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Review of MySQL (VII): use of tables

2022-06-12 18:20:00 【BKSW.】

MySql Review ( 7、 ... and )： The use of a watch

Create table

Grammar format

create table tableName(
   columnName dataType(length),
   ………………..
   columnName dataType(length)
);
set character_set_results='gbk';

show variables like '%char%';

--  When creating a table , There are fields in the table , Each field has ：
--     *  Field name 
--     *  Field data type 
--     *  Field length limit 
--     *  Field constraints

Common data types

type	describe
Char( length )	Fixed length string , The storage space is fixed , Suitable as primary key or foreign key
Varchar( length )	Variable length string , The storage space is equal to the actual data space
double( Number of significant digits , Decimal places )	Numerical type
Float( Number of significant digits , Decimal places )	Numerical type
Int( length )	integer
bigint( length )	Long integer
Date	Date type Specific date
DateTime	Date type Specific date Minutes and seconds millisecond
time	Date type Minutes and seconds
BLOB	Binary Large OBject（ Binary big object ）
CLOB	Character Large OBject（ Character large object ）
Other …………………

Create student table

 Create the syntax format of the table statement ：
	create table  Table name (
		 Field name 1  data type ,
		 Field name 2  data type ,
		 Field name 3  data type ,
		....
	);

	create table t_student(
		number bigint,
		name varchar(255),
		sex char(1),
		classno varchar(255),
		birth char(10)
	);

Table structure operation

utilize alter table To operate , Does not affect the data in the table

Add fields Add

You need to add the contact number field to the student table , The field name is ：contatct_tel The type is varchar(40)

alter table t_student add  contact_tel varchar(40);

Modify fields Modify

Modify the length of the field ： Student names do not meet the needs , The length needs to be changed to 100

alter table t_student modify student_name varchar(100) ;

Change the name of the field ：sex The field doesn't feel easy to use gender Replace

alter table t_student change sex gender char(2) not null ;

Delete field Drop

Delete the contact number field

alter table t_student drop contact_tel;

Operation of table data （DML）

add to Insert

Grammar format
Grammar format ：
insert into Table name ( Field name 1, Field name 2, Field name 3,…) values( value 1, value 2, value 3,…)
requirement ： The number of fields is the same as the number of values , And the data types should be the same .
Omit the insertion of fields
This method is not recommended , Because when the field position in the database table changes, it will affect insert sentence
```
insert into emp values(9999,'zhangsan','MANAGER', null, null,3000, 500, 10);
```

Specify the insertion of the field ( It is recommended to use )

insert into t_student(name) values('wangwu'); --  except name Outside the field , All remaining fields are automatically inserted NULL.

Insert more than one row of data

insert into t_student
	(no,name,sex,classno,birth) 
values
	(3,'rose','1','gaosi2ban','1952-12-14'),(4,'laotie','1','gaosi2ban','1955-12-14');

Be careful ：
When one insert After the statement is executed successfully , There must be one more line in the table .

Even if there are more than one line, some fields in the record are NULL, There is no way to carry out it later

insert Statement inserts data , Only use update updated .

Insert date

The inserted date format is consistent with the displayed date format

insert into emp(empno, ename, job, mgr, hiredate, sal, comm, deptno) values(9997,'zhangsan','MANAGER', null, '1981-06-12',3000, 500, 10);

use str_to_date

insert into emp(empno, ename, job, mgr, hiredate, sal, comm, deptno) valu
es(9996,'zhangsan','MANAGER',null,str_to_date('1981-06-12','%Y-%m-%d'),3000, 500, 10);

Add system date (now())

insert into emp(empno, ename, job, mgr, hiredate, sal, comm, deptno) values(9995,'zhangsan','MANAGER',null,now() ,3000, 500, 10);

Replication of tables

Grammar format
create table Table name as select sentence ;
Create a query as a result .

create table emp_bak as select empno,ename,sal from emp;

Insert the query results into a table

insert into emp_bak select * from emp where sal=3000;

modify Update

Data can be modified , You can modify the data according to the conditions

Grammar format
update Table name set Field name 1= value 1, Field name 2= value 2… where Conditions ;

If conditions are missing , The entire table will be updated ！！

take job by manager The wages of our employees have risen 10 element

update emp set sal = sal+sal*0.1 where job = 'MANAGE '

Department 10 Of LOC It is amended as follows SHANGHAI, Change the name of the Department to RENSHIBU

update dept1 set loc = 'SHANGHAI',dname = 'RENSHIBU' where deptno = 10;

Delete Delete

You can delete data , Data can be deleted according to conditions

Grammar format ：
delete from Table name where Conditions ;
Delete allowance as 500 The employees'
delete from emp where comm=500;
Delete the employee whose allowance is empty
delete from emp where comm is null;

How to delete a large table ？

truncate table Table name ; // The watch is cut off , Cannot roll back . Permanent loss .

Table constraints

What is a table constraint ？
When creating tables , You can add corresponding constraints to the fields of the table , The purpose of adding constraints is to ensure the consistency of data in the table
Legitimacy 、 effectiveness 、 integrity .

Common constraints are ？

Non empty constraint (not null)： Constraint field cannot be NULL
Unique constraint (unique)： Constraint fields cannot be repeated
Primary key constraint (primary key)： The constraint field cannot be either NULL, It can't be repeated （ abbreviation PK）
Foreign key constraints (foreign key)：…（ abbreviation FK）
Check constraint (check)： Be careful Oracle The database has check constraint , however mysql No, , at present mysql The constraint is not supported .

Non empty constraint NOT NULL

Set the value of a field that is not empty , If the student's name is not empty

drop table if exists t_student; 
create table t_student(
	student_id  	int(10),
	student_name 	varchar(20) not null,
	sex		char(2)  default  'm',
	birthday	date, 
	email		varchar(30),
	classes_id	int(3)	
)

--  Wrong insertion , Student name cannot be blank 
insert into t_student(student_id, birthday, email, classes_id) 
values
(1002, '1988-01-01', '[email protected]', 10)

Unique constraint ,Unique

Uniqueness constraint , It can make the value of a field not repeatable , Such as ：email Can't repeat ：

drop table if exists t_student; 
create table t_student(
	student_id  	int(10),
	student_name 	varchar(20) not null,
	sex		char(2)  default  'm',
	birthday	date, 
	email		varchar(30)  unique,
	classes_id	int(3)	
)

--  A duplicate... Was inserted email, Cause an error 
insert into t_student(student_id, student_name , sex, birthday, email, classes_id) 
values
(1001,'zhangsan','m', '1988-01-01', '[email protected]', 10)

Primary key constraint Primary key

Each table should have a primary key , A primary key can identify the uniqueness of a record , Primary keys are divided into single primary keys and composite primary keys （ union ） Primary key , A single primary key consists of a single field , Reunite with （ union ） A primary key consists of multiple fields

rop table if exists t_student; 
create table t_student()
	student_id  	int(10)  primary key,/* Column level constraints */
	student_name 	varchar(20) not null,
	sex		char(2)  default  'm',
	birthday	date, 
	email		varchar(30)  ,
	classes_id	int(3)	
)
--  Add student ID to the above table 1001 Two records of , The following error occurred , Because the primary key constraint is added 
insert into t_student(student_id, student_name , sex, birthday, email, classes_id) 
values
(1001,'zhangsan','m', '1988-01-01', '[email protected]', 10)

You can name constraints through table level constraints

drop table if exists t_student; 
create table t_student(
	student_id  	int(10),
	student_name 	varchar(20) not null,
	sex		char(2)  default  'm',
	birthday	date, 
	email		varchar(30)  ,
	classes_id	int(3),
    CONSTRAINT p_id PRIMARY key (student_id)
)

Foreign key constraints Foreign key

Foreign keys are mainly used to maintain the relationship between tables , Mainly to ensure referential integrity , If a field in the table is a foreign key field , The value of this field must come from the primary key of the referenced table , Such as ：emp Medium deptno The value must come from dept In the table deptno field value .

Establish the connection between students and class schedule

First, create a class table

drop table if exists t_classes;
create table t_classes(
	classes_id int(3),
	classes_name varchar(40),
	constraint pk_classes_id primary key(classes_id)
)

stay t_student Adding foreign key constraints to

drop table if exists t_student;
create table t_student(
	student_id  	int(10),
	student_name 	varchar(20),
	sex		char(2),
	birthday	date,
	email		varchar(30),
	classes_id	int(3),
	constraint      student_id_pk primary key(student_id),
constraint	fk_classes_id foreign key(classes_id) references t_classes(classes_id)	   
)

towards t_student Add data to

insert into t_student(student_id, student_name, sex, birthday, email, classes_id) values(1001, 'zhangsan', 'm', '1988-01-01', '[email protected]', 10)
--  Because there is no class number in the class table 10 class , Foreign key constraints work 
--  A table with foreign keys is a child table , The referenced table is the parent table , So there is a parent-child relationship , That is, the master-slave relationship , The main table is the class table , From the table is the student table

The foreign key value can be NULL？
Foreign key can be null.
When a foreign key field references a field in another table , Must the referenced field be a primary key ？
Be careful ： The referenced field does not have to be a primary key , But at least it has unique constraint , Have uniqueness , Do not repeat ！

Cascade update and cascade delete

on update cascade

mysql It is troublesome to modify some constraints , So we can delete , Add again

alter table t_student drop foreign key fk_classes_id;

alter table t_student add constraint fk_classes_id_1 foreign key(classes_id) references t_classes(classes_id)  on update cascade;

We only modified the data in the parent table , The data in the sub table will also change

on delete cascade;

mysql It is not supported when modifying some constraints , So we can delete , Add again

alter table t_student drop foreign key fk_classes_id;

alter table t_student add constraint fk_classes_id_1 foreign key(classes_id) references t_classes(classes_id)  on delete cascade;
delete from t_classes where classes_id = 20;