MySQL learning diary (II)

SQL Detailed explanation of connection mode ：https://www.cnblogs.com/janneystory/p/5618140.html

MYSQL data type :
integer
tinyint(m)            1 Bytes Range (-128~127)
smallint(m)        2 Bytes Range (-32768~32767)
mediumint(m)        3 Bytes Range (-8388608~8388607)
int(m)            4 Bytes Range (-2147483648~2147483647)
bigint(m)            8 Bytes Range (+-9.22*10 Of 18 Power )
[ notes ： Add in front unsigned, Then the maximum value in the range 、 The minimum values are doubled .（m） It refers to the display width in the query result set ]
floating-point
float(m,d)             Single precision floating point 8 Bit accuracy (4 byte ) m Total number ,d Decimal places
double(m,d)         Double precision floating point 16 Bit accuracy (8 byte ) m Total number ,d Decimal places
Fixed-point number
decimal(m,d)          Parameters m<65 It's the total number ,d<30 And d<m It's a decimal place
character string (char,varchar,_text)
char(n)         Fixed length , most 255 Characters
varchar(n)     Fixed length , most 65535 Characters
tinytext         Variable length , most 255 Characters
text         Variable length , most 65535 Characters
mediumtext     Variable length , most 2 Of 24 Power -1 Characters
longtext         Variable length , most 2 Of 32 Power -1 Characters
[ notes ：varchar The query speed ratio of text fast ,char The query speed ratio of varchar fast ]
MySQL It will be automatically modified to... Before the non-conforming value is inserted into the table 0!!!!!

Simple use of tables ：
NULL : Allow null value
NOT NULL : Null value... Is not allowed , When inserting or updating rows , The column must have a value , When inserting a column without a value , Will return an error
【 notes ： Space ““ Not equal to null value 】
DEFAULT : Specify default , Without assigning a value to the column , By default, the system assigns DEFAULT The number after

CREATE TABLE OrderItems
(
    order_num    INTEGER        NOT NULL,
    order_item    INTEGER        NOT NULL,
    prod_id    CHAR(10)    NOT NULL,
    quantity    INTEGER        NOT NULL    DEFAULT 1,
    item_price    DECIMAL(8,2)    NOT NULL
);

The assumption is that quantity There is no assignment in a cell , The system assigns values automatically 0;
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Update table ：
ALTER TABLE( surface )
------------------------------
Add a column to the existing table ：
alter table Table name add Name data type ;（ Such as ,vend_phone）
------------------------------
Delete a known column for the existing table ：
alter table Table name drop Name ;
------------------------------
Delete an existing table ：
drop table Table name ;
------------------------------
Change the table name of an existing table ：
alter table The old name of the table rename to/as The new name of the table ;  
------------------------------

Update the existing data in the table ：
give an example ：
Update a column ：
update customers
set cust_email = '[email protected]'
where cust_id = 1000000005;
Update multiple columns ：
update  customers
set cust_contact = 'Sam Roberts',
      cust_email = '[email protected]'
where cust_id = 1000000006;
When you want to delete the value of a column , It can be changed to cust_contact = NULL ,cust_email = NULL
------------------------------
Delete data ：
give an example ：
delect from customers
where cust_id = 1000000006;
This is intended to delete the table customers in cust_id=1000000006 A row of data , Note that if you delete a specific column , use update sentence ;
If there is no “where cust_id = 1000000006;” Delete customers All rows in , But the form still exists .