MySQL数据库之select查询语句

select查询语句

语法
- select [选项] 列名 [from 表名] [where 条件] [group by 分组] [order by 排序][having 条件] [limit 限制]

select字段表达式

可以直接输出内容

MariaDB [sel]> select 'Sunny is a boy!';
+-----------------+
| Sunny is a boy! |
+-----------------+
| Sunny is a boy! |
+-----------------+
# `1 row in set (0.000 sec)`

输出表达式

MariaDB [sel]> select 10*27;
+-------+
| 10*27 |
+-------+
|   270 |
+-------+
# `1 row in set (0.000 sec)`

MariaDB [sel]> select name,chinese,math,chinese+math from grades;
+-------+---------+------+--------------+
| name  | chinese | math | chinese+math |
+-------+---------+------+--------------+
| Sunny |      93 |   96 |          189 |
| Jerry |      97 |   91 |          188 |
| Marry |      95 |   94 |          189 |
| Tommy |      98 |   94 |          192 |
+-------+---------+------+--------------+
# `4 rows in set (0.000 sec)`

输出函数表达式

MariaDB [sel]> select rand();
+-------------------+
| rand()            |
+-------------------+
| 0.294372641755615 |
+-------------------+
# `1 row in set (0.007 sec)`

通过as给字段取别名
- as可以省略

MariaDB [sel]> select 'Sunny' as 'name';
+-------+
| name  |
+-------+
| Sunny |
+-------+
# `1 row in set (0.000 sec)`

MariaDB [sel]> select name,chinese,math,chinese+math as '总分' from grades;
+-------+---------+------+------+
| name  | chinese | math | 总分 |
+-------+---------+------+------+
| Sunny |      93 |   96 |  189 |
| Jerry |      97 |   91 |  188 |
| Marry |      95 |   94 |  189 |
| Tommy |      98 |   94 |  192 |
+-------+---------+------+------+
# `4 rows in set (0.007 sec)`

MariaDB [sel]> select name,chinese,math,chinese+math '总分' from grades;
+-------+---------+------+------+
| name  | chinese | math | 总分 |
+-------+---------+------+------+
| Sunny |      93 |   96 |  189 |
| Jerry |      97 |   91 |  188 |
| Marry |      95 |   94 |  189 |
| Tommy |      98 |   94 |  192 |
+-------+---------+------+------+
# `4 rows in set (0.001 sec)`

from子句

from [表名]
- from后面跟的是数据源
- 数据源可以有多个,返回笛卡尔积

-- 创建数据表
MariaDB [sel]> create table stu1(
    -> name varchar(20)
    -> )charset=gbk;
# `Query OK, 0 rows affected (0.026 sec)`

MariaDB [sel]> insert into stu1 values ('Sunny'),('Jerry');
# `Query OK, 2 rows affected (0.012 sec)`
# `Records: 2  Duplicates: 0  Warnings: 0`

MariaDB [sel]> create table stu2(
    -> age int
    -> )charset=gbk;
# `Query OK, 0 rows affected (0.023 sec)`

MariaDB [sel]> insert into stu2 values (20),(24);
# `Query OK, 2 rows affected (0.012 sec)`
# `Records: 2  Duplicates: 0  Warnings: 0`

-- from子句查询
MariaDB [sel]> select * from stu1;
+-------+
| name  |
+-------+
| Sunny |
| Jerry |
+-------+
# `2 rows in set (0.000 sec)`

-- from返回笛卡尔积
MariaDB [sel]> select * from stu1,stu2;
+-------+------+
| name  | age  |
+-------+------+
| Sunny |   20 |
| Jerry |   20 |
| Sunny |   24 |
| Jerry |   24 |
+-------+------+
# `4 rows in set (0.000 sec)`

dual表

概念
- dual表是一个伪表
- 在有些特定情况下，没有具体的表的参与
- 为了保证select语句的完整又必须要一个表名,这时候就使用伪表

MariaDB [sel]> select 10*27 as '结果' from dual;
+------+
| 结果 |
+------+
|  270 |
+------+
# `1 row in set (0.007 sec)`

where子句

概念
- where后面跟的是条件,在数据源中进行筛选
- 返回条件为真记录
MySQL支持的运算符
- 比较运算符
  - > 大于
  - < 小于
  - >= 大于等于
  - <= 小于等于
  - = 等于
  - != 不等于
- 逻辑运算符
  - and 与
  - or 或
  - not 非
- 其他
  - in | not in 字段的值在枚举范围内
  - between...and | not between...and 字段的值在数字范围内
  - is null | is not null 字段的值不为空

-- 比较运算判断
MariaDB [sel]> select * from grades where math=94;
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Marry |      95 |   94 |
| Tommy |      98 |   94 |
+-------+---------+------+
# `2 rows in set (0.007 sec)`

-- 输出所有数据
MariaDB [sel]> select * from grades where 1;
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Sunny |      93 |   96 |
| Jerry |      97 |   91 |
| Marry |      95 |   94 |
| Tommy |      98 |   94 |
+-------+---------+------+
# `4 rows in set (0.000 sec)`

-- 不输出数据
MariaDB [sel]> select * from grades where 0;
# `Empty set (0.000 sec)`

-- 逻辑运算判断
MariaDB [sel]> select * from grades where math=96 or math=91;
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Sunny |      93 |   96 |
| Jerry |      97 |   91 |
+-------+---------+------+
# `2 rows in set (0.001 sec)`

MariaDB [sel]> select * from grades where math in (91,96);
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Sunny |      93 |   96 |
| Jerry |      97 |   91 |
+-------+---------+------+
# `2 rows in set (0.000 sec)`

MariaDB [sel]> select * from grades where math not in (91,96);
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Marry |      95 |   94 |
| Tommy |      98 |   94 |
+-------+---------+------+
# `2 rows in set (0.000 sec)`

查找年龄在20~25之间

-- 方法一：
mysql> select * from stu where stuage>=20 and stuage<=25;

-- 方法二：
mysql> select * from stu where not(stuage<20 or stuage>25);

-- 方法三：between...and...
mysql> select * from stu where stuage between 20 and 25;

-- 年龄不在20~25之间
mysql> select * from stu where stuage not between 20 and 25;

查找缺考的学生

mysql> select * from stu where ch is null or math is null;
+--------+----------+--------+--------+---------+------------+------+------+
| stuNo  | stuName  | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+----------+--------+--------+---------+------------+------+------+
| s25301 | 张秋丽    | 男     |    18 |       1 | 北京        |   80 | NULL |
| s25304 | 欧阳俊雄  | 男     |    28 |       4 | 天津        | NULL |   74 |
+--------+----------+--------+--------+---------+------------+------+------+

查找没有缺考的学生

mysql> select * from stu where ch is not null and math is not null;
+--------+----------+--------+--------+---------+------------+------+------+
| stuNo  | stuName  | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+----------+--------+--------+---------+------------+------+------+
| s25302 | 李文才    | 男     |     31 |       3 | 上海       |   77 |   76 |
| s25303 | 李斯文    | 女     |     22 |       2 | 北京       |   55 |   82 |
| s25305 | 诸葛丽丽  | 女     |     23 |       7 | 河南       |   72 |   56 |
| s25318 | 争青小子  | 男     |     26 |       6 | 天津       |   86 |   92 |
| s25319 | 梅超风    | 女     |     23 |       5 | 河北       |   74 |   67 |
| s25320 | Tom      | 男     |     24 |       8 | 北京       |   65 |   67 |
| s25321 | Tabm     | 女     |     23 |       9 | 河北       |   88 |   77 |
+--------+----------+--------+--------+---------+------------+------+------+
# `7 rows in set (0.00 sec)`

查找需要补考的学生

mysql> select * from stu where ch<60 or math<60 or ch is null or math is null;
+--------+----------+--------+--------+---------+------------+------+------+
| stuNo  | stuName  | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+----------+--------+--------+---------+------------+------+------+
| s25301 | 张秋丽    | 男    |     18 |       1 | 北京        |   80 | NULL |
| s25303 | 李斯文    | 女    |     22 |       2 | 北京        |   55 |   82 |
| s25304 | 欧阳俊雄  | 男    |     28 |       4 | 天津        | NULL |   74 |
| s25305 | 诸葛丽丽  | 女    |     23 |       7 | 河南        |   72 |   56 |
+--------+----------+--------+--------+---------+------------+------+------+
4 rows in set (0.00 sec)

查找学号是s25301,s25302,s25303的学生

mysql> select * from stu where stuno in ('s25301','s25302','s25303');
+--------+---------+--------+--------+---------+------------+------+------+
| stuNo  | stuName | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+---------+--------+--------+---------+------------+------+------+
| s25301 | 张秋丽   | 男     |     18 |       1 | 北京       |   80 | NULL |
| s25302 | 李文才   | 男     |     31 |       3 | 上海       |   77 |   76 |
| s25303 | 李斯文   | 女     |     22 |       2 | 北京       |   55 |   82 |
+--------+---------+--------+--------+---------+------------+------+------+
# `3 rows in set (0.00 sec)`

查找年龄是18~20的学生

mysql> select * from stu where stuage between 18 and 20;
+--------+---------+--------+--------+---------+------------+------+------+
| stuNo  | stuName | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+---------+--------+--------+---------+------------+------+------+
| s25301 | 张秋丽  | 男     |     18 |       1 | 北京        |   80 | NULL |
+--------+---------+--------+--------+---------+------------+------+------+
1 row in set (0.00 sec)

group by [分组查询]

概念
- 将查询的结果分组，分组查询目的在于统计数据
- 如果是分组查询，查询字段是分组字段和聚合函数
- 查询字段是普通字段，只取第一个值
- group_concat()将同一组的数据连接起来

-- 查询男生和女生的各自语文平均分
mysql> select stusex,avg(ch) '平均分' from stu group by stusex;
+--------+---------+
| stusex | 平均分  |
+--------+---------+
| 女     | 72.2500 |
| 男     | 77.0000 |
+--------+---------+
# `2 rows in set (0.00 sec)`

-- 查询男生和女生各自多少人
mysql> select stusex,count(*) 人数 from stu group by stusex;
+--------+------+
| stusex | 人数 |
+--------+------+
| 女     |    4 |
| 男     |    5 |
+--------+------+
# `2 rows in set (0.00 sec)`

-- 查询每个地区多少人
mysql> select stuaddress,count(*) from stu group by stuaddress;
+------------+----------+
| stuaddress | count(*) |
+------------+----------+
| 上海       |        1 |
| 北京       |        3 |
| 天津       |        2 |
| 河北       |        2 |
| 河南       |        1 |
+------------+----------+
# `5 rows in set (0.00 sec)`

-- 每个地区的数学平均分
mysql> select stuaddress,avg(math) from stu group by stuaddress;
+------------+-----------+
| stuaddress | avg(math) |
+------------+-----------+
| 上海       |   76.0000 |
| 北京       |   74.5000 |
| 天津       |   83.0000 |
| 河北       |   72.0000 |
| 河南       |   56.0000 |
+------------+-----------+
# `5 rows in set (0.00 sec)`

group_concat() 函数
- 将同一组的值连接起来显示

mysql> select group_concat(stuname),stusex,avg(math) from stu group by stusex;
+-------------------------------------+--------+-----------+
| group_concat(stuname)               | stusex | avg(math) |
+-------------------------------------+--------+-----------+
| 李斯文,诸葛丽丽,梅超风,Tabm           | 女     |   70.5000 |
| 张秋丽,李文才,欧阳俊雄,争青小子,Tom    | 男    |   77.2500 |
+-------------------------------------+--------+-----------+
# `2 rows in set (0.00 sec)`

多列分组

mysql> select stuaddress,stusex,avg(math) from stu group by stuaddress,stusex;
+------------+--------+-----------+
| stuaddress | stusex | avg(math) |
+------------+--------+-----------+
| 上海       | 男     |   76.0000 |
| 北京       | 女     |   82.0000 |
| 北京       | 男     |   67.0000 |
| 天津       | 男     |   83.0000 |
| 河北       | 女     |   72.0000 |
| 河南       | 女     |   56.0000 |
+------------+--------+-----------+
# `6 rows in set (0.00 sec)`

order by [排序]

asc 升序 [默认]
desc 降序

MariaDB [sel]> select * from grades order by math desc;
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Sunny |      93 |   96 |
| Marry |      95 |   94 |
| Tommy |      98 |   94 |
| Jerry |      97 |   91 |
+-------+---------+------+
# `4 rows in set (0.001 sec)`

-- 按总分降序排列
MariaDB [sel]> select *,chinese+math  from grades order by math+chinese desc;
+-------+---------+------+--------------+
| name  | chinese | math | chinese+math |
+-------+---------+------+--------------+
| Tommy |      98 |   94 |          192 |
| Sunny |      93 |   96 |          189 |
| Marry |      95 |   94 |          189 |
| Jerry |      97 |   91 |          188 |
+-------+---------+------+--------------+
# `4 rows in set (0.001 sec)`

MariaDB [sel]> select *,chinese+math '总分'  from grades order by math+chinese desc;
+-------+---------+------+------+
| name  | chinese | math | 总分 |
+-------+---------+------+------+
| Tommy |      98 |   94 |  192 |
| Sunny |      93 |   96 |  189 |
| Marry |      95 |   94 |  189 |
| Jerry |      97 |   91 |  188 |
+-------+---------+------+------+
# `4 rows in set (0.000 sec)`

多列排序
- 前列为主，后列为辅

MariaDB [sel]> select * from grades order by math desc,chinese asc;
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Sunny |      93 |   96 |
| Marry |      95 |   94 |
| Tommy |      98 |   94 |
| Jerry |      97 |   91 |
+-------+---------+------+
# `4 rows in set (0.000 sec)`

having条件

概念
- 是在结果集上进行条件筛选
having和where的区别
- where是对原始数据进行筛选
- having是对记录集进行筛选

-- having与where相同
MariaDB [sel]> select * from grades where math=94;
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Marry |      95 |   94 |
| Tommy |      98 |   94 |
+-------+---------+------+
# `2 rows in set (0.000 sec)`

MariaDB [sel]> select * from grades having math=94;
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Marry |      95 |   94 |
| Tommy |      98 |   94 |
+-------+---------+------+
# 2 rows in set (0.000 sec)

-- having与where不同
MariaDB [sel]> select name from grades where math=94;
+-------+
| name  |
+-------+
| Marry |
| Tommy |
+-------+
# `2 rows in set (0.000 sec)`

MariaDB [sel]> select name from grades having math=94;
# `ERROR 1054 (42S22): Unknown column 'math' in 'having clause'`

limit

语法
- limit [起始位置]，显示长度

-- 从第0个位置开始取，取3条记录
mysql> select * from stu limit 0,3;

-- 从第2个位置开始取，取3条记录
mysql> select * from stu limit 2,3;
+--------+----------+--------+--------+---------+------------+------+------+
| stuNo  | stuName  | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+----------+--------+--------+---------+------------+------+------+
| s25303 | 李斯文    | 女     |     22 |       2 | 北京       |   55 |   82 |
| s25304 | 欧阳俊雄  | 男     |     28 |       4 | 天津       | NULL |   74 |
| s25305 | 诸葛丽丽  | 女     |     23 |       7 | 河南       |   72 |   56 |
+--------+----------+--------+--------+---------+------------+------+------+
# `3 rows in set (0.00 sec)`

起始位置可以省略，默认是从0开始

mysql> select * from stu limit 3;
+--------+---------+--------+--------+---------+------------+------+------+
| stuNo  | stuName | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+---------+--------+--------+---------+------------+------+------+
| s25301 | 张秋丽  | 男     |     18 |       1 | 北京        |   80 | NULL |
| s25302 | 李文才  | 男     |     31 |       3 | 上海        |   77 |   76 |
| s25303 | 李斯文  | 女     |     22 |       2 | 北京        |   55 |   82 |
+--------+---------+--------+--------+---------+------------+------+------+
# `3 rows in set (0.00 sec)`

找出班级总分前三名

mysql> select *,ch+math total from stu order by (ch+math) desc limit 0,3;
+--------+----------+--------+--------+---------+------------+------+------+-------+
| stuNo  | stuName  | stuSex | stuAge | stuSeat | stuAddress | ch   | math | total |
+--------+----------+--------+--------+---------+------------+------+------+-------+
| s25318 | 争青小子  | 男     |     26 |       6 | 天津       |   86 |   92 |   178 |
| s25321 | Tabm     | 女     |     23 |       9 | 河北       |   88 |   77 |   165 |
| s25302 | 李文才   | 男     |     31 |       3 | 上海        |   77 |   76 |   153 |
+--------+----------+--------+--------+---------+------------+------+------+-------+
# `3 rows in set (0.00 sec)`

limit在update和delete语句中也是可以使用的

-- 前3名语文成绩加1分
mysql> update stu set ch=ch+1 order by ch+math desc limit 3;
# `Query OK, 3 rows affected (0.00 sec)`
# `Rows matched: 3  Changed: 3  Warnings: 0`

-- 前3名删除
mysql> delete from stu order by ch+math desc limit 3;
# `Query OK, 3 rows affected (0.00 sec)`

查询语句中的选项distinct

查询语句中的选项有两个
- all 显示所有数据 [默认]
- distinct 去除结果集中重复的数据

-- 显示列所有数据
MariaDB [sel]> select all math from grades;
+------+
| math |
+------+
|   96 |
|   91 |
|   94 |
|   94 |
+------+
# `4 rows in set (0.001 sec)`

-- 单列去除重复的项
MariaDB [sel]> select distinct math from grades;
+------+
| math |
+------+
|   96 |
|   91 |
|   94 |
+------+
# `3 rows in set (0.001 sec)`

posted @ 2020-12-15 12:10 wing1377 阅读(411) 评论(0) 编辑收藏举报

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MySQL数据库之select查询语句

select查询语句

select字段表达式

from子句

dual表

where子句

group by [分组查询]

order by [排序]

having条件

limit

查询语句中的选项distinct

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