mysql 伪表查询语句_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)`
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