当前位置：网站首页>MySQL foundation --- query (learn MySQL foundation in 1 day)

CREATE TABLE emp(
id int COMMENT 'ID' PRIMARY KEY,
name VARCHAR(10) COMMENT ' full name ',
age int COMMENT ' Age ',
job VARCHAR(20) COMMENT ' Work ',
salary int COMMENT ' salary ',
entrydate VARCHAR(20) COMMENT ' date ',
managerid int COMMENT 'managerid',
dept_id int COMMENT 'dept_id'
) COMMENT ' The employee table ';

After running, you can see , There is an extra watch like this ;

2. Operate the meter (DDL)

2.1 modify

Modify the data type syntax of the table ：

ALERT TABLE Table name MODIFY Field name New data types ;

【 Take the salary of the above table salary from int Type into varchar（8） type 】

alter table emp modify salary varchar(8);

After execution SQL After the statement , It can be seen that ,salary The data type of has changed .

Modify the field name and field type syntax ：

ALTER TABLE Table name CHANGE Old field name new field name New field type 【COMMENT notes 】;

【 take emp In the table entrydate Change to sex（varchar（1）） And annotated with gender 】

alter table emp change entrydate sex varchar(1) comment " Gender ";

It can be seen that , Set up successfully ！

Modify the syntax of the table name ：

ALTER TABLE indicate RENAME The new name of the table ;

【 take emp The name of the table is changed to emp1】

alter table emp rename emp1

so , The modification was successful ！

2.2 Delete

The syntax for deleting fields ：

ALTER TABLE Table name DROP Field name ：

【 Delete emp1 In the table salary Field 】

alter table emp drop salary;

This will be salary The field is deleted .

Delete table Syntax ：

DROP TABLE [IF EXISTS] indicate ;

【 Delete emp1 surface 】

drop table if exists emp1;

emp1 The table is deleted .

Delete the specified table , And recreate the syntax of the table ：

TRUNCATE TABLE Table name ;

truncate table emp;

2.3 add to

Add field Syntax

ALTER TABLE Table name ADD Field name Field type ;

【 Go to emp Add... To the table sex The type of the field is varchar(1)】

alter table emp add sex varchar(1);

To sum up ：DDL Statement is to operate on the table structure .

2. Add data to the table (DML)

grammar ：

INSERT INTO Table name （ Field name 1, Field name 2, Field name 3.....）VALUES ( Content 1),（ Content 2）.....;

Built above emp Add data to the table

INSERT INTO emp(id,name,age,job,salary,entrydate,managerid,dept_id) VALUES (1," Jin yong ",66," President ",20000,"2000-01-10",NULL,1),
(2," zhang wuji ",20," project manager ",20000,"2000-01-10",1,5),
(3," Yang Xiao ",33," Development ",20000,"2000-01-10",2,5),
(4," Xiangr ",48," Development ",20000,"2000-01-10",2,5),
(5," chang yuchun ",43," Development ",20000,"2000-01-10",2,5);

After running, you can see , Data added successfully ！

4. Operate on the data in the table （DML）

4.1 to update

Syntax for updating some data in the table ：

UPDATE Table name SET Field name 1= value 1, Field name 2= value 2,.....【WHERE Conditions 】

【 Raise the salary of the employee named Jin Yong to 30000】

UPDATE emp SET salary=30000 WHERE `name`=" Jin yong ";

This completes the modification .

4.2 Delete

Delete data syntax ：

DELETE FROM Table name 【WHERE Conditions 】

Be careful ： If you don't add conditions , All data in the table will be cleared ！

【 Delete older than 45 Year old employees 】

DELETE FROM emp WHERE age>45;

According to the prompt, you can see , Older than 45 There are two people aged , And the data has been deleted .

5. Table related queries （DQL）

5.1 Alias the queried data

Add after the field list AS Or the AS Omission is also acceptable .

Be careful ："+" The role of mysql It only acts as an operation ！

5.2 IF NULL Use

grammar ：

IF NULL( Judge the object , If null How will );

【 Inquire about emp For everyone in the employee table salary】

It can be seen that , Wei Yixiao's salary has changed from empty to 0; In the process of adding values ,null Add to any number null, Sometimes you need to convert like this .

5.3 Conditions of the query

grammar ：

SELECT Query list FROM Table name WHERE Conditions ;

Three filtering methods of conditional query ：

1. Filter by conditional expression

Conditional expression operators ：>,<,=,!=,<>,>=,<=;

2. Filter by logical expression

not,and,or;

3. Fuzzy query

like,between....and,in;

【 Inquire about managerid by 2 All the information about our employees 】

【 The screening age is less than 40 Or the position is the name and salary of the President 】

Wildcards are required for fuzzy queries ：

% For any character （ Can match any number of ）

_ Express Any character

【 The second character in the filtered employee list is _ Employee name 】

5.4 How to judge whether it is null

1. use is perhaps is not;

2. Use the safety equal sign <=>;

【 Screening manageid by null All the information of our employees 】

5.5 Aggregate functions

1.count Ask for quantity ;

2.max For maximum

3.min For the minimum

4.avg averaging

5.sum Sum up

Just show one of them here , The application is similar .

【 Query the average salary of all employees 】

It can also be seen from here ,null No participation avg The operation of a function .

5.6 Group query

grammar ：

SELECT Field list name

FEOM Table name

WHERE Conditions

GROUP BY Group field name

HAVING Filter conditions after grouping

Be careful ：having You can use aggregate functions

The order of execution is ：WHERE > Aggregate functions > HAVING

【 Query age is greater than 45 Year old employees , And according to dept_id Grouping , Obtain more than two employees dept_id】

5.7 Sort query

grammar :

SELECT Query list FROM Table name WHERE filter ORDER BY Sort list 【ASC/DESC】

Be careful ：ASC In ascending order ,DESC Arrange... In descending order （ Do not add this suffix , The default is ASC）

【 Inquire about emp All the information in the table , Rank according to age from low to high, followed by id The numbers are arranged from large to small 】

5.8 Paging query

grammar ：

SELECT Query list FROM Table name LIMIT Starting index , Number of query records

Be careful ：

Starting index from 0 Start , Starting index =（ Query the number of pages -1）* Actual records per page

【 Check the contents of the second page , each page 3 Group data 】

5.9 DQL The order in which statements are executed

FROM List of table names

WHERE List of conditions

GROUP BY Group field list

HAVING Condition list after grouping

SELECT Field list

ORDER BY Arrange field list

LIMIT Paging parameters

Be careful ：

Here is the order of execution , Instead of writing in order .

DQL The sentence ends here ,DQL It mainly queries the data in the table ;

6. User permission Settings （DCL）

6.1 User management

6.1.1 Create user

grammar ：

CREATE USER ' user name '@' With that machine, you can access ' IDENTIFIED BY ' password ';

Set up the success , And successfully log in , But no permission is set for it , Later, we will talk about setting permissions ;

6.1.2 Change Password

grammar ：

ALTER USER ' user name '@' With that machine, you can access ' IDENTIFIED WITH

mysql_native_password BY ' New password ';

After the modification, the original password cannot be logged on , The modified password can log in

6.1.3 Delete user

grammar ：

DROP USER ' user name '@' With that machine, you can access ';

6.2 Access control ：

ALL,ALL PRIVILEGES All permissions

SELSET Query data

INSERT insert data

UPDATE Modifying data

DELETE Delete data

ALTER Modify table

DROP Delete database / surface / View ;

CREAT Create database / surface

7. Common functions

7.1 String function

LENGTH Get the number of parameter value bytes

CONCAT String concatenation

UPPER Turn lowercase letters to uppercase

LOWWER Turn letters from uppercase to lowercase

SUBSTR/SUBSTRING Intercepting string , Index from 1 Start

INSERT Returns the index of the first occurrence of a string , No return found 0

TRIM Remove all characters specified before and after the substring （ Default removal “ ”）

LPAD Realize left padding with specified characters

RPAD Realize right filling with specified characters

REPLACE Replace

Try any two examples

7.2 Mathematical functions

ROUND rounding

CEIL Round up

DLOOR Rounding down

TRUNCATE truncation

RAND return 0~1 The random number

MOD modulus

【 When obtaining the verification code , Will produce a 6 Bit verification code , How is this verification code generated ？】

7.3 Date function

CURDATE Return the current system date , Not including time

CURTIME Returns the current time of the system , No date included

NOW Return current time + date

DATE_ADD(date , INTERVAL+ Time )

DATEDIFF(DATE1,DATE2) Returns the number of days of the interval

7.4 Process control functions

IF(value,t,f) If value by true, Child return t, Otherwise return to f;

IFNULL(value1,value2) If value1 Not empty , Then return to value1, Otherwise return to value2;

CASE WHEN [VAL1] THEN [RES1]....ELSE [DEFAULT] END;

If VAL1 by true, Then return to RES1........ Otherwise, return to the default DEFAULT

CASE [EXPE] WHEN [VAL1] THEN [RES1]...ELSE [DEFAULT] END;

If EXPE And VAL1 equal , Then return to RES1........ Otherwise, return to the default DEFAULT

Because it's too long , therefore mysql The foundation is divided into two phases , constraint , Operations such as multi table query will be put into the second phase ;

Study in recent days mysql Summary of , It's full of dry goods , You can collect it , Many things need to be remembered , It's easy to forget after a long time .

原网站

版权声明
本文为[Xiao Wei practiced the algorithm hard]所创，转载请带上原文链接，感谢
https://yzsam.com/2022/183/202207020509326336.html