MySQL review: create tables, MySQL data types, primary key constraints, primary key

1. Syntax for creating tables ：

create table Table name （）;

Create rows and columns in the table ：
Name data type primary key, //primary key： This is the primary key. A table can only have one primary key , And no NULL value .( Insertion time , This Sno It can't be NULL) This is Sno It's a primary key constraint , You can modify （ A way of writing ）
Name data type ( No addition not null The default value can be NULL)
Name data type

perhaps
Name data type
Name data type
Name data type primary key, // Put the primary key constraint in the last column

create table Student1(
    Sno   varchar(10)  primary key,  
     name varchar(10),
    SSex char(1),
phonenumber varchar(10)

);

If there is no primary key constraint, an error will be reported –primary key

also ：not null What does that mean? ？

CREATE TABLE SC(
Sno VARCHAR(5) NOT NULL,
Cao VARCHAR(5) NOT NULL,
Primary key( Sno) 
);

Primary key( Sno) This means ： The primary key constraint is ：Sno This column .（ The second way ）
Sno VARCHAR(5) NOT NULL, //NOT NULL, The value of this column ： You can't have null values , Insert value cannot be empty ： Namely NULL;( The default is NULL value )

You can see below ：name It can be null

mysql The data type here ：

int integer Integers ：1,6,11,44,865,78872 wait
VARCHAR(5) A character array amount to char :5 It means that this has 5 A character array of characters .
varchar(10): Yes 10 A character array of characters ： Such as ： ‘1111’, ‘helloworld’,‘fthwfw’,' written words ’ wait

2. Insert statement ：（ Method 1）

`

insert into student1 values('1',NULL ,NULL ,NULL);// direct values（ Column name variable value , Column name variable value ...）;

2. check ：

Look up the variables in the table and directly define the table statement ：desc Table name ;
Look up the tables in this database ： Direct presentation table ： show tables;( Before checking Switch database use database )

3. Run script ：

mysql>source D:\dp2019\SPJ.txt// There must be ：source;(cmd Inside )