1、mysql死锁
2、查看时区：
show variables like "%time_zone%";
3、mysql datetime字段使用DATE_FORMAT()函数，我是链接
4、MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD hh:mm:ss' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59' . TheTIMESTAMP data type is used for values that contain both date and time parts.TIMESTAMP has a range of '1970-01-01 00:00:01' UTC to '2038-01-19 03:14:07' UTC.
5、linux没有设置systemctl服务的话，需要去安装的bin下执行nohup mysqld &；
6、
解决报错:Error Code: 1786 Statement violates GTID consistency: CREATE TABLE ... SELECT.
7、简单的游标定义的存储过程
CREATE DEFINER=`root`@`%` PROCEDURE `savexxxxx`(
in position_ids varchar(10),
in project_id bigint,
out exe_result int
)
BEGIN
DECLARE done INT DEFAULT 0;
DECLARE facility_id,tmp_i BIGINT DEFAULT 0;
DECLARE cur1 CURSOR FOR select d.id from tb_a d,tb_b t where d.facility_type = t.id and t.need_rel_area = 1 and d.position_id = position_ids;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
select position_ids;
select project_id;
OPEN cur1;
REPEAT
FETCH cur1 INTO facility_id;
IF NOT done THEN
#select facility_id;
INSERT INTO tb_project_item(project_id,type,type_id) VALUES (project_id,3,facility_id);
END IF;
UNTIL done END REPEAT;
CLOSE cur1;
select count(*) from tb_c into exe_result;
END
mysql shell
https://dev.mysql.com/downloads/shell/
mysql表结构中关于时间字段可以同时存在一个datetime字段，一个unix time的long值字段
mysql索引原理
测试环境的5.7.24版本的my.cnf设置：
#my.cnf [client] port = 3306 socket = /tmp/mysql.sock [mysql] prompt="\u@\h [\d]>" #pager="less -i -n -S" #tee=/opt/mysql/query.log no-auto-rehash [mysqld] federated sql_mode='' user = mysql basedir = /usr/local/mysql datadir = /data1/mysql3306/data port = 3306 socket = /tmp/mysql.sock event_scheduler = on tmpdir = /data1/mysql3306/tmp skip_name_resolve = 1 disable-partition-engine-check log_timestamps = SYSTEM max_allowed_packet = 20M wait_timeout=86400 interactive_timeout = 86400 #character set character-set-server = utf8mb4 log_warnings = 1 lower_case_table_names=1 max_connections = 1000 max_connect_errors = 100000 #logs slow_query_log = 1 slow_query_log_file = slow.log lock_wait_timeout = 3600 log-error = error.log pid-file = mysql.pid long_query_time = 1 #log-slow-admin-statements = 1 #log-queries-not-using-indexes = 1 log-slow-slave-statements = 1 binlog_format = row server-id = 173306 log-bin = /data1/mysql3306/logs/mysql-bin sync_binlog = 1 expire_logs_days = 7 log_bin_trust_function_creators=1 #slave setup gtid_mode = on enforce_gtid_consistency = 1 master_info_repository = TABLE relay_log_info_repository = TABLE skip_slave_start = 1 relay_log_purge = 1 relay_log_recovery = 1 relay-log=relay-bin relay-log-index=relay-bin.index log_slave_updates #buffers & cache query_cache_size = 0 query_cache_type = 0 #innodb innodb_data_file_path = ibdata1:1G:autoextend innodb_log_file_size = 100M innodb_log_files_in_group = 2 innodb_max_undo_log_size = 1G innodb_max_undo_log_size = 1M innodb_buffer_pool_size = 200M innodb_flush_log_at_trx_commit = 1 innodb_rollback_on_timeout innodb_status_file = 1 innodb_io_capacity = 100 innodb_io_capacity_max = 100 transaction_isolation = READ-COMMITTED innodb_flush_method = O_DIRECT
mysql group by分组查询count时，对count(null)得到结果值是0，结合case when then可以查询多个字段count的值:
SELECT CONVERT ( from_base64 ( u.true_name ) USING utf8 ) AS userName, count( CASE WHEN a.origin_status = 1 THEN NULL ELSE a.origin_status END) as autoCount, count( CASE when a.origin_status = 1 then null WHEN a.check_status = '0' THEN NULL ELSE a.check_status END ) as autoCompleteCount, count( CASE WHEN a.origin_status = 0 THEN NULL ELSE a.origin_status END) as selfCount FROM tb_a a,tb_sys_user u where a.check_person = u.id and DATE_FORMAT( a.create_date, '%Y' ) = 2019 AND DATE_FORMAT( a.create_date, '%m' ) = 12 GROUP BY u.true_name,u.id;
https://dev.mysql.com/doc/refman/5.7/en/case.html
2020-01-13
01-15
使用float字段类型时，需要注意设置字段的小数点精度；
慕课网mysql优化，还有可以去参考阿里巴巴java开发手册中相关内容
explain返回各列的含义：
type：表示从表中查询到行所执行的方式，查询方式是 SQL 优化中一个很重要的指标，结果值从好到差依次是：system > const > eq_ref > ref > range > index > ALL。
possible_keys：可能使用到的索引。
key：实际使用到的索引。
key_len：当前使用的索引的长度，不损失精度的情况，越小越好
ref：关联 id 等信息。
rows：查找到记录所扫描的行数。
filtered 过滤掉数据占所有数据的比例
extra列需要注意的返回值：
Using filesort:看到这个，查询就需要优化了。Mysql需要进行额外的步骤来发现如何对返回的行排序。
Using temporary: 需要优化，Mysql需要创建一个临时表来存储结果，这通常发生在对不同的列集进行order by上，而不是group by 上；
#eg1:
select max(payment_date) from payment;
#通过对payment_date建立索引，提高查询效率
create index id_paydate on payment(payment_date);
#正确 select count(release_year = '2006' or null) as '2006film_count', count(release_year = '2007' or null) as '2007file_count' from file;
子查询的优化：
通常情况下，需要把子查询优化为join查询，但在优化时需要注意关联键是否有一对多关系，要注意重复数据。
explain select title,release_year,LENGTH FROM film where film_id in (select film_id from file_actor where actor_id in (select actor_id from actor where first_name = 'sandra')); select * from t where t.id in (select t1.tid from t1); select t.id from t join t1 on t.id = t1.tid;
sql数据库优化
explain select actor.first_name,actor.last_name,count(*) from sakila.film_actor inner join sakila.actor using(actor_id) group by file_actor.actor_id; #优化后 参演电影最多的演员 explain select actor.first_name,actor.last_name,c.cnt from sakila.actor inner join(select actor_id,count(*) as cnt from sakila.film_actor group by actor_id) as c using (actor_id);
sql优化limit查询，时常会伴随order by从句使用，因此大多数时候会使用filesorts，这样会造成大量的IO问题。
select film_id,desc from sakila.film order by title limit 50,5;
这个查询的优化步骤1：使用有索引的列或者主键进行order by 操作
select film_id,desc from sakila.film order by id limit 50,5;
优化步骤2：记录上一次返回的主键，在下次查询时使用主键过滤(中心思想：避免了数据量大时扫描过多的记录)：
select film_id,desc from sakila.film where film_id > 600 and file_id <=605 order by film_id limit 1,5;
sql及索引优化：
如何选择合适的列建立索引？
1、在where从句，group by 从句,order by 从句，on从句中出现的列
2、索引字段越小越好
3、离散度大的列放在联合索引的前面
select * from payment where staff_id = 2 and customer_id = 584;
由于customer_id的离散度更大，所以应该使用index(customer_id,staff_id)
判断离散度的方法
select count(customer_id),count(staff_id) from payment
哪个大，哪个离散度大
索引的维护和优化：
--查找重复和冗余的索引：
pt-duplicate-key-checker -u root -p '' -h 127.0.0.1
--删除不用索引
mariadb中查看index_statistics表，mysql中通过慢日志配合pt-index-usage工具来进行索引使用情况分析
pt-index-usage -u root -p '' mysql-slow.log
数据库结构优化
选择合适的数据类型：
1、选在最小的数据类型
2、使用简单的数据类型；
3、使用not null定义字段
4、少于text类型；
create table test(id int auto_increament not null,timestr int,primary key(id)); insert into test(timestr) values (unix_timestamp('2014-06-01 13:12:00')); select from_unixtime(timestr) from test
可以使用bigint来存ip地址，使用inet_aton(),inet_ntoa()两个函数来进行转换；
https://tech.meituan.com/2017/03/09/sqladvisor-pr.html
https://github.com/Meituan-Dianping/SQLAdvisor
and unix_timestamp(end_time) > unix_timestamp(NOW());