MySQL 使用经验合辑

一. MySQL Dump:

命令行下具体用法如下：  mysqldump -u用戶名 -p密码 -d 數據库名 表名 脚本名;

1、导出數據库為dbname的表结构（其中用戶名為root,密码為dbpasswd,生成的脚本名為db.sql）

mysqldump -uroot -pdbpasswd -d dbname >db.sql;

2、导出數據库為dbname某张表(test)结构

mysqldump -uroot -pdbpasswd -d dbname test>db.sql;

3、导出數據库為dbname所有表结构及表數據（不加-d）

mysqldump -uroot -pdbpasswd  dbname >db.sql;

4、导出數據库為dbname某张表(test)结构及表數據（不加-d）

mysqldump -uroot -pdbpasswd dbname test>db.sql;

EG: 

mysqldump  -h XXXX.XXX.XXX.XX -uUSERNAME_XXXX -pPASSWORD_XXX DBNAME TABLENAME>file.sql;

 

二. MySQL查询随机一行数据:

SELECT AAA.ID, AAA.Name, AAA.Age 
FROM MySqlTable AS AAA JOIN ( 
     SELECT ROUND(RAND() * 
            ((SELECT MAX(ID) FROM MySqlTable)-(SELECT MIN(ID) FROM MySqlTable))+ 
            (SELECT MIN(ID) FROM MySqlTable) 
     ) AS RandomID) AS BBB 
WHERE AAA.ID >= BBB.RandomID 
ORDER BY AAA.ID LIMIT 1; 

 在7W的数据量下,  查询平均花费 0.0014 秒(已测)

 

三. MySQL 查询缓存:select @@query_cache_type;

1. 要么打开缓存;

2. 要么关闭缓存;

3. 要么使用SQL的显示缓存 select SQL_CACHE user_name from users where user_id = '100';

http://blog.sina.com.cn/s/blog_75ad10100101by7j.html

 

四. MySQ Lleft(right)join筛选条件在on and与where差异:

http://xianglp.iteye.com/blog/868957

 

五. MySQL计算集合之间的差集和交集;

普通做法就是IN; 但是大数据下太慢; 所以1, 2:

1. 差集;

SELECT ID FROM (
SELECT DISTINCT A.AID AS ID FROM TABLE_A A
UNION ALL
SELECT DISTINCT B.BID AS ID FROM TABLE_B B
)TEMP GROUP BY ID HAVING COUNT(ID) = 1

 2. 交集;

SELECT ID FROM (
SELECT DISTINCT A.AID AS ID FROM TABLE_A A
UNION ALL
SELECT DISTINCT B.BID AS ID FROM TABLE_B B
)TEMP GROUP BY ID HAVING COUNT(ID) = 2

 3. .... LEFT JOIN解决问题:

http://www.cnblogs.com/cy163/archive/2008/11/03/1325400.html

 

六. MySQL死锁解决:

JAVA异常:

2014-02-11 12:20:29,947 - SYS [http-bio-52000-exec-9] ERROR  Log4jUtil.exception(Log4jUtil.java:47) 

- PreparedStatementCallback; SQL []; Deadlock found when trying to get lock; 

try restarting transaction; nested exception is com.mysql.jdbc.exceptions.jdbc4.MySQLTransactionRollbackException: Deadlock found when trying to get lock;  try restarting transaction

mysql> show processlist ;

mysql> kill mysqlprocessid;

原因: 数据批量更新时,执行一个Update语句时, 使用了一个耗时较大的SQL, SQL中用了IN, IN中的数据大约是2000条;

 

七. mysql group by order by

select * from test group by category_id order by `date`

 

select * from (select * from `test` order by `date` desc) `temp`  group by category_id order by `date` desc

第一个SQL,是基于 category_id分组后, 取分组的数据为顺序排列的第一条数据.  最后在根据每个组的数据基于date排序;

第二个SQL, 是取category_id分组中date最大的值的行, 最后再根据每个组基于date排序

 http://www.cnblogs.com/not-code/archive/2011/11/01/2231493.html

 

八. 循环插入表数据

#创建测试的test表
DROP TABLE IF EXISTS test; 
CREATE TABLE test( 
    ID INT(10) NOT NULL, 
    `Name` VARCHAR(20) DEFAULT '' NOT NULL, 
    PRIMARY KEY( ID ) 
)ENGINE=INNODB DEFAULT CHARSET utf8; 

#创建生成测试数据的存储过程
DROP PROCEDURE IF EXISTS pre_test; 
DELIMITER //
CREATE PROCEDURE pre_test() 
BEGIN 
DECLARE i INT DEFAULT 0; 
SET autocommit = 0; 
WHILE i<10000000 DO 
INSERT INTO test ( ID,`Name` ) VALUES( i, CONCAT( 'Carl', i ) ); 
SET i = i+1; 
IF i%2000 = 0 THEN 
COMMIT; 
END IF; 
END WHILE; 
END; //
DELIMITER ;

#执行存储过程生成测试数据
CALL pre_test();

 

九. 查询排名:

/*表结构*/
CREATE TABLE  `t1` (                   
          `xingming` char(10) default NULL,   
          `chengji` decimal(6,2) default NULL,
           `mingci` int(3) NOT NULL,            
          KEY `t1` (xingming,chengji)            
        ) ;
/*数据*/
insert into t1 (xingming,chengji)
      values
      ('张一',50),
      ('张二',80),
      ('张三',70),
      ('张四',20),
      ('张五',90),
      ('张六',90),
      ('张七',70);
/*思路
排名是这样的
先按分数排序
如果分数一样,则并列第几名
分数不一样,则是他的顺序号
用三个变量实现
@chengji  保存成绩
@mingci 保存名次
@shunxu 保存顺序
*/
set @chengji=0;
set @mingci=1;
set @shunxu=0;
/*sql语句*/
update t1,
(select xingming,chengji,
@shunxu:=@shunxu+1,
if(chengji=@chengji,@mingci,@mingci:=@shunxu) as mingci,
@chengji:=chengji
from t1 order by chengji desc) as t2
set t1.mingci=t2.mingci
WHERE t1.xingming=t2.xingming and t1.chengji=t2.chengji;


select xingming,chengji,mingci from t1 order by mingci desc;
/*查询结果
"张四"	"20.00"	"7"
"张一"	"50.00"	"6"
"张三"	"70.00"	"4"
"张七"	"70.00"	"4"
"张二"	"80.00"	"3"
"张五"	"90.00"	"1"
"张六"	"90.00"	"1"
*/

 同理, 给每行数据返回中添加自增的ID的SQL;

SET    @VAR_Rank=0;
SELECT AA.DataColumn,
       @VAR_Rank:=@VAR_Rank+1 AS ID
FROM   TABLE_XXXXX AA
where  AA.DataColumn>1000
ORDER  BY AA.DataColumn ASC
LIMIT  100

 http://bbs.csdn.net/topics/250014224

   十. 数据库表数据备份:

#!/bin/bash

da=`date '+%Y%m%d'`
#echo "$da"
mysqldump -udbusername -ppasswd110 -h127.0.0.1 dbname tablename > /data/tablename/tablename_$da.sql

 

评论

1 楼 greatwqs 2018-11-07  

ALTER TABLE `mydb`.`user` 
MODIFY COLUMN `CreateTime` datetime(0) NOT NULL DEFAULT CURRENT_TIMESTAMP,
MODIFY COLUMN `UpdateTime` datetime(0) NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP;

创建时间默认值, 更新时间默认值 + 更新默认值