MySQL learning record - II. MySQL create table command

Create table command

1.1 Sign in

[[email protected]]# mysql -u root -p   
Enter password:******  #  Enter the terminal after logging in 
mysql> create DATABASE RUNOOB;
// establish 
CREATE DATABASE  Database name ;

1.2 Create database

CREATE TABLE table_name (column_name column_type);
mysqladmin -u root -p create < Database name >
Enter password:******

1.3 Delete database
drop Command to delete the database

drop database < Database name >;

mysqladmin Delete database

mysqladmin -u root -p drop  Database name 
// Check the data 
desc emo
// Show the bank 
show linesize
// Set line payment 
set linesize 120
// Set column width 
col ename for a8
col sal for 999
/

mysql Index means

1.Unique Set non repeatable fields ( unique index );
2.Normal Speed up query ( General index );
3.Full Text Article segment query ( Full-text index );

Index method

1.B-Tree
B-Tree Is the most common type of index , All worth （ Indexed columns ） It's all sorted out , The distance from each leaf node to the following node is equal . therefore B-Tree It is suitable for finding data in a certain range , And it can directly support data sorting （ORDER BY）
B-Tree stay MyISAM The form and Innodb not quite the same ：
MyISAM The table data file and the index file are separate , The index file only stores the disk address of the data record
InnoDB The table data file itself is the primary index , Leaf nodes data Domain holds complete data records
2.HASH
1. Support only "=",“IN" and ”<=>" Precise query , Can't use range query ：
because Hash Index comparison is to carry out Hash After the operation Hash value , So it can only be used for equivalent filtering , Can't be used for range based filtering , Because after the corresponding Hash After algorithm processing Hash
2. Sort is not supported ：
because Hash What the index stores is the process Hash After calculation Hash value , and Hash The magnitude of the value does not necessarily relate to Hash The key value before the operation is exactly the same , So the database can't use the index data to avoid any sort operation
3. Table scanning cannot be avoided at any time ：
because Hash Index comparison is to carry out Hash After the operation Hash value , So even if you meet someone Hash Number of records of key value data , I can't go from Hash Direct query in index , You still need to access the actual data in the table for corresponding comparison , And get the corresponding result
4. High retrieval efficiency , The index can be retrieved at one time , Unlike B-Tree The index needs to be from the root node to the branch node , Finally, you can access the page node for many times IO visit , therefore Hash The query efficiency of index is much higher than B-Tree Indexes
5. Only Memory The engine supports explicit Hash Indexes , But it's Hash yes nonunique Of , Too many conflicts will also affect the search performance .Memory The default index type of the engine is Hash Indexes , Although it also supports B-Tree Indexes

mysql Index these three ,nomal The most commonly used ,full text Generally used in article search ,Unique Generally, it means that it cannot be reused
How to optimize database ：
1． Choose a unique index
2． Order for frequent need 、 Index fields for grouping and combining operations
3． Index fields that are often used as query criteria
4． Limit the number of indexes
5． Try to use indexes with less data
6． Try to use prefixes to index
7． Delete indexes that are no longer or rarely used

Reference article ： https://www.cnblogs.com/maowenqiang/p/10383835.html