MySQL common skills

1. Common operation statements

Connect

mysql -hxx -Pxx -uxx  -pxx

Build table

 CREATE TABLE `user` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `age` int(10) DEFAULT '0',
  `pos` varchar(30) DEFAULT NULL,
  `phone` varchar(11) DEFAULT NULL,
  `created_time` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_name_age_pos_phone` (`name`,`age`,`pos`,`phone`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

Operation table

select  *  from student left join score on student.id=score.s_id order by score_val desc  limit 15

select DISTINCT course_name from student_table where grade=1;

select * ,count(b) as sum  from table  where a='' group by s having sum>10 order by c desc  limit

select s.id, c.name from student s left join course c on s.id=c.student_id

insert into table (a,b) values (1,2)

delete from table where id>10

update table set a=1 where id=1

alter table app_apply add log_index varchar(255) default Null // change 、 Add fields 
alter table  Table name  add column  Name  varchar(20) not null first;
alter table  Table name  add column  Name  varchar(20) not null after user1;
alter table app_apply drop column def ;
alter table app_apply change  def unit char;

Aggregate functions

–COUNT： Count the number of lines

–SUM： Get the total value of a single column

–AVG： Calculate the average value of a column

–MAX： Calculate the maximum value of the column

–MIN： Calculate the minimum value of the column

SELECT student_class,AVG(student_age) AS  Average age  FROM t_student GROUP BY (student_class) HAVING AVG(student_age)>20; 

Add index

// Add primary key index  
ALTER TABLE `table_name` ADD PRIMARY KEY (`column`) 

// Add unique index 
ALTER TABLE `table_name` ADD UNIQUE (`column`) 

// Add full text index 
ALTER TABLE `table_name` ADD FULLTEXT (`column`) 

// Add a normal index 
ALTER TABLE `table_name` ADD INDEX index_name (`column` ) 

// Add Composite Index  
ALTER TABLE `table_name` ADD INDEX index_name (`column1`, `column2`, `column3`)

##2. The slow query

Check the database status

status;
show tables;
select * from information_schema.tables where table_name ='table_name'
show variables  like 'port'; 
show status like 'Threads%'; // Check the current number of connections to the database , Concurrency number 
show variables like '%max_connections%'; // See the maximum number of connections 
set global max_commections = 1000; // Modify the maximum number of connections  
show grants for 'root'@'localhost'; //  View the permissions of a specific user 

Index failure

• If the index has multiple columns （ Joint index a,b,c） Follow the leftmost prefix rule , That is, the query starts at the top left of the index and does not skip the columns in the index
• The column to the right of the index range condition ,< 、> The index column to the right of the index range condition will become invalid
• select *
• Do operations on index columns , Such as calculation 、 function 、（ Automatically or Manual ） Type conversion, etc , Will result in index invalidation and full table scan
• keyword or 、not in 、!=、<> It's not equal to
• Use LIKE Keyword query statement , If it starts with a wildcard （'%abc'） Index failure . Only “%” Index doesn't work until it's in the first place
• String without single quotes
• order by Violation of the leftmost prefix rule , Causes additional file sorting （ It can degrade performance ）
• group by Violation of the leftmost prefix rule , Resulting in a temporary watch （ It can degrade performance ）

adopt explain analysis sql sentence

explain select * from user where age = 20 and phone = '18730658760' and pos = 'cxy';

Show results analysis ：

table | type | possible_keys | key |key_len | ref | rows | Extra

EXPLAIN Explanation of columns ：

table Show which table this row's data is about

type This is an important column , Shows what type of connection is used . The best to worst connection type is const、eq_reg、ref、range、indexhe and ALL

rows Display the number of lines to scan

key Index used

perform show processlist;

see mysql Running sql Threads , You can see sql Statement execution time

show processlist

show full processlist

mysqladmin processlist

select * from information_schema.processlist // View all current process

select * from information_schema.processlist where info is not null order by time desc // View the query currently in progress and reverse it according to the execution time

see mysql Slow query log （ By default ,mysql Do not open slow query log ）

Log analysis tool mysqldumpslow

Get the most returned recordset 10 individual SQL.

mysqldumpslow -s r -t 10 /database/mysql/mysql06_slow.log

The most visited 10 individual SQL

mysqldumpslow -s c -t 10 /database/mysql/mysql06_slow.log

The number of slow queries is saved in mysql In the library slow_log surface .

SELECT * FROM slow_log where start_time > '2019/05/19 00:00:00';

Business logic

The data is huge

Deep paging Limit Causes a full table scan

Uneven data distribution

Network environment factors

One request execution 30 strip Sql Transmission time increases

network bandwidth Network card full

Packet loss retry

https://segmentfault.com/a/1190000021464570