Mysql database (II) DML data operation statements and basic DQL statements

1,DML sentence

（1） Data increase ：INSERT, The main function is to add records to the database table .

（2） Data deletion ：DELETE, The main function is to delete the existing records in the database table . It can be modified according to the conditions .

（3） Data modification ：UPDATE, The main function is to modify the existing records in the database table . It can be modified according to the conditions .

2, Data addition ：INSERT 

        grammar ：INSERT INTO Table name [( Field 1, Field 2, Field 3... Field n)] VALUE/VALUES ( value 1, value 2, value 3... value n)[( value 1, value 2, value 3... value n)];

# DML
-- INSERT INTO  Table name  （ Field name , Field name , Field name ）values （ value , value , value ）
--  Add a student 

--  Date should be represented by string 
INSERT INTO student(sid,sname,birthday,ssex,classid) VALUE(9,' Zhang San ','1988-1-1',' male ',2);

--  Select a field to insert （ Data cannot be inserted if there is a default value ）
INSERT INTO student(sname) VALUE(' Li Si ');

--  For full field insertion, it is not necessary to write the segment name after the table name 
INSERT INTO student VALUE(11,' Wang Wu ','1999-1-1',' Woman ',1);

-- DEFAULT,NULL  Default value for auto increment of primary key 
-- NULL  Only the primary key 
INSERT INTO student(sid) VALUES(NULL);

3, Data addition ：INSERT INTO SELECT

        grammar ：INSERT INTO table2( Field 1, Field 2...) SELECT  Field 1, Field 2... FEOM table1;

--  Insert multiple pieces of data at once 
--  Mode one ：
-- VALUES  Insert more than one  VALUE  Insert a 

INSERT INTO student(sname) VALUES(' Li Yun '),(' Zhou Mei '),(' Wu Lan '),(' Zheng Zhu '),(' Wangju ');

INSERT INTO student VALUES(NULL,' Li Yun ','1990-08-06',' male ',2),(NULL,' Zhou Mei ','1991-12-01',' Woman ',1),(NULL,' Wu Lan ','1992-03-01',' Woman ',2),(NULL,' Zheng Zhu ','1989-07-01',' Woman ',1),(NULL,' Wangju ','1990-01-20',' Woman ',2);

--  Mode two ：
-- INSERT INTO SELECT
--  Tables exist 

CREATE TABLE newstu(
	xingming VARCHAR(10),
	xingbie VARCHAR(10)
);

INSERT INTO newstu(xingming,xingbie) SELECT sname,ssex FROM student;

--  Mode three ：
-- CREATE TABLE  New table  SELECT;

CREATE TABLE stu SELECT sid,sname,birthday FROM student;
SHOW CREATE TABLE stu;

4, Data modification ：UPDATE

        grammar ：UPDATE Table name SET Field 1 = value 1 [SET Field 2 = value ...] [WHERE Conditions ]

#  modify 
-- WHERE  Must write 
-- UPDATE  Table name  SET  Field name = value , Field name = value ... Field name = value  [WHERE  Clause ]
UPDATE newstu SET xingbie=' male ' WHERE xingming=' Qian Dian ';
UPDATE stu SET birthday='2022-06-07' WHERE sid <> 3;
UPDATE stu SET birthday='2000-01-01' WHERE sid >= 4 AND sid <=8;
UPDATE stu SET birthday='1888-01-01' WHERE sid BETWEEN 4 AND 8;

5,WHERE Clause  

Operator	meaning	give an example	result
=	be equal to	5 = 6	false
< > or ! =	It's not equal to	5 != 6	true
>	Greater than	5 > 6	false
<	Less than	5 < 6	true
>=	Greater than or equal to	5 >= 6	false
<=	Less than or equal to	5 <= 6	true
BETWEEN	In a certain range	BETWEEN 5 AND 10	-
AND	also	5 > 1 AND 1 > 2	false
OR	or	5 > 1 OR 1 > 2	true
NOT	Not

        =：set The latter represents assignment ,where Said judgment  

6, Data deletion ：DELETE

        grammar ：DELETE FROM Table name [WHERE Conditions ];

#  Delete 
-- DELETE FROM  Table name  [WHERE  Clause ]
--  All data in the table is deleted 
DELETE FROM newstu;
DELETE FROM stu WHERE sname=' Zhao Lei ';
DELETE FROM stu WHERE sid >=2 AND sid <= 6;
-- BETWEEN  Followed by small data  AND  Followed by big data 
DELETE FROM stu WHERE sid BETWEEN 7 AND 10;

7, Data clearing ：TRUNCATE

        TRUNCATE [TABLE] Table name ;

--  Clear the table 
-- TRUNCATE  Table name 
TRUNCATE stu;
DELETE FROM student;
TRUNCATE student;
-- DELETE TRUNCATE   difference 		
-- DELETE  Do not delete index values 

8,MySQL Database recovery method

Console recovery ：mmysql –h localhost -P 3306 –u root –p day001 < /path/db_name.sql;

day001 ： Name of the database to recover .

/path/db_name.sql ： To recover the absolute path of the database .

Console backup ：mysqldump -h localhost –P 3306 –u root –p day001 > /path/db_name.sql;

day001 ： Name of the database to be backed up .

/path/db_name.sql ： The absolute path of the database to be backed up .

9, The role and value of database backup and recovery

High availability ： Minimize the number of database failures , So as to maintain the maximum availability of the database .

Security ： Computer virus 、 Trojan horse type 、“ hackers ” Invasive 、 Logic bomb type will cause information loss , Lost data needs to be recovered in time .

integrity ： When the database fails , Ensure that as little or no data is lost , So that the data has the greatest integrity .

10, Single table query

select [distinct] 

{ * or   surface 1.* or [ surface 1. Field 1 [ as Field alias 1] [, surface 1. Field 2 [as Field alias 2]] [......]  ]  }

from surface 1 [as Table alias ]

[left or rigth or inner join surface 2 on  The relationship between tables ]

[where]

[group by]

[limit {[ Position offset ,] Row number }];

keyword	effect
distinct	Remove duplicate records in the table
as	Alias the field name or table name
group by	Display the queried data by group classification
having	Play the role of conditional restriction after grouping
order by	Sort the queried data
limit	Limit the number of results displayed

#  Inquire about 
--  The simplest query 
SELECT 'abc';

--  Query from a data table 

--  Full field query 
-- SELECT  Field name ..., Field name  FROM  Table name ;

--  recommend 
SELECT sid,sname,birthday,ssex,classid FROM student;
--  Not recommended , Do not conform to the sql Optimization principle 
SELECT * FROM student;
--  Partial field query 
SELECT sname,ssex,' Ape academy ' FROM student;
--  Alias fields 
SELECT sname AS ' full name ',ssex ' Gender ',' Ape academy '  School  FROM student;

11, Remove duplicate data ：distinct

grammar ：select distinct Field name 1, Field name 2... from Table name

effect ： Get rid of select Duplicate records in the returned record result of query （ All return columns have the same value ）, Just return one .

-- DISTINCT  Remove duplication （ Query all fields the same , Remove the repetitive ones ）

SELECT DISTINCT sname,birthday FROM student;

12,where 

grammar ：select * from Table name [where Conditions ];

--  Conditional query 
SELECT * FROM student WHERE ssex = ' male ';
SELECT * FROM student WHERE ssex = ' male ' AND classid = 2;
SELECT * FROM student WHERE ssex = ' Woman ' AND classid != 2;

-- sid(4-7)
SELECT * FROM student WHERE sid BETWEEN 4 AND 7;
SELECT * FROM student WHERE sid >= 4 AND Sid <= 7;

-- sid !(4-7)
SELECT * FROM student WHERE sid <= 4 OR sid >= 7;

--  Find a birthday greater than 1990 Of the students ( Dated )
SELECT * FROM student WHERE birthday < '1990-1-1';
INSERT INTO student (sname) VALUES (' Teacher wang '),(' Teacher Wang next door '),(' Handsome teacher Wang '),(' Lao Wang '),(' Wang Yanyi ');

13,like Fuzzy query

grammar ：select * from Table name where Field like Conditions ;

-- LIKE  Fuzzy query 
--  Fuzzy symbols  
-- %  Any number of any characters 
-- _  An arbitrary character 
SELECT * FROM student WHERE Sname LIKE ' king %';
SELECT * FROM student WHERE Sname LIKE '% king %';
SELECT * FROM student WHERE sname LIKE ' king _';

14,in Range queries

grammar ：select * from Table name where Field in ( value 1, value 2, value 3...);

-- IN  In this range 
-- 2,4,5,7,9,200
SELECT * FROM student WHERE Sid IN (2,4,5,7,9,200);

15,null 

grammar ：select * from Table name where Field is null or is not null;

--  Yes null Conditions 
SELECT * FROM student WHERE birthday is NULL;
SELECT * FROM student WHERE birthday is NOT NULL;

16, Aggregate functions

Function name	Return value
avg(col)	Returns the average value of the specified column
count(col)	Returns the value of the specified column null The number of values
min(col)	Returns the minimum value of the specified column
max(col)	Returns the maximum value of the specified column
sum(col)	Returns the sum of all specified columns

--  Aggregate functions 
-- COUNT( Field name )  Number of Statistics 
-- COUNT  No statistics null
--  It is recommended to use the primary key to count the number 
-- *  recommend 
--  Constant   recommend 

--  Count out how many male students there are in the student list 
SELECT COUNT(Ssex) FROM student WHERE Ssex= ' male ';

--  Are all statistical 
-- AVG( Field name )  averaging 
SELECT AVG(score) FROM sc;

-- MAX( Field name )  Maximum 
SELECT MAX(score) FROM sc;

-- MIN( Field name )  minimum value 
SELECT MIN(score) FROM sc;

-- SUM( Field name )  The sum of the 
SELECT SUM(score) FROM sc;

--  Count the total number of exams in the transcript , The highest , Lowest score , average , Total score 
SELECT COUNT(*)  frequency ,MAX(score)  The highest ,MIN(score)  Lowest score ,AVG(score)  average ,SUM(score)  Total score  FROM sc;

17,group by and having grouping

group by：

        1、 Group all data .

        2、 There can be multiple fields for grouping , And group them in turn .

having：

        And group by Use a combination of , Data filtering after grouping

--  grouping  GROUP BY
--  Count the number of different genders in the student table 
SELECT ssex,COUNT(1) FROM student GROUP BY Ssex;

--  Count the number of different classes in the student table 
SELECT classid  class , COUNT(*) FROM student GROUP BY classid;

--  The average score of each student in the transcript 
SELECT sid,AVG(score) FROM sc GROUP BY Sid;

--  Statistics results   Each student is greater than 60 Total score of points 
SELECT sid,SUM(score) FROM sc WHERE score >=60 GROUP BY Sid;

--  Filter after grouping  HAVING
--  The total score is greater than 200 branch 
-- WHERE  There can be no aggregated data behind 
-- HAVING  The latter condition is the screening after aggregation 
--  Not alone , It has to be with  GROUP BY  Come together 
-- WHERE  and  HAVING The difference between 

SELECT sid,SUM(score) FROM sc WHERE score >= 60 GROUP BY Sid HAVING SUM(score) > 200;

--  Find the average score of each course , It shows that the average score is greater than 85 Courses with scores and average scores 

SELECT cid,AVG(score) FROM sc GROUP BY cid HAVING AVG(score) >85;

18,order by Sort

grammar ：select * from Table name order by Field name [desc or asc];

--  Sort 
-- DESC  Descending 
-- ASC  Ascending 
--  The rules  ： Write first, arrange first , Write back 

SELECT * FROM student ORDER BY Sid;
SELECT * FROM sc ORDER BY score DESC,Cid DESC;

19,limit Pagination

grammar ：selsct * from Table name limit [n,m];

-- LIMIT  Pagination 
-- LIMIT num  The first few 
-- LIMIT num（ Location ）,num（ step ） 

SELECT * FROM student LIMIT 0,3;

--  The formula  ：( Page number -1)* step 

--  Wrong writing 
SELECT * FROM student LIMIT (1-1)*3,3;

--  Find out the students whose average score in the exam is the second in the score table 
SELECT sid,AVG(score) FROM sc GROUP BY sid ORDER BY AVG(score) DESC LIMIT 1,1;