing key is a primary key
CREATE TABLE ExamDB.stuinfo(
 StuName  VARCHAR(8) NOT NULL ,
 StuNo VARCHAR(20) NOT NULL ,
 StuSex VARCHAR(4) NOT NULL DEFAULT ' male ',
 StuAge INT(4) NOT NULL,
 StuSeat INT(4) NOT NULL AUTO_INCREMENT ,
 StuAddress varchar(100) DEFAULT ' The address is unknown ',
  PRIMARY KEY (`StuSeat`)
);

# surface 2 StuMarks
CREATE TABLE ExamDB.StuMarks(
 ID  INT(4) NOT NULL AUTO_INCREMENT,
 ExamNo VARCHAR(22) NOT NULL,
 StuNo VARCHAR(20) ,
 WrittenExam INT(4) NOT NULL DEFAULT 0,
 LabExam INT(4) NOT NULL NULL DEFAULT 0,
 PRIMARY KEY (`ID`)
);

# Add data
INSERT INTO ExamDB.stuinfo (
StuName,StuNo,StuSex,StuAge,StuSeat,StuAddress
)
VALUE(
' Zhang Qiuli ',
'2013000001',
' male ',
18,
1,
' Beijing haidian '
);
INSERT INTO ExamDB.stuinfo (
StuName,StuNo,StuSex,StuAge,StuSeat,StuAddress
)
VALUE(
' Lisbon ',
'2013000002',
' Woman ',
22,
2,
' Luoyang, Henan '
);
INSERT INTO ExamDB.stuinfo (
StuName,StuNo,StuSex,StuAge,StuSeat
)
VALUE(
' Li Wencai ',
'2013000003',
' male ',
21,
3
);
INSERT INTO ExamDB.stuinfo (
StuName,StuNo,StuSex,StuAge,StuSeat,StuAddress
)
VALUE(
' Ouyang Junxiong ',
'2013000004',
' male ',
18,
4,
' Karamay, Xinjiang '
);

# insert data League tables
INSERT INTO ExamDB.StuMarks (
 ExamNo,
 StuNo,
 WrittenExam ,
 LabExam
)
VALUE(
'E2013000001',
'2013000001',
80,
58
);

INSERT INTO ExamDB.StuMarks (
 ExamNo,
 StuNo,
 WrittenExam 

)
VALUE(
'E2013000002',
'2013000002',
50

);
INSERT INTO ExamDB.StuMarks (
 ExamNo,
 StuNo,
 WrittenExam ,
 LabExam
)
VALUE(
'E2013000003',
'2013000003',
97,
82
);

# 3、 ... and 、 Data addition, deletion, modification and query
-- #（ One ） Query operation
-- 1） Query the data of two tables
SELECT *  FROM  ExamDB.stumarks;
SELECT *  FROM  ExamDB.stuinfo;
-- 2） Check the list of male students
SELECT *  FROM  stuinfo WHERE StuSex=' male ' ;
-- 3） Inquire about the students with excellent written examination results （75～100 branch ）
SELECT *  FROM  ExamDB.stumarks WHERE WrittenExam BETWEEN 75 AND 100;
-- 4） Query the student's grades for reference , Include student name , Written test results , Machine test results
SELECT a.StuName,b.WrittenExam,b.LabExam  FROM  stuinfo a LEFT JOIN StuMarks b ON a.StuNo=b.StuNo ;
-- 5） Count the average scores of written test and machine test
SELECT AVG(WrittenExam) as WrittenExam,AVG(LabExam) as LabExam FROM  ExamDB.stumarks;
-- 6） Count the number of students participating in this exam
SELECT count(*) from  ExamDB.stumarks;
-- 7） Check the number of people who failed the exam （ The written test or machine test is less than 60 branch ）
SELECT count(*) from  ExamDB.stumarks where WrittenExam<60 or LabExam>60 ;
-- 8） Query students' scores , Show student number , Written test results , Machine test results , average
SELECT StuNo,WrittenExam, LabExam,(WrittenExam+LabExam)/2 as avg FROM  ExamDB.stumarks GROUP BY StuNo;
-- 9） Ranking （ Sort by average from high to low ）, Show student number 、 average
SELECT StuNo,(WrittenExam+LabExam)/2 as avg  FROM  ExamDB.stumarks   GROUP BY StuNo order by avg DESC;

-- 10） a key ： Ranking ( Sort by average from high to low ), Show name , Written test results , Machine test results , average
SELECT a.StuName,b.WrittenExam,b.LabExam,(b.WrittenExam+b.LabExam)/2 as avgsScore  FROM  ExamDB.stuinfo a 
      LEFT JOIN ExamDB.stumarks b on a.StuNo=b.StuNo order by avgsScore DESC ;
-- 11） a key ： According to the average score , Display the top two messages , Including name, 、 Written test results 、 Machine test results 、 average  limit
SELECT a.StuName,b.WrittenExam,b.LabExam,(b.WrittenExam+b.LabExam)/2 as avgsScore  FROM  ExamDB.stuinfo a 
      LEFT JOIN ExamDB.stumarks b on a.StuNo=b.StuNo order by avgsScore DESC  LIMIT 2;
-- （ Two ）DML operation
-- 1） a key ： Add all written examination results 5 branch , But not more than 100 branch .
UPDATE stuMarks SET WrittenExam= if(WrittenExam>95,100,WrittenExam + 5 );
-- 2） Name is “ Ouyang Junxiong ” The student's name was changed to “ Ouyang Maimaiti ”
update ExamDB.stuinfo set StuName= ' Ouyang Maimaiti ' WHERE StuName =' Ouyang Junxiong ';