Mysqls on Coding

MySQL对包含NULL字段排序

Wed, 07 Mar 2018 02:54:57 +0000

CREATE TABLE `sorted` (
	`id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
	`idx` INT(11) UNSIGNED NULL DEFAULT NULL,
	PRIMARY KEY (`id`)
)

INSERT INTO `sorted` (`id`, `idx`) VALUES (1, 2);
INSERT INTO `sorted` (`id`, `idx`) VALUES (2, NULL);
INSERT INTO `sorted` (`id`, `idx`) VALUES (3, 3);
INSERT INTO `sorted` (`id`, `idx`) VALUES (4, 1);
INSERT INTO `sorted` (`id`, `idx`) VALUES (5, 1);
INSERT INTO `sorted` (`id`, `idx`) VALUES (6, 4);
INSERT INTO `sorted` (`id`, `idx`) VALUES (7, 3);
INSERT INTO `sorted` (`id`, `idx`) VALUES (8, NULL);
INSERT INTO `sorted` (`id`, `idx`) VALUES (9, 0);

NULL被视为低于任何非NULL值。

mysql> select * from sorted;
+----+------+
| id | idx  |
+----+------+
|  1 |    2 |
|  2 | NULL |
|  3 |    3 |
|  4 |    1 |
|  5 |    1 |
|  6 |    4 |
|  7 |    3 |
|  8 | NULL |
|  9 |    0 |
+----+------+
9 rows in set (0.00 sec)

方案一：

mysql> select id, idx from sorted order by -idx desc;
+----+------+
| id | idx  |
+----+------+
|  9 |    0 |
|  4 |    1 |
|  5 |    1 |
|  1 |    2 |
|  3 |    3 |
|  7 |    3 |
|  6 |    4 |
|  2 | NULL |
|  8 | NULL |
+----+------+
9 rows in set (0.00 sec)

方案二：

mysql> select id,idx from sorted order by isnull(idx), idx;
+----+------+
| id | idx  |
+----+------+
|  9 |    0 |
|  4 |    1 |
|  5 |    1 |
|  1 |    2 |
|  3 |    3 |
|  7 |    3 |
|  6 |    4 |
|  2 | NULL |
|  8 | NULL |
+----+------+
9 rows in set (0.00 sec)

方案三：

mysql> select id,idx from sorted order by if(isnull(idx), 0, 1) desc, idx;
+----+------+
| id | idx  |
+----+------+
|  9 |    0 |
|  4 |    1 |
|  5 |    1 |
|  1 |    2 |
|  3 |    3 |
|  7 |    3 |
|  6 |    4 |
|  2 | NULL |
|  8 | NULL |
+----+------+
9 rows in set (0.00 sec)

ONLY_FULL_GROUP_BY

Wed, 22 Nov 2017 06:42:11 +0000

单机启动多个MySQL实例

Wed, 22 Nov 2017 06:42:11 +0000

一：利用mysql_install_db生成数据库

mysql_install_db --datadir=/var/db/my3307/data --user=mysql
mysql_install_db --datadir=/var/db/my3308/data --user=mysql

二：配置文件

vi /etc/my_multi.conf

[mysqld_multi]
[mysqld3307] 
server-id = 3307
datadir  = /var/db/my3307/data
socket = /var/db/my3307/mysql.sock
pid-file  = /var/db/my3307/mysqld.pid
log-bin	= /var/db/my3307/logs/mysql-bin.log
log-error = /var/db/my3307/logs/mysqld.log

[mysqld3308]
server-id = 3308
datadir   = /var/db/my3308/data
socket    = /var/db/my3308/mysql.sock
pid-file  = /var/db/my3308/mysqld.pid
log-bin   = /var/db/my3308/logs/mysql-bin.log
log-error = /var/db/my3308/logs/mysqld.log

三：启动

/usr/local/bin/mysqld_multi --defaults-extra-file=/etc/my_multi.cnf stop|start|report

遇到的问题：

FATAL ERROR: Could not find ./bin/my_print_defaults If you compiled from source, you need to run make install to copy the software into the correct location ready for operation. If you are using a binary release, you must either be at the top level of the extracted archive, or pass the basedir option

解决方案：

在/usr/local下执行或者指定basedir。

/usr/local/bin/mysql_install_db
--user=mysql
--basedir=/usr/local
--datadir=/var/db/my3307/data
--defaults-file=/etc/my.cnf