解決 MySQL 分頁數據錯亂重複
前言
一天，小明很着急地在通信工具上說：這邊線上出現了個奇怪的問題，麻煩 DBA 大大鑒定下，執行語句 select xx from table_name wheere xxx order by 字段A limit offset;，
表數據總共 48 條，分頁數量正常，但出現告終果混雜的狀況，第一頁的數據出如今了第二頁上；若是 order by 字段B 就不會出現這個現象，怎麼會這樣呢！html
其實，這個問題很簡單，若是你有仔細閱讀官檔的話。~^_^~mysql
咱們先來看看官檔是怎麼說的：sql
If multiple rows have identical values in the ORDER BY columns, the server is free to return those rows in any order, and may do so differently depending on the overall execution plan. In other words, the sort order of those rows is nondeterministic with respect to the nonordered columns.ide
One factor that affects the execution plan is LIMIT, so an ORDER BY query with and without LIMIT may return rows in different orders.工具
問題重現
本次實驗使用社區版 MySQL 5.6.26（由於小明出現問題的環境就是這個版本O(∩_∩)O~），下面先建立實驗環境和初始化測試數據：測試
root@localhost [(none)]>select @@version;
+------------+
| @@version |
+------------+
| 5.6.26-log |
+------------+
1 row in set (0.00 sec)
root@localhost [(none)]>show variables like "sql_mode";
+---------------+------------------------+
| Variable_name | Value |
+---------------+------------------------+
| sql_mode | NO_ENGINE_SUBSTITUTION |
+---------------+------------------------+
1 row in set (0.00 sec)
root@localhost [(none)]>create database glon_ho;
Query OK, 1 row affected (0.04 sec)
root@localhost [(none)]>use glon_ho
Database changed
root@localhost [glon_ho]>create table glon(
-> id int not null auto_increment primary key,
-> name varchar(20) not null,
-> create_time datetime not null,
-> age tinyint unsigned default 18
-> );
Query OK, 0 rows affected (0.01 sec)
root@localhost [glon_ho]>INSERT INTO `glon` VALUES (1, 'Eason Chan', '2017-05-02 08:10:10', 19),(2, 'Glon Ho', '2017-05-03 12:10:10', 18),(3, '趙敏', '2017-05-03 14:10:10', 17),(4, 'Jacky Cheung', '2017-05-02 14:00:00', 22),(5, '周芷若', '2017-05-02 14:00:00', 16),(6, 'Andy Lau', '2017-05-02 14:00:00', 50),(7, '至尊寶', '2017-05-02 14:00:00', 20),(8, '劉三姐', '2017-05-02 14:00:00', 19);
Query OK, 8 rows affected (0.01 sec)
Records: 8 Duplicates: 0 Warnings: 0
root@localhost [glon_ho]>select * from glon;
+----+--------------+---------------------+------+
| id | name | create_time | age |
+----+--------------+---------------------+------+
| 1 | Eason Chan | 2017-05-02 08:10:10 | 19 |
| 2 | Glon Ho | 2017-05-03 12:10:10 | 18 |
| 3 | 趙敏 | 2017-05-03 14:10:10 | 17 |
| 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 |
| 5 | 周芷若 | 2017-05-02 14:00:00 | 16 |
| 6 | Andy Lau | 2017-05-02 14:00:00 | 50 |
| 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 |
| 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 |
+----+--------------+---------------------+------+
8 rows in set (0.00 sec)
這裏建立了一個 glon 表，字段有自增 id, 姓名 name, 年齡 age, 及用戶註冊時間 create_time。spa
接着來複現問題：code
- 根據用戶註冊時間 create_time 來排序：
root@localhost [glon_ho]>select * from glon ORDER BY create_time limit 0, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | +----+-----------+---------------------+------+ 4 rows in set (0.00 sec)
能夠看到兩次查詢結果中都出現了 id 爲 8 的劉三姐，從上面初始化數據來看，總共有 8 條數據，如今不但分頁出現重複數據，還丟了一條！server
問題確實重現了，不過先不急，咱們再來試多幾組其餘的排序方式。htm
- create_time 和 age 組合排序
root@localhost [glon_ho]>select * from glon ORDER BY create_time,age limit 0, 4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | +----+------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,age limit 4, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec)
- create_time 和 id 組合排序
root@localhost [glon_ho]>select * from glon ORDER BY create_time,id limit 0, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,id limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | +----+-----------+---------------------+------+ 4 rows in set (0.00 sec)
- 主鍵 id 排序
root@localhost [glon_ho]>select * from glon ORDER BY id limit 0, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY id limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | +----+-----------+---------------------+------+ 4 rows in set (0.00 sec)
看到，後面的幾組排序方式都沒有再出現問題了，結合官檔，咱們知道 order by 排序的時候，若是排序字段中有多行相同的列值，則排序結果是不肯定的。因此後面的幾組組合形式的排序或者是主鍵 id 的排序，由於惟一性高，因此排序是肯定的，不會出現結果混亂的問題。
那是否是能夠就此結束了呢，no way， 咱們再來看下面的實驗，繼續鞏固一下：
- 根據年齡 age 來排序：
root@localhost [glon_ho]>select * from glon ORDER BY age limit 0, 4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | +----+------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age limit 4, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec)
咦，這個排序也只是根據一個字段 age 來排序，怎麼就沒有出問題呢？不急，還有招：
root@localhost [glon_ho]>insert into glon values (9,'喬峯','2017-05-03 13:10:10',22),(10,'段譽','2017-05-03 15:10:10',19),(11,'郭靖','2017-05-03 17:10:10',20),(12,'黃蓉','2017-05-03 08:10:10',19); Query OK, 4 rows affected (0.01 sec) Records: 4 Duplicates: 0 Warnings: 0 root@localhost [glon_ho]>select * from glon; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 9 | 喬峯 | 2017-05-03 13:10:10 | 22 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | +----+--------------+---------------------+------+ 12 rows in set (0.00 sec)
我又給 glon 表新增了幾條數據，而後再來看看：
root@localhost [glon_ho]>select * from glon ORDER BY create_time limit 0, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | +----+-----------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time limit 8, 4; +----+--------+---------------------+------+ | id | name | create_time | age | +----+--------+---------------------+------+ | 9 | 喬峯 | 2017-05-03 13:10:10 | 22 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | +----+--------+---------------------+------+ 4 rows in set (0.00 sec)
根據 create_time 排序，沒有問題了，再來：
root@localhost [glon_ho]>select * from glon ORDER BY age limit 0, 4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | +----+------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | +----+-----------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age limit 8, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 9 | 喬峯 | 2017-05-03 13:10:10 | 22 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec)
能夠看到根據年齡 age 排序，問題出現了。
而後在看看組合的排序：
root@localhost [glon_ho]>select * from glon ORDER BY create_time,id limit 0, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,id limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | +----+-----------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,id limit 8, 4; +----+--------+---------------------+------+ | id | name | create_time | age | +----+--------+---------------------+------+ | 9 | 喬峯 | 2017-05-03 13:10:10 | 22 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | +----+--------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,age limit 0, 4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | +----+------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,age limit 4, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,age limit 8, 4; +----+--------+---------------------+------+ | id | name | create_time | age | +----+--------+---------------------+------+ | 9 | 喬峯 | 2017-05-03 13:10:10 | 22 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | +----+--------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age,id limit 0, 4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | +----+------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age,id limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | +----+-----------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age,id limit 8, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 9 | 喬峯 | 2017-05-03 13:10:10 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec)
思考
既然排序不定，那麼給排序字段加上索引會不會有用呢？
root@localhost [glon_ho]>alter table glon add index ix_age(age);
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
root@localhost [glon_ho]>show create table glon\G
*************************** 1. row ***************************
Table: glon
Create Table: CREATE TABLE `glon` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(20) NOT NULL,
`create_time` datetime NOT NULL,
`age` tinyint(3) unsigned DEFAULT '18',
PRIMARY KEY (`id`),
KEY `ix_age` (`age`)
) ENGINE=InnoDB AUTO_INCREMENT=13 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
root@localhost [glon_ho]>select * from glon order by age limit 0,4;
+----+------------+---------------------+------+
| id | name | create_time | age |
+----+------------+---------------------+------+
| 5 | 周芷若 | 2017-05-02 14:00:00 | 16 |
| 3 | 趙敏 | 2017-05-03 14:10:10 | 17 |
| 2 | Glon Ho | 2017-05-03 12:10:10 | 18 |
| 1 | Eason Chan | 2017-05-02 08:10:10 | 19 |
+----+------------+---------------------+------+
4 rows in set (0.00 sec)
root@localhost [glon_ho]>select * from glon order by age limit 4,4;
+----+-----------+---------------------+------+
| id | name | create_time | age |
+----+-----------+---------------------+------+
| 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 |
| 10 | 段譽 | 2017-05-03 15:10:10 | 19 |
| 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 |
| 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 |
+----+-----------+---------------------+------+
4 rows in set (0.00 sec)
root@localhost [glon_ho]>select * from glon order by age limit 8,4;
+----+--------------+---------------------+------+
| id | name | create_time | age |
+----+--------------+---------------------+------+
| 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 |
| 9 | 喬峯 | 2017-05-03 13:10:10 | 22 |
| 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 |
| 6 | Andy Lau | 2017-05-02 14:00:00 | 50 |
+----+--------------+---------------------+------+
4 rows in set (0.00 sec)
也能夠不在 order by 後面加多一列增長惟一性，能夠改寫成下面的形式：
root@localhost [glon_ho]>select * from (select distinct g.* from glon g order by age) t limit 0,4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | +----+------------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from (select distinct g.* from glon g order by age) t limit 4,4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | +----+-----------+---------------------+------+ 4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from (select distinct g.* from glon g order by age) t limit 8,4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 9 | 喬峯 | 2017-05-03 13:10:10 | 22 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+ 4 rows in set (0.00 sec)
總之，若是發生了，最簡單的方法就是在排序列（如 create time）上加索引，而後在 order by 上明示 primary key，這個問題就很是圓滿的解決了。
總結
MySQL 使用 limit 進行分頁時，可能會出現重複數據，經過加入 order by 子句能夠解決，可是須要注意的是，若是排序字段有相同值的狀況下，因爲排序字段數據重複,可能會致使每次查詢排序後結果順序不一樣，分頁仍是會出現重複數據，這時能夠加入第二個排序字段，提升排序的惟一性，最好保證排序的字段在表中的值是惟一的，這樣就能夠少寫一個排序字段，增長查詢效率，由於 order by 後面有多個排序字段時，沒法用到索引。
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