Several points in MySQL that are easy to ignore and forget

MySQL

Backup and recovery

Database backup ：
The graphical interface is often easy to forget how to write commands after a long time

dos Enter... Under the window

mysqldump -u root -p -B databaseName1 [databaseName2 databaseNamen] > d:\\bak.sql

Table backup

mysqldump -u root -p databaseName tableName1 [tableName2 tableNamen] > d:\\bak.sql

recovery

mysql Input in

source d:\\bak.sql;

Common data types

varchar

Range 0-65535 byte Maximum 65532 byte （1~3 Bytes are used to record the size ）

• If the table code is utf8 varchar(size) size = (65535 - 3) / 3 = 21844

• If the table code is gbk varchar(size) size = (65535 - 3) / 2 = 32766

That is, the maximum number of characters that can be specified when creating a table is the above two

char(4) and varchar(4) It means the character , Not bytes , There are four characters in both numbers and Chinese characters , The byte space occupied depends on the encoding .

timestamp

You can create a table with a statement If this field does not specify a value during creation and update, the current timestamp is used .

NOT NULL 
DEFAULT CURRENT_TIMESTAMP
ON UPDATE CURRENT_TIMESTAMP;

CRUD

count(*) And count( Column ) difference

• count(*)： Return the number of rows of records that meet the conditions
• count( Column ) The statistical conditions of , How many in a column , But it will be ruled out null

Business

View the isolation level of the current session

SELECT @@tx_isolation;

View the current isolation level of the system

SELECT @@global.tx_isolation;

Set the isolation level of the current session

SET SESSION TRANSACTION ISOLATION LEVEL read uncommitted --  It can be followed by four isolation levels   for example  READ UNCOMMITTED

Set the current isolation level of the system

SET GLOBAL TRANSACTION ISOLATION LEVEL read uncommitted --  It can be followed by four isolation levels   for example  READ UNCOMMITTED

Global modification , stay my.ini Profile's [mysqld] Add or modify

#  The equal sign is followed by four isolation levels   for example  READ UNCOMMITTED
transaction-isolation = read uncommitted

The transaction ACID characteristic

• Atomicity （Atomicity）

A transaction is an indivisible unit of work , Either the operations in the transaction occur , Or none at all .

• Uniformity （Consistency）

Transactions must transform the database from one consistent state to another .

• Isolation, （Isolation）

Transaction isolation is when multiple users access the database concurrently , A transaction opened by a database for each user , Can't be disturbed by the operation data of other transactions , Multiple concurrent transactions should be isolated from each other .

• persistence （Durability）

Once a transaction is committed , Its changes to database data are permanent , Next, even if the database fails, it should not have any impact .

Storage engine

Modify the storage engine

ALTER TABLE `tablename` ENGINE = innodb   --  For example, it needs to be changed to innodb