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MySQL查询数据之:单表查询

MySQL查询数据之:单表查询

单表查询指从一张表数据中查询所需的数据。
(1)查询所有字段
(2)查询指定字段
(3)查询指定记录
(4)带in关键字的查询
(5)带between and的范围的查询
(6)带like的字符匹配查询
(7)查询空值
(8)带and的多条件查询
(9)带or的多条件查询
(10)查询结果不重复
(11)对查询结果排序
(12)分组查询
(13)使用limit限制查询结果的数量


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(1)查询所有字段

1.select * from fruits
2.select f_id,s_id,f_name,f_price from fruits;

上面两个语句执行后的结果都是查询所有字段:

mysql> select * from fruits;+------+------+------------+---------+| f_id | s_id | f_name     | f_price |+------+------+------------+---------+| 12   |  104 | lemon      |    6.40 || a1   |  101 | apple      |    5.20 || a2   |  103 | apricot    |    2.20 || b1   |  101 | blackberry |   10.20 || b2   |  104 | berry      |    7.60 || b5   |  107 | xxxx       |    3.60 || bs1  |  102 | orange     |   11.20 || bs2  |  105 | melon      |    8.20 || c0   |  101 | cherry     |    3.20 || m1   |  106 | mango      |   15.70 || m2   |  105 | xbabay     |    2.60 || m3   |  105 | xxtt       |   11.60 || o2   |  103 | coconut    |    9.20 || t1   |  102 | banana     |   10.30 || t2   |  102 | grape      |    5.30 || t4   |  107 | xbabay     |    3.60 |+------+------+------------+---------+16 rows in set (0.00 sec)
(2)查询指定字段

1.查询单个字段:select 列名 from 表名;

【例】查询fruits表中f_name列所有水果名称,SQL语句如下:

mysql> select f_name from fruits;+------------+| f_name     |+------------+| lemon      || apple      || apricot    || blackberry || berry      || xxxx       || orange     || melon      || cherry     || mango      || xbabay     || xxtt       || coconut    || banana     || grape      || xbabay     |+------------+16 rows in set (0.00 sec)

2.查询多个字段:select 字段名1,字段名2,...字段名n from 表名;
【例】从fruits表中获取名称为f_name和f_price两列,SQL语句如下:

mysql> select f_name,f_price from fruits;+------------+---------+| f_name     | f_price |+------------+---------+| lemon      |    6.40 || apple      |    5.20 || apricot    |    2.20 || blackberry |   10.20 || berry      |    7.60 || xxxx       |    3.60 || orange     |   11.20 || melon      |    8.20 || cherry     |    3.20 || mango      |   15.70 || xbabay     |    2.60 || xxtt       |   11.60 || coconut    |    9.20 || banana     |   10.30 || grape      |    5.30 || xbabay     |    3.60 |+------------+---------+16 rows in set (0.00 sec)
(3)查询指定记录
select 字段名1,字段名2,。。。,字段名n
from 表名
where 查询条件

【例1】查询价格为10.2元的水果名称,SQL语句如下:

mysql> select f_name,f_price    -> from fruits    -> where f_price = 10.2;+------------+---------+| f_name     | f_price |+------------+---------+| blackberry |   10.20 |+------------+---------+1 row in set (0.00 sec)

【例2】查找名称为"apple"的水果价格,SQL语句如下:

mysql> select f_name,f_price    -> from fruits    -> where f_name = 'apple';+--------+---------+| f_name | f_price |+--------+---------+| apple  |    5.20 |+--------+---------+1 row in set (0.00 sec)

【例3】查询价格小于10的水果名称,SQL语句如下:

mysql> select f_name,f_price    -> from fruits    -> where f_price<10.00;+---------+---------+| f_name  | f_price |+---------+---------+| lemon   |    6.40 || apple   |    5.20 || apricot |    2.20 || berry   |    7.60 || xxxx    |    3.60 || melon   |    8.20 || cherry  |    3.20 || xbabay  |    2.60 || coconut |    9.20 || grape   |    5.30 || xbabay  |    3.60 |+---------+---------+11 rows in set (0.00 sec)
(4)带in关键字的查询
  • in操作符用来查询满足指定范围内的条件的记录,使用in操作符,将所有检索条件用括号括起来,检索条件之间用逗号分隔开,只要满足条件范围内的一个值即为匹配项。

【例1】s_id为101和102的记录,SQL语句如下;

mysql> select s_id,f_name,f_price    -> from fruits    -> where s_id in(101,102)
    -> order by f_name;+------+------------+---------+| s_id | f_name     | f_price |+------+------------+---------+|  101 | apple      |    5.20 ||  102 | banana     |   10.30 ||  101 | blackberry |   10.20 ||  101 | cherry     |    3.20 ||  102 | grape      |    5.30 ||  102 | orange     |   11.20 |+------+------------+---------+6 rows in set (0.00 sec)

【例2】查询所有s_id既不等于101也不等于102的记录,SQL语句如下:

mysql> select s_id,f_name,f_price    -> from fruits    -> where s_id not in (101,102)
    -> order by f_name;+------+---------+---------+| s_id | f_name  | f_price |+------+---------+---------+|  103 | apricot |    2.20 ||  104 | berry   |    7.60 ||  103 | coconut |    9.20 ||  104 | lemon   |    6.40 ||  106 | mango   |   15.70 ||  105 | melon   |    8.20 ||  105 | xbabay  |    2.60 ||  107 | xbabay  |    3.60 ||  105 | xxtt    |   11.60 ||  107 | xxxx    |    3.60 |+------+---------+---------+10 rows in set (0.00 sec)
(5)带between and的范围的查询
  • between and 用来查询某个范围内的值,该操作符需要两个参数,即范围的开始值和结束值。

【例1】查询价格在2.00元到10.20元之间的水果名称和价格,SQL语句如下:

mysql> select f_name,f_price from fruits where f_price between 2.00 and 10.20;+------------+---------+| f_name     | f_price |+------------+---------+| lemon      |    6.40 || apple      |    5.20 || apricot    |    2.20 || blackberry |   10.20 || berry      |    7.60 || xxxx       |    3.60 || melon      |    8.20 || cherry     |    3.20 || xbabay     |    2.60 || coconut    |    9.20 || grape      |    5.30 || xbabay     |    3.60 |+------------+---------+12 rows in set (0.00 sec)

【例2】查询价格在2.00元到10.20元之外的水果名称和价格,SQL语句如下;

mysql> select f_name,f_price    -> from fruits    -> where f_price not between 2.00 and 10.20;+--------+---------+| f_name | f_price |+--------+---------+| orange |   11.20 || mango  |   15.70 || xxtt   |   11.60 || banana |   10.30 |+--------+---------+4 rows in set (0.00 sec)
(6)带like的字符匹配查询

1.%通配符,匹配任意长度的字符,甚至包括零字符。
【例1】查找所有以"b"字母开头的水果,SQL语句如下:

mysql> select f_id,f_name    -> from fruits    -> where f_name like 'b%';+------+------------+| f_id | f_name     |+------+------------+| b1   | blackberry || b2   | berry      || t1   | banana     |+------+------------+3 rows in set (0.00 sec)

【例2】在fruits表中,查询f_name中包含字母"g"的记录,SQL语句如下:

mysql> select f_id,f_name    -> from fruits    -> where f_name like '%g%';+------+--------+| f_id | f_name |+------+--------+| bs1  | orange || m1   | mango  || t2   | grape  |+------+--------+3 rows in set (0.00 sec)

【例3】查询以"b"开头,并以‘y’结尾的水果的名称,SQL语句如下:

mysql> select f_id,f_name    -> from fruits    -> where f_name like 'b%y';+------+------------+| f_id | f_name     |+------+------------+| b1   | blackberry || b2   | berry      |+------+------------+2 rows in set (0.00 sec)

2._通配符,一次只能匹配任意一个字符。

【例】在fruits表中,查询以字母‘y’结尾,且‘y’前面只有4个字母的记录,SQL语句如下:

mysql> select f_id,f_name    -> from fruits    -> where f_name like '____y';+------+--------+| f_id | f_name |+------+--------+| b2   | berry  |+------+--------+1 row in set (0.00 sec)
(7)查询空值
  • 数据表创建时,可以指定某列中是否可以包含空值null。空值不同于0,也不同于空字符串。空值一般表示数据位置、不使用或将在以后添加数据。在select语句中使用is null子句,可以查询某字段内容为空记录。

下面创建数据表customers来演示:

create table customers(
	c_id int not null auto_increment,
	c_name char(50) not null,
	c_address char(50) null,
	c_city char(50) null,
	c_zip char(10) null,
	c_contact char(50) null,
	c_email char(255) null,
	primary key (c_id)
	);

插入下列语句:

mysql> insert into customers(c_id,c_name,c_address,c_city,c_zip,c_contact,c_email)
    -> values
    -> (10001,'redhool','200 Street ','Tianjin','300000','LiMing','LMing@163.com'),
    -> (10002,'Stars','333 Fromage Lane','Dalian','116000','Zhangbo','Jerry@hotmail.com'),
    -> (10003,'Netbhood','1 Sunny Place','Qingdao','266000','LuoCong',null),
    -> (10004,'JOTO','829 Riverside Drive','Haikou','570000','YangShan','sam@hotmail.com');Query OK, 4 rows affected (0.06 sec)Records: 4  Duplicates: 0  Warnings: 0

【例1】查询customers表中c_email为空的记录的c_id、c_name和c_email字段值,SQL语句如下:

mysql> select c_id,c_name,c_email from customers where c_email is null;+-------+----------+---------+| c_id  | c_name   | c_email |+-------+----------+---------+| 10003 | Netbhood | NULL    |+-------+----------+---------+1 row in set (0.00 sec)

【例2】查询customers表中c_email不为空的记录的c_id、c_name、和c_email字段值,SQL语句如下:

mysql> select c_id,c_name,c_email    -> from customers    -> where c_email is not null;+-------+---------+-------------------+| c_id  | c_name  | c_email           |+-------+---------+-------------------+| 10001 | redhool | LMing@163.com     || 10002 | Stars   | Jerry@hotmail.com || 10004 | JOTO    | sam@hotmail.com   |+-------+---------+-------------------+3 rows in set (0.00 sec)
(8)带and的多条件查询

【例1】在fruits表中查询s_id=101,且f_price大于5的水果的价格和名称,SQL语句如下:

mysql> select f_id,f_price,f_name
    -> from fruits
    -> where s_id = '101' and f_price>=5;
+------+---------+------------+
| f_id | f_price | f_name     |
+------+---------+------------+
| a1   |    5.20 | apple      |
| b1   |   10.20 | blackberry |
+------+---------+------------+
2 rows in set (0.00 sec)

【例2】在fruits表中查询s_id=101或者102,且f_price大于5,且f_name='apple’的水果价格和名称,SQL语句如下:

mysql> select f_id ,f_price , f_name    -> from fruits    -> where s_id in ('101','102') and f_price >=5 and f_name = 'apple';+------+---------+--------+| f_id | f_price | f_name |+------+---------+--------+| a1   |    5.20 | apple  |+------+---------+--------+1 row in set (0.05 sec)
(9)带or的多条件查询

【例1】查询s_id=101或者s_id=102的水果供应商的f_price和f_name,SQL语句如下:

mysql> select s_id,f_name,f_price    -> from fruits    -> where s_id=101 or s_id=102;+------+------------+---------+| s_id | f_name     | f_price |+------+------------+---------+|  101 | apple      |    5.20 ||  101 | blackberry |   10.20 ||  102 | orange     |   11.20 ||  101 | cherry     |    3.20 ||  102 | banana     |   10.30 ||  102 | grape      |    5.30 |+------+------------+---------+6 rows in set (0.00 sec)

【例2】查询s_id=101或者s_id=102的水果供应商的f_price和f_name,SQL语句如下:

mysql> select s_id,f_name,f_price    -> from fruits    -> where s_id in(101,102);+------+------------+---------+| s_id | f_name     | f_price |+------+------------+---------+|  101 | apple      |    5.20 ||  101 | blackberry |   10.20 ||  102 | orange     |   11.20 ||  101 | cherry     |    3.20 ||  102 | banana     |   10.30 ||  102 | grape      |    5.30 |+------+------------+---------+6 rows in set (0.00 sec
  • 注意
    or可以和and一起使用,但是要注意两者的优先级,由于and的优先级高于or,旖旎次先对and两边的操作数进行操作,再与or中的操作数结合。
(10)查询结果不重复
  • 语法格式:select distinct 字段名 from 表名;

【例】查询fruits表中s_id字段的值,返回s_id字段值且不得重复,SQL语句如下;

mysql> select distinct s_id from fruits;+------+| s_id |+------+|  104 ||  101 ||  103 ||  107 ||  102 ||  105 ||  106 |+------+7 rows in set (0.05 sec)
(11)对查询结果排序

1.单列排序order by

【例】查询fruits表的f_name字段值,并对其进行排序,SQL语句如下:

mysql> select f_name from fruits order by f_name;+------------+| f_name     |+------------+| apple      || apricot    || banana     || berry      || blackberry || cherry     || coconut    || grape      || lemon      || mango      || melon      || orange     || xbabay     || xbabay     || xxtt       || xxxx       |+------------+16 rows in set (0.00 sec)

2.多列排序

  • 多列排序首先排序的第一列必须有相同的列值,才会对第二列进行排序。如果第一列数据中所有值都是唯一的,将不再对第二列进行排序。

【例】查询fruits表中的f_name和f_price字段,先按f_name排序,再按f_price排序,SQL语句如下:

mysql> select f_name,f_price    -> from fruits    -> order by f_name, f_price;+------------+---------+| f_name     | f_price |+------------+---------+| apple      |    5.20 || apricot    |    2.20 || banana     |   10.30 || berry      |    7.60 || blackberry |   10.20 || cherry     |    3.20 || coconut    |    9.20 || grape      |    5.30 || lemon      |    6.40 || mango      |   15.70 || melon      |    8.20 || orange     |   11.20 || xbabay     |    2.60 || xbabay     |    3.60 || xxtt       |   11.60 || xxxx       |    3.60 |+------------+---------+16 rows in set (0.00 sec)

3.指定排序方向desc

  • desc是降序排列,与之对应的是asc升序排列,但asc是默认的,可以不加。

【例1】查询fruits表中的f_name和f_price字段,对结果按f_price降序方式排序,SQL语句如下:

mysql> select f_name,f_price    -> from fruits    -> order by f_price desc;+------------+---------+| f_name     | f_price |+------------+---------+| mango      |   15.70 || xxtt       |   11.60 || orange     |   11.20 || banana     |   10.30 || blackberry |   10.20 || coconut    |    9.20 || melon      |    8.20 || berry      |    7.60 || lemon      |    6.40 || grape      |    5.30 || apple      |    5.20 || xxxx       |    3.60 || xbabay     |    3.60 || cherry     |    3.20 || xbabay     |    2.60 || apricot    |    2.20 |+------------+---------+16 rows in set (0.00 sec)

【例2】查询fruits表,先按f_price降序排序,再按f_name字段升序排序,SQL语句如下:

mysql> select f_price,f_name    -> from fruits    -> order by f_price desc,f_name;+---------+------------+| f_price | f_name     |+---------+------------+|   15.70 | mango      ||   11.60 | xxtt       ||   11.20 | orange     ||   10.30 | banana     ||   10.20 | blackberry ||    9.20 | coconut    ||    8.20 | melon      ||    7.60 | berry      ||    6.40 | lemon      ||    5.30 | grape      ||    5.20 | apple      ||    3.60 | xbabay     ||    3.60 | xxxx       ||    3.20 | cherry     ||    2.60 | xbabay     ||    2.20 | apricot    |+---------+------------+16 rows in set (0.00 sec)
(12)分组查询
  • 分组查询是对数据按照某个或多个字段进行分组,MySQL中使用group by 关键字对数据进行分组,基本语法形式为:[group by 字段][having<条件表达式>]
  • 字段值为进行分组时所依据的列名称,"having<条件表达式>"指定满足表达式限定条件的结果将被显示。

1.创建分组

【例1】根据s_id对fruits表中的数据进行分组,SQL语句如下;

mysql> select s_id,count(*) as total    -> from fruits    -> group by s_id;+------+-------+| s_id | total |+------+-------+|  104 |     2 ||  101 |     3 ||  103 |     2 ||  107 |     2 ||  102 |     3 ||  105 |     3 ||  106 |     1 |+------+-------+7 rows in set (0.05 sec)

可以看到group by 子句按照s_id排序并对数据分组。

  • 如果需要查看每个供应商提供的水果种类名称,可以在group by子句中使用group_concat()函数,将每个分组中各个字段的值显示出来。

【例2】根据s_id对fruits表中的数据进行分组,将每个供应商的水果名称显示出来,SQL语句如下:

mysql> select s_id,group_concat(f_name) as Names    -> from fruits    -> group by s_id;+------+-------------------------+| s_id | Names                   |+------+-------------------------+|  101 | apple,blackberry,cherry ||  102 | orange,banana,grape     ||  103 | apricot,coconut         ||  104 | lemon,berry             ||  105 | melon,xbabay,xxtt       ||  106 | mango                   ||  107 | xxxx,xbabay             |+------+-------------------------+7 rows in set (0.05 sec)

2.使用having过滤分组

  • groub by可以和having一起限定显示记录所需满足的条件,只有满足条件的分组才会被显示。
  • having和where都是用来过滤数据的,having在数据分组之后进行过滤来选择分组,而where在分组之前用来选择记录。where排除的记录不再包括在分组中。

【例】根据s_id对fruits表中的数据进行分组,并显示水果种类大于1的分组信息,SQL语句如下:

mysql> select s_id,group_concat(f_name) as Names    -> from fruits    -> group by s_id having count(f_name) >1;+------+-------------------------+| s_id | Names                   |+------+-------------------------+|  101 | apple,blackberry,cherry ||  102 | orange,banana,grape     ||  103 | apricot,coconut         ||  104 | lemon,berry             ||  105 | melon,xbabay,xxtt       ||  107 | xxxx,xbabay             |+------+-------------------------+6 rows in set (0.00 sec)

3.在group by 子句中使用with rollup

  • 使用with rolluo关键字之后,在所有查询出的分组记录之后增加一条记录,该记录计算查询出的所有记录的总和,即统计记录数量。

【例】根据s_id对fruits表中的数据进行分组,并显示记录数量,SQL语句如下:

mysql> select s_id,count(*) as Total    -> from fruits    -> group by s_id with rollup;+------+-------+| s_id | Total |+------+-------+|  101 |     3 ||  102 |     3 ||  103 |     2 ||  104 |     2 ||  105 |     3 ||  106 |     1 ||  107 |     2 || NULL |    16 |+------+-------+8 rows in set (0.05 sec)

4.多字段分组

  • 使用group by可以对多个字段进行分组,group by 关键字后面跟需要分组的字段,MySQL根据多字段的值来进行层次分组,分组层次从左到右,即先按第1个字段分组,然后在第1个字段值相同的记录中,再根据第2个字段的值进行分组,以此类推。

【例】根据s_id和f_name字段对fruits表中的数据进行分组,SQL语句如下:

mysql> select * from fruits group by s_id,f_name;+------+------+------------+---------+| f_id | s_id | f_name     | f_price |+------+------+------------+---------+| 12   |  104 | lemon      |    6.40 || a1   |  101 | apple      |    5.20 || a2   |  103 | apricot    |    2.20 || b1   |  101 | blackberry |   10.20 || b2   |  104 | berry      |    7.60 || b5   |  107 | xxxx       |    3.60 || bs1  |  102 | orange     |   11.20 || bs2  |  105 | melon      |    8.20 || c0   |  101 | cherry     |    3.20 || m1   |  106 | mango      |   15.70 || m2   |  105 | xbabay     |    2.60 || m3   |  105 | xxtt       |   11.60 || o2   |  103 | coconut    |    9.20 || t1   |  102 | banana     |   10.30 || t2   |  102 | grape      |    5.30 || t4   |  107 | xbabay     |    3.60 |+------+------+------------+---------+16 rows in set (0.00 sec)

5.group by 和order by一起使用

  • 某些情况下需要对分组进行排序,order by用来对查询的记录排序,如果和group by一起使用可以完成对分组的排序。

创建数据表演示:

mysql> create table orderitems    -> (
    -> o_num int not null,
    -> o_item int not null,
    -> f_id char(10) not null,
    -> quantity int not null,
    -> item_price decimal(8,2) not null,
    -> primary key (o_num,o_item)
    -> );Query OK, 0 rows affected (0.13 sec)mysql> insert into orderitems(o_num,o_item,f_id,quantity,item_price)
    -> values(30001,1,'a1',10,5.2),
    -> (30001,2,'b2',3,7.6),
    -> (30001,3,'bs1',5,11.2),
    -> (30001,4,'bs2',15,9.2),
    -> (30002,1,'b3',2,20.0),
    -> (30003,1,'c0',100,10),
    -> (30004,1,'o2',50,2.50),
    -> (30005,1,'c0',5,10),
    -> (30005,2,'b1',10,8.99),
    -> (30005,3,'a2',10,2.2),
    -> (30005,4,'m1',5,14.99);Query OK, 11 rows affected (0.06 sec)Records: 11  Duplicates: 0  Warnings: 0

【例】查询订单价格大于100的订单号和总订单价格,SQL语句如下;

mysql> select o_num,sum(quantity * item_price) as orderTotal    -> from orderitems    -> group by o_num    -> having sum(quantity * item_price) >=100;+-------+------------+| o_num | orderTotal |+-------+------------+| 30001 |     268.80 || 30003 |    1000.00 || 30004 |     125.00 || 30005 |     236.85 |+-------+------------+4 rows in set (0.00 sec)

可以看到orderTotal列的总订单价格并没有按照一定的顺序显示,接下来使用order by关键字按总订单价格排列显示结果,SQL语句如下:

mysql> select o_num,sum(quantity * item_price) as orderTotal    -> from orderitems    -> group by o_num    -> having sum(quantity * item_price)>=100
    -> order by orderTotal;+-------+------------+| o_num | orderTotal |+-------+------------+| 30004 |     125.00 || 30005 |     236.85 || 30001 |     268.80 || 30003 |    1000.00 |+-------+------------+4 rows in set (0.00 sec)

可以看到,group by 子句按订单号对数据进行分组,sum()函数便可以返回总的订单价格,having子句对分组数据进行过滤,使得只返回总价格大于100的订单,最后使用order by子句排序输出。

(13)使用limit限制查询结果的数量

select返回所有匹配的行,有可能是表中所有的行,如仅仅需要返回第一行或者前几行,使用limit关键字,基本语法如下:

limit [位置偏移量,] 行数
  • 第一个"位置偏移量"参数只是MySQL从哪一行开始显示,是一个可选参数,如果不指定"位置偏移量",将会从表中的第一条记录开始(第一条记录的位置偏移量是0,第二条记录的位置偏移量是1,以此类推);第二个参数"行数指示返回的记录条数"。

【例1】显示fruits表查询结果的前4行,SQL语句如下:

mysql> select * from fruits limit 4;+------+------+------------+---------+| f_id | s_id | f_name     | f_price |+------+------+------------+---------+| 12   |  104 | lemon      |    6.40 || a1   |  101 | apple      |    5.20 || a2   |  103 | apricot    |    2.20 || b1   |  101 | blackberry |   10.20 |+------+------+------------+---------+4 rows in set (0.00 sec)

【例2】在fruits表中,使用limit子句,返回从第5个记录开始的、行数长度为3的记录,SQL语句如下:

mysql> select * from fruits limit 4,3;+------+------+--------+---------+| f_id | s_id | f_name | f_price |+------+------+--------+---------+| b2   |  104 | berry  |    7.60 || b5   |  107 | xxxx   |    3.60 || bs1  |  102 | orange |   11.20 |+------+------+--------+---------+3 rows in set (0.00 sec)

也可以使用"limit 4 offset 3"也是获取从第5条记录开始后面的3条记录。

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