mysql表结构说明只能为1 8_SQL基础
1 SQL语句分类
名称
作用
语法关键字
DDL
数据定义语言
CREATE,DROP,ALTER
DML
数据操纵语言
INSERT,DELETE,UPDATE
DQL
数据查询语言
SELECT
DCL
数据控制语言
GRANT,REVOKE,COMMIT,ROLLBACK
2 数据类型
数值类型
日期/时间类型
字符串(字符)类型
2.1 数值类型
2.1.1 整数型
类型
储存空间(字节)
范围
tinyint(m)
1
-128~127
smallint(m)
2
-32768~32767
mediumint(m)
3
-8388608~8388607
int(m)
4
-2147483648~2147483647
bigint(m)
8
-2^63~2^63-1
上述数据类型,如果加修饰符unsigned后,则最大值翻倍,如:tinyint unsigned的取值范围为(0~255)
int(m)里的m是表示SELECT查询结果集中的显示宽度,并不影响实际的取值范围,规定了MySQL的一些交互工具(例如MySQL命令行客户端)用来显示字符的个数。对于存储和计算来说,Int(1)和Int(20)是相同的
BOOL,BOOLEAN:布尔型,是TINYINT(1)的同义词。zero值被视为假,非zero值视为真
2.1.2 浮点型(float和double),近似值
float(m,d)单精度浮点型8位精度(4字节)m总个数,d小数位
double(m,d)双精度浮点型16位精度(8字节)m总个数,d小数位
假设一个字段定义为float(6,3),如果插入一个数123.45678,实际数据库里存的是123.457,但总个数还以实际为准,即6位
2.1.3 定点数
在数据库中存放的是精确值,存为十进制
decimal(m,d)参数m<65 是总个数,d<30且 d
MySQL5.0和更高版本将数字打包保存到一个二进制字符串中(每4个字节存9个数字)。
例如:
decimal(18,9)小数点两边将各存储9个数字,一共使用9个字节:其中,小数点前的9个数字用4个字节,小数点后的9个数字用4个字节,小数点本身占1个字节
浮点类型在存储同样范围的值时,通常比decimal使用更少的空间。float使用4个字节存储。double占用8个字节
因为需要额外的空间和计算开销,所以应该尽量只在对小数进行精确计算时才使用decimal,例如存储财务数据。但在数据量比较大的时候,可以考虑使用bigint代替decimal
2.2 日期/时间类型
数据类型
值
DATE
'0000-00-00'
TIME
'00:00:00'
DATETIME
'0000-00-00 00:00:00'
TIMESTAMP
'0000-00-00 00:00:00'
YEAR(2),YEAR(4)
00,0000
2.3 字符串类型
数据类型
值
char(n) 固定长度
最多255个字符
varchar(n) 可变长度
最多65535个字符
tinytext 可变长度
最多255个字符
text 可变长度
最多65535个字符
mediumtext 可变长度
最多2的24次方-1个字符
longtext 可变长度
最多2的32次方-1个字符
BINARY(M) 固定长度
可存二进制或字符,长度为0-M字节
VARBINARY(M) 可变长度
可存二进制或字符,允许长度为0-M字节
内建类型:ENUM枚举,SET集合
ENUM是一个字符串对象,其值是从允许值的列表中选择的,这些值在表创建时在列规范中明确枚举
SET是可以具有零个或多个值的字符串对象,每个值都必须从创建表时指定的允许值列表中选择。 SET由多个set成员组成的列值用用逗号(,)分隔的成员指定。这样的结果是 SET成员值本身不应包含逗号。
2.3.1 char和varchar比较
1.char(n) 若存入字符数小于n,则以空格补于其后,查询之时再将空格去掉,所以char类型存储的字符串末尾不能有空格,varchar不限于此
2.char(n) 固定长度,char(4)不管是存入几个字符,都将占用4个字节,varchar是存入的实际字符数+1个字节(n< n>255),所以varchar(4),存入3个字符将占用4个字节
3.char类型的字符串检索速度要比varchar类型的快
2.3.2 varchar和text比较
1.varchar可指定n,text不能指定,内部存储varchar是存入的实际字符数+1个字节(n< n>255),text是实际字符数+2个字节。
2.text类型不能有默认值
3.varchar可直接创建索引,text创建索引要指定前多少个字符。varchar查询速度快于text
数据类型
2.3.3 BLOB和TEXT比较
1.BLOB和test存储方式不同,text以文本方式存储,英文存储区分大小写,而blob以二进制方式存储,不区分大小写
2.BLOB存储的数据只能整体读出
3.text可以指定字符集,blob不用指定字符集
2.4 修饰符
2.4.1 适合所有类型的修饰符:
名称
作用
NULL
数据列可包含NULL值
NOT NULL
数据列不允许包含NULL值
DEFAULT
默认值
PRIMARY KEY
主键
UNIQUE KEY
唯一键
CHARACTER SET name
指定一个字符集
2.4.2 适用数值型的修饰符
名称
作用
AUTO_INCREMENT
自动递增,适用于整数类型,不支持列的负值。
UNSIGNED
无符号,可用于仅允许一列中使用非负数,或者在您需要该列的较大的较高数字范围时使用
3 DDL语句
3.1 创建表
HELP CREATE TALBE;
创建表的方法
3.1.1 直接创建
CREATE TABLE [IF NOT EXISTS] ‘tbl_name’ (col1 type1 修饰符, col2 type2 修饰符,
...)
#字段信息
col type1
PRIMARY KEY(col1,...)
INDEX(col1, ...)
UNIQUE KEY(col1, ...)
#表选项:
ENGINE [=] engine_name
ROW_FORMAT [=] {DEFAULT|DYNAMIC|FIXED|COMPRESSED|REDUNDANT|COMPACT}
注意:
Storage Engine是指表类型,也即在表创建时指明其使用的存储引擎
同一库中不同表可以使用不同的存储引擎
同一个库中表建议要使用同一种存储引擎类型
范例:
mysql> CREATE TABLE student (id int UNSIGNED AUTO_INCREMENT PRIMARY KEY,name VARCHAR(20) NOT NULL,age TINYINT UNSIGNED);
Query OK, 0 rows affected (0.03 sec)
mysql> DESC student;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
mysql> CREATE TABLE employee (id int UNSIGNED NOT NULL,name VARCHAR(20) NOT NULL,age TINYINT UNSIGNED,PRIMARY KEY(id,name));
Query OK, 0 rows affected (0.02 sec)
3.1.2 通过查询现存表创建,新表会被直接插入查询而来的数据
语法:
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name [(create_definition,...)]
[table_options]
[partition_options] select_statement
【例】通过查询现存表student的数据来创建新表new_student:
mysql> desc student;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)
mysql> select * from student;
+----+----------+------+
| id | name | age |
+----+----------+------+
| 1 | xiaoming | 18 |
| 2 | xiaohong | 20 |
+----+----------+------+
2 rows in set (0.00 sec)
mysql> create table new_student select * from student;
Query OK, 2 rows affected (0.02 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from new_student;
+----+----------+------+
| id | name | age |
+----+----------+------+
| 1 | xiaoming | 18 |
| 2 | xiaohong | 20 |
+----+----------+------+
2 rows in set (0.00 sec)
mysql> desc new_student;
+-------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id | int(10) unsigned | NO | | 0 | |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
注:这种创建表的方法虽然把现存表的查询结果插入到了新表中,但是没有把现存表的表结构的修饰符插入到新表中。
3.1.3 通过复制现存的表的表结构创建,但不复制数据。
语法:
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name { LIKE old_tbl_name | (LIKE
old_tbl_name) }
【例】通过查询现存表student的数据来创建新表new2_student:
mysql> desc student;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
mysql> select * from student;
+----+----------+------+
| id | name | age |
+----+----------+------+
| 1 | xiaoming | 18 |
| 2 | xiaohong | 20 |
+----+----------+------+
2 rows in set (0.00 sec)
mysql> create table new2_student like student;
Query OK, 0 rows affected (0.30 sec)
mysql> desc new2_student;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)
mysql> select * from new2_student;
Empty set (0.00 sec)
注:从上面的结果中可以发现,此种创建新表的方法,只是复制了现在表的表结构,而不复制数据。
3.2 表查看
3.2.1 查看支持的engine类型
mysql> show engines;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine | Support | Comment | Transactions | XA | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| InnoDB | DEFAULT | Supports transactions, row-level locking, and foreign keys | YES | YES | YES |
| MRG_MYISAM | YES | Collection of identical MyISAM tables | NO | NO | NO |
| MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO |
| BLACKHOLE | YES | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO |
| MyISAM | YES | MyISAM storage engine | NO | NO | NO |
| CSV | YES | CSV storage engine | NO | NO | NO |
| ARCHIVE | YES | Archive storage engine | NO | NO | NO |
| PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO |
| FEDERATED | NO | Federated MySQL storage engine | NULL | NULL | NULL |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
9 rows in set (0.00 sec)
注:默认是innodb存储引擎
3.2.2 查看当前数据库有哪些表
SHOW TABLES [FROM db_name]
3.2.3 查看表结构
DESC [db_name.]tb_name
SHOW COLUMNS FROM [db_name.]tb_name
3.2.4 查看表创建命令
SHOW CREATE TABLE tbl_name;
3.2.5 查看表状态
SHOW TABLE STATUS LIKE 'tbl_name';
3.2.6 查看库中所有表状态
show table status from db_name;
3.3 修改和删除表
3.3.1 删除表
DROP TABLE [IF EXISTS] 'tbl_name';
3.3.2 修改表
语法:
ALTER TABLE tbl_name
获取帮助:
help alter table
# 在表中添加字段:add
语法:
ADD [COLUMN] col_name column_definition [FIRST | AFTER col_name]
# 删除表中字段:drop
语法:
DROP [COLUMN] col_name
# 修改表中字段:
change(字段名)
语法:
CHANGE [COLUMN] old_col_name new_col_name column_definition [FIRST|AFTER col_name]
modify(字段属性)
MODIFY [COLUMN] col_name column_definition [FIRST | AFTER col_name]
范例:
3.3.2.1 修改student表名为S1
mysql> alter table student rename s1;
Query OK, 0 rows affected (0.01 sec)
3.3.2.2 在s1表中的name字段后面添加phone字段,并且设置数据类型为可变长的字符串。
mysql> desc s1;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
mysql> alter table s1 add phone varchar(11) after name;
Query OK, 0 rows affected (0.07 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc s1;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| phone | varchar(11) | YES | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
3.3.2.3 修改s1表中的phone字段的数据类型为int
mysql> alter table s1 modify phone int;
Query OK, 2 rows affected (7.02 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> desc s1;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| phone | int(11) | YES | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
3.3.2.4 修改s1表中的phone字段的名称为mobile,并且将其数据类型修改为定长字符串char
mysql> alter table s1 change column phone mobile char(11);
Query OK, 2 rows affected (0.32 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> desc s1;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| mobile | char(11) | YES | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+--------+---------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
3.3.2.5 删除s1表中的mobile字段
mysql> alter table s1 drop column mobile;
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc s1;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
修改s1表的字符集类型为utf8
mysql> alter table s1 character set utf8;
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
修改s1表中的name字段名为new_name,并且为其设置字符集为utf8
mysql> alter table s1 change name new_name varchar(20) character set utf8;
Query OK, 2 rows affected (0.05 sec)
Records: 2 Duplicates: 0 Warnings: 0
4 DML语句
INSERT,DELETE,UPDATE
4.1 INSERT语句
功能:一次插入一行或多行数据
获取帮助:help insert
简化语法:
INSERT tbl_name [(col1,...)] VALUES (val1,...), (val2,...)
4.2 UPDATE语句
获取帮助:help update
语法:
UPDATE [LOW_PRIORITY] [IGNORE] table_reference
SET assignment_list
[WHERE where_condition]
[ORDER BY ...]
[LIMIT row_count]
注:一定要有(where)限制条件,否则将修改所有行的指定字段
可以利用以下选项避免此错误
mysql -U | --safe-updates| --i-am-a-dummy
范例
4.2.1 没加限制条件直接更新s1表的age为25,将导致所有行的age段都为25
mysql> select * from s1;
+----+----------+------+-------+
| id | new_name | age | geder |
+----+----------+------+-------+
| 1 | xiaoming | 22 | NULL |
| 2 | xiaohong | 20 | NULL |
+----+----------+------+-------+
2 rows in set (0.00 sec)
mysql> update s1 set age=25 ;
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2 Changed: 2 Warnings: 0
mysql> select * from s1;
+----+----------+------+-------+
| id | new_name | age | geder |
+----+----------+------+-------+
| 1 | xiaoming | 25 | NULL |
| 2 | xiaohong | 25 | NULL |
+----+----------+------+-------+
2 rows in set (0.00 sec)
4.2.2 加where限制条件,只更新id为1的age值为18,以避免上面所发生的错误
mysql> update s1 set age=18 where id=1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from s1;
+----+----------+------+-------+
| id | new_name | age | geder |
+----+----------+------+-------+
| 1 | xiaoming | 18 | NULL |
| 2 | xiaohong | 25 | NULL |
+----+----------+------+-------+
2 rows in set (0.00 sec)
4.2.3 登录MySQL服务器的时候加上-U选项(也可以是其它两个),然后更新age字段的值为30,如果不加限制条件将直接报错,加上此选项从而避免了更新所有行的指定字段值的错误
[root@CentOS7-01 ~]#mysql -U -p123456
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 16
Server version: 5.7.28 MySQL Community Server (GPL)
Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> use mytest
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> select * from s1;
+----+----------+------+-------+
| id | new_name | age | geder |
+----+----------+------+-------+
| 1 | xiaoming | 18 | NULL |
| 2 | xiaohong | 25 | NULL |
+----+----------+------+-------+
2 rows in set (0.00 sec)
mysql> update s1 set age=30;
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column.
mysql> update s1 set age=30 where id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
4.3 DELETE语句
获取帮助:help delete;
语法:
DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM tbl_name
[PARTITION (partition_name [, partition_name] ...)]
[WHERE where_condition]
[ORDER BY ...]
[LIMIT row_count]
注意:一定要有限制条件,否则将清空表中的所有数据
如果只想清空表,但保留表结构,可以使用下面语句
truncate table tbl_name;
5 DQL语句
5.1 单表查询
获取帮助:help select
语法:
SELECT
[ALL | DISTINCT | DISTINCTROW ]
[HIGH_PRIORITY]
[STRAIGHT_JOIN]
[SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
[SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
select_expr [, select_expr ...]
[FROM table_references
[PARTITION partition_list]
[WHERE where_condition]
[GROUP BY {col_name | expr | position}
[ASC | DESC], ... [WITH ROLLUP]]
[HAVING where_condition]
[ORDER BY {col_name | expr | position}
[ASC | DESC], ...]
[LIMIT {[offset,] row_count | row_count OFFSET offset}]
[PROCEDURE procedure_name(argument_list)]
[INTO OUTFILE 'file_name'
[CHARACTER SET charset_name]
export_options
| INTO DUMPFILE 'file_name'
| INTO var_name [, var_name]]
[FOR UPDATE | LOCK IN SHARE MODE]]
说明:
1)字段显示可以使用别名
字段1 AS 字段1别名,字段2 AS 字段2别名
2)WHERE字句:指明过滤条件以实现“选择”的功能。
过滤条件:布尔型表达式
算术操作符:+,-,*,/,%
比较操作符:=,<=>(相等或都为空),<>,!=(非标准SQL),>,>=,
BETWEEN min_num AND max_num # 在两个值之间
IN(element1,element2,...) # ()中的任意单个值,相当于列表
IS NULL #为空
IS NOT NULL #不为空
DISTINCT #去除重复列,范例:SELECT DISTINCT gender FROM students;
LIKE:
% 任意长度的任意字符
_ 任意单个字符
RLIKE:正则表达式,索引失效,不建议使用
REGEXP:匹配字符串可用正则表达式书写模式,同上
逻辑操作符:NOT,AND,OR,XOR
3)GROUP:根据指定的条件把查询结果进行“分组”以用于做“聚合”运算,先过滤再分组
常见聚合函数:avg(), max(), min(), count(), sum()
HAVING: 对分组聚合运算后的结果指定过滤条件,先分组再过滤
4)ORDER BY: 根据指定的字段对查询结果进行排序
升序:ASC
降序:DESC
5)LIMIT [[offset,]row_count]:对查询的结果进行输出行数数量限制
6)对查询结果中的数据请求施加“锁”
FOR UPDATE: 写锁,独占或排它锁,只有一个读和写操作
LOCK IN SHARE MODE: 读锁,共享锁,同时多个读操作
5.1.1 练习
使用students表进行练习,表内容如下
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | NULL | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | NULL | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 45 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
+-------+---------------+-----+--------+---------+-----------+
1.查询年龄大于25岁,且为男性的同学的名字和年龄
mysql> select name,age,gender from students where age > 25 and gender='m';
+-----------+-----+--------+
| name | age | gender |
+-----------+-----+--------+
| Xie Yanke | 53 | M |
| Ding Dian | 32 | M |
| Yu Yutong | 26 | M |
| Shi Qing | 46 | M |
+-----------+-----+--------+
4 rows in set (0.00 sec)
2.以ClassID为分组依据,显示每组的平均年龄
mysql> select classid,avg(age) from students group by classid;
+---------+----------+
| classid | avg(age) |
+---------+----------+
| NULL | 36.0000 |
| 1 | 22.0000 |
| 2 | 22.0000 |
| 3 | 35.5000 |
| 4 | 32.0000 |
| 5 | 46.0000 |
| 6 | 20.0000 |
| 7 | 17.0000 |
+---------+----------+
8 rows in set (0.00 sec)
3.显示第2题中平均年龄大于30的分组及平均年龄
mysql> select classid,avg(age) from students where age>30 group by classid;
+---------+----------+
| classid | avg(age) |
+---------+----------+
| NULL | 53.0000 |
| 3 | 45.0000 |
| 4 | 32.0000 |
| 5 | 46.0000 |
+---------+----------+
4 rows in set (0.00 sec)
4.显示以L开头的名字的同学的信息
mysql> select * from students where name like 'l%';
+-------+-----------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-----------+-----+--------+---------+-----------+
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
+-------+-----------+-----+--------+---------+-----------+
1 row in set (0.00 sec)
5.显示TeacherID非空的同学的相关信息
mysql> select * from students where teacherid is not null;
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | NULL | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
+-------+-------------+-----+--------+---------+-----------+
5 rows in set (0.00 sec)
6.以年龄排序后,显示年龄最大的前3位同学的信息
mysql> select * from students order by age desc limit 3;
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
| 3 | Xie Yanke | 53 | M | NULL | 16 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 10 | Yue Lingshan | 45 | F | 3 | NULL |
+-------+--------------+-----+--------+---------+-----------+
3 rows in set (0.00 sec)
7.查询年龄大于等于20岁,小于等于25岁的同学的信息
mysql> select * from students where age between 20 and 25;
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
+-------+--------------+-----+--------+---------+-----------+
3 rows in set (0.00 sec)
8.以ClassID分组,显示每班的同学的人数
mysql> select classid as 班级id,count(stuid) as 班级人数 from students group by classid;
+----------+--------------+
| 班级id | 班级人数 |
+----------+--------------+
| NULL | 2 |
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 1 |
| 5 | 1 |
| 6 | 1 |
| 7 | 1 |
+----------+--------------+
8 rows in set (0.00 sec)
9.以Gender分组,显示其年龄之和
mysql> select gender 性别,sum(age) 年龄之和 from students group by gender;
+--------+--------------+
| 性别 | 年龄之和 |
+--------+--------------+
| F | 101 |
| M | 201 |
+--------+--------------+
2 rows in set (0.00 sec)
10.以ClassID分组,显示其平均年龄大于25的班级
mysql> select classid,avg(age) 平均年龄 from students group by classid having 平均年龄 > 25;
+---------+--------------+
| classid | 平均年龄 |
+---------+--------------+
| NULL | 36.0000 |
| 3 | 35.5000 |
| 4 | 32.0000 |
| 5 | 46.0000 |
+---------+--------------+
4 rows in set (0.00 sec)
11.以Gender分组,显示各组中年龄大于25的学员的年龄之和
mysql> select gender,sum(age) from students where age > 25 group by gender;
+--------+----------+
| gender | sum(age) |
+--------+----------+
| F | 45 |
| M | 157 |
+--------+----------+
2 rows in set (0.00 sec
12.显示其年龄大于平均年龄的同学的名字
mysql> select name,age from students where age > (select avg(age) from students);
+--------------+-----+
| name | age |
+--------------+-----+
| Xie Yanke | 53 |
| Ding Dian | 32 |
| Shi Qing | 46 |
| Yue Lingshan | 45 |
+--------------+-----+
4 rows in set (0.00 sec)
13.对classid正序排序,NULL记录排在最后
mysql> select * from students order by -classid desc;
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 10 | Yue Lingshan | 45 | F | 3 | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 3 | Xie Yanke | 53 | M | NULL | 16 |
| 7 | Xi Ren | 19 | F | NULL | NULL |
+-------+--------------+-----+--------+---------+-----------+
10 rows in set (0.00 sec)
5.1.2 SQL注入
使用user表进行练习,表内容如下
+----+-------+--------+----------+
| id | name | gender | password |
+----+-------+--------+----------+
| 2 | root | M | 88888 |
| 3 | admin | M | 66666 |
+----+-------+--------+----------+
【例1】
mysql> select * from user where name='admin' and password='' or 1=1;
+----+-------+--------+----------+
| id | name | gender | password |
+----+-------+--------+----------+
| 2 | root | M | 88888 |
| 3 | admin | M | 66666 |
+----+-------+--------+----------+
2 rows in set (0.01 sec)
【例2】
mysql> select * from user where name='admin'; -- and password='abc123';
+----+-------+--------+----------+
| id | name | gender | password |
+----+-------+--------+----------+
| 3 | admin | M | 66666 |
+----+-------+--------+----------+
1 row in set (0.00 sec)
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