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MySQL学习，详解分页查询(limit)
limit介绍
limit⽤来限制select查询返回的⾏数，常⽤于分页等操作。
语法：
select 列 from 表 limit [offset,] count;
说明：
offset：表⽰偏移量，通俗点讲就是跳过多少⾏，offset可以省略，默认为0，表
⽰跳过0⾏；范围：[0,+∞)。
count：跳过offset⾏之后开始取数据，取count⾏记录；范围：[0,+∞)。
limit中offset和count的值不能⽤表达式。
下⾯我们列⼀些常⽤的⽰例来加深理解。
获取前n⾏记录
select 列 from 表 limit 0,n;
或者
select 列 from 表 limit n;
⽰例，获取订单的前2条记录，如下：
mysql> create table t_order(
-> id int not null auto_increment comment '订单编号',
-> price decimal(10,2) not null default 0 comment '订单⾦额',
-> primary key(id)
-> )comment '订单表';
Query OK, 0 rows affected (0.01 sec)
mysql> insert into t_order (price) values (88.95),(100.68),(500),
(300),(20.88),(200.5);Query OK, 6 rows affected (0.01 sec)
Records: 6 Duplicates: 0 Warnings: 0
mysql> select * from t_order;
+----+--------+
| id | price |
+----+--------+
| 1 | 88.95 |
| 2 | 100.68 |
| 3 | 500.00 |
| 4 | 300.00 |
| 5 | 20.88 |
| 6 | 200.50 |
+----+--------+
6 rows in set (0.00 sec)
mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a limit 2;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 1 | 88.95 |
| 2 | 100.68 |
+--------------+--------------+
2 rows in set (0.00 sec)
mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a limit 0,2;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 1 | 88.95 |
| 2 | 100.68 |
+--------------+--------------+
2 rows in set (0.00 sec)
获取最⼤的⼀条记录
我们需要获取订单⾦额最⼤的⼀条记录，可以这么做：先按照⾦额降序，然后取第⼀条记
录，如下：mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a order by
a.price desc;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 3 | 500.00 |
| 4 | 300.00 |
| 6 | 200.50 |
| 2 | 100.68 |
| 1 | 88.95 |
| 5 | 20.88 |
+--------------+--------------+
6 rows in set (0.00 sec)
mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a order by
a.price desc limit 1;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 3 | 500.00 |
+--------------+--------------+
1 row in set (0.00 sec)
mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a order by
a.price desc limit 0,1;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 3 | 500.00 |
+--------------+--------------+
1 row in set (0.00 sec)
获取排名第n到m的记录
我们需要先跳过n-1条记录，然后取m-n+1条记录，如下：
select 列 from 表 limit n-1,m-n+1;如：我们想获取订单⾦额最⾼的3到5名的记录，我们需要跳过2条，然后获取3条记录，
如下：
mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a order by
a.price desc;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 3 | 500.00 |
| 4 | 300.00 |
| 6 | 200.50 |
| 2 | 100.68 |
| 1 | 88.95 |
| 5 | 20.88 |
+--------------+--------------+
6 rows in set (0.00 sec)
mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a order by
a.price desc limit 2,3;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 6 | 200.50 |
| 2 | 100.68 |
| 1 | 88.95 |
+--------------+--------------+
3 rows in set (0.00 sec)
分⻚查询
开发过程中，分页我们经常使⽤，分页⼀般有2个参数：
page：表⽰第⼏页，从1开始，范围[1,+∞)
pageSize：每页显⽰多少条记录，范围[1,+∞)
如：page = 2，pageSize = 10，表⽰获取第2页10条数据。
我们使⽤limit实现分页，语法如下：select 列 from 表名 limit (page - 1) * pageSize,pageSize;
需求：我们按照订单⾦额降序，每页显⽰2条，依次获取所有订单数据、第1页、第2页、
第3页数据，如下：
mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a order by
a.price desc;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 3 | 500.00 |
| 4 | 300.00 |
| 6 | 200.50 |
| 2 | 100.68 |
| 1 | 88.95 |
| 5 | 20.88 |
+--------------+--------------+
6 rows in set (0.00 sec)
mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a order by
a.price desc limit 0,2;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 3 | 500.00 |
| 4 | 300.00 |
+--------------+--------------+
2 rows in set (0.00 sec)
mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a order by
a.price desc limit 2,2;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 6 | 200.50 |
| 2 | 100.68 |
+--------------+--------------+
2 rows in set (0.00 sec)mysql> select a.id 订单编号,a.price 订单⾦额 from t_order a order by
a.price desc limit 4,2;
+--------------+--------------+
| 订单编号 | 订单⾦额 |
+--------------+--------------+
| 1 | 88.95 |
| 5 | 20.88 |
+--------------+--------------+
2 rows in set (0.00 sec)
避免踩坑
limit中不能使⽤表达式
mysql> select * from t_order where limit 1,4+1;
ERROR 1064 (42000): You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near 'limit 1,4+1' at line 1
mysql> select * from t_order where limit 1+0;
ERROR 1064 (42000): You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near 'limit 1+0' at line 1
mysql>
结论：limit后⾯只能够跟明确的数字。
limit后⾯的2个数字不能为负数
mysql> select * from t_order where limit -1;
ERROR 1064 (42000): You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near 'limit -1' at line 1
mysql> select * from t_order where limit 0,-1;
ERROR 1064 (42000): You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near 'limit 0,-1' at line 1
mysql> select * from t_order where limit -1,-1;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right
syntax to use near 'limit -1,-1' at line 1
排序分⻚存在的坑
准备数据：
mysql> insert into test1 (b) values (1),(2),(3),(4),(2),(2),(2),(2);
Query OK, 8 rows affected (0.01 sec)
Records: 8 Duplicates: 0 Warnings: 0
mysql> select * from test1;
+---+---+
| a | b |
+---+---+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
| 5 | 2 |
| 6 | 2 |
| 7 | 2 |
| 8 | 2 |
+---+---+
8 rows in set (0.00 sec)
mysql> select * from test1 order by b asc;
+---+---+
| a | b |
+---+---+
| 1 | 1 |
| 2 | 2 |
| 5 | 2 |
| 6 | 2 |
| 7 | 2 |
| 8 | 2 |
| 3 | 3 |
| 4 | 4 |+---+---+
8 rows in set (0.00 sec)
下⾯我们按照b升序，每页2条数据，来获取数据。
下⾯的sql依次为第1页、第2页、第3页、第4页、第5页的数据，如下：
mysql> select * from test1 order by b asc limit 0,2;
+---+---+
| a | b |
+---+---+
| 1 | 1 |
| 2 | 2 |
+---+---+
2 rows in set (0.00 sec)
mysql> select * from test1 order by b asc limit 2,2;
+---+---+
| a | b |
+---+---+
| 8 | 2 |
| 6 | 2 |
+---+---+
2 rows in set (0.00 sec)
mysql> select * from test1 order by b asc limit 4,2;
+---+---+
| a | b |
+---+---+
| 6 | 2 |
| 7 | 2 |
+---+---+
2 rows in set (0.00 sec)
mysql> select * from test1 order by b asc limit 6,2;
+---+---+
| a | b |
+---+---+
| 3 | 3 |
| 4 | 4 |
+---+---+2 rows in set (0.00 sec)
mysql> select * from test1 order by b asc limit 7,2;
+---+---+
| a | b |
+---+---+
| 4 | 4 |
+---+---+
1 row in set (0.00 sec)
上⾯有2个问题：
问题1：看⼀下第2个sql和第3个sql，分别是第2页和第3页的数据，结果出现了相同的数
据，是不是懵逼了。
问题2：整个表只有8条记录，怎么会出现第5页的数据呢，又懵逼了。
我们来分析⼀下上⾯的原因：主要是b字段存在相同的值，当排序过程中存在相同的值
时，没有其他排序规则时，mysql懵逼了，不知道怎么排序了。
就像我们上学站队⼀样，按照⾝⾼排序，那⾝⾼⼀样的时候如何排序呢？⾝⾼⼀样的就乱
排了。
建议：排序中存在相同的值时，需要再指定⼀个排序规则，通过这种排序规则不存在⼆义
性，⽐如上⾯可以再加上a降序，如下：
mysql> select * from test1 order by b asc,a desc;
+---+---+
| a | b |
+---+---+
| 1 | 1 |
| 8 | 2 |
| 7 | 2 |
| 6 | 2 |
| 5 | 2 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
+---+---+8 rows in set (0.00 sec)
mysql> select * from test1 order by b asc,a desc limit 0,2;
+---+---+
| a | b |
+---+---+
| 1 | 1 |
| 8 | 2 |
+---+---+
2 rows in set (0.00 sec)
mysql> select * from test1 order by b asc,a desc limit 2,2;
+---+---+
| a | b |
+---+---+
| 7 | 2 |
| 6 | 2 |
+---+---+
2 rows in set (0.00 sec)
mysql> select * from test1 order by b asc,a desc limit 4,2;
+---+---+
| a | b |
+---+---+
| 5 | 2 |
| 2 | 2 |
+---+---+
2 rows in set (0.00 sec)
mysql> select * from test1 order by b asc,a desc limit 6,2;
+---+---+
| a | b |
+---+---+
| 3 | 3 |
| 4 | 4 |
+---+---+
2 rows in set (0.00 sec)
mysql> select * from test1 order by b asc,a desc limit 8,2;
Empty set (0.00 sec)看上⾯的结果，分页数据都正常了，第5页也没有数据了。
总结
• order by ... [asc|desc]⽤于对查询结果排序，asc：升序，desc：降序，asc|desc可以省
略，默认为asc
• limit⽤来限制查询结果返回的⾏数，有2个参数（offset，count），offset：表⽰跳过
多少⾏，count：表⽰跳过offset⾏之后取count⾏
• limit中offset可以省略，默认值为0
• limit中offset 和 count都必须⼤于等于0
• limit中offset和count的值不能⽤表达式
• 分页排序时，排序不要有⼆义性，⼆义性情况下可能会导致分页结果乱序，可以在后
⾯追加⼀个主键排序