SQL语言的兴起与语法标准
20世纪70年代，IBM开发出SQL，用于DB2
1981年，IBM推出SQL/DS数据库
业内标准微软和Sybase的T-SQL，Oracle的PL/SQL
SQL作为关系型数据库所使用的标准语言，最初是基于IBM的实现在1986年被批准的。1987年，“国际标准化组织(ISO)”把ANSI(美国国家标准化组织) SQL作为国际标准。
SQL：ANSI SQL
SQL-86, SQL-89, SQL-92, SQL-99, SQL-03
SQL语言规范
在数据库系统中，SQL语句不区分大小写(建议用大写)
但字符串常量区分大小写
SQL语句可单行或多行书写，以“;”结尾
关键词不能跨多行或简写
用空格和缩进来提高语句的可读性
子句通常位于独立行，便于编辑，提高可读性
注释：
SQL标准：
/*注释内容*/ 多行注释
-- 注释内容 单行注释，注意有空格
MySQL注释：
除了SQL标准外还可以用#
数据库对象
数据库的组件(对象)：
数据库、表、索引、视图、用户、存储过程、函数、触发器、事件调度器等
命名规则：
必须以字母开头
可包括数字和三个特殊字符（# _ $，#和$尽量别用）
不要使用MySQL的保留字
同一database(Schema)下的对象不能同名，即使是不同类型的对象
SQL语句分类
SQL语句分类：
DDL: Data Defination Language
CREATE, DROP, ALTER
DML: Data Manipulation Language
INSERT, DELETE, UPDATE
DCL：Data Control Language
GRANT, REVOKE
DQL：Data Query Language
SELECT
SQL语句构成
SQL语句构成：
Keyword组成clause（子句）
多条clause组成语句
示例：
SELECT * SELECT子句
FROM products FROM子句
WHERE price>400 WHERE子句
说明：此为一组SQL语句，由三个子句构成，SELECT,FROM和WHERE是关键字
数据库操作
创建数据库：
CREATE DATABASE|SCHEMA [IF NOT EXISTS] 'DB_NAME'; #创建数据库
CHARACTER SET 'character set name' #指定字符集
COLLATE 'collate name' #指定排序方式
删除数据库
DROP DATABASE|SCHEMA [IF EXISTS] 'DB_NAME';
查看支持所有字符集：SHOW CHARACTER SET;
查看支持所有排序规则：SHOW COLLATION;
获取命令使用帮助：
mysql> HELP KEYWORD;
例：HELP CREATE DATABASE;
看到基本用法及帮助文档的链接，可以去看
URL: http://dev.mysql.com/doc/refman/5.5/en/create-database.html
查看数据库列表：
mysql> SHOW DATABASES;
[root@centos7 ~]#cat /var/lib/mysql/db1/db.opt
default-character-set=latin1
default-collation=latin1_swedish_ci
表
表：二维关系
设计表：遵循规范
定义：字段，索引
字段：字段名，字段数据类型，修改符
约束，索引：应该创建在经常用作查询条件的字段上
创建表
创建表：CREATE TABLE
(1) 直接创建
(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
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name [(create_definition,...)] [table_options]
[partition_options] select_statement
(3) 通过复制现存的表的表结构创建，但不复制数据
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name { LIKE old_tbl_name | (LIKE old_tbl_name) }
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
创建表
CREATE TABLE [IF NOT EXISTS] 'tbl_name' (col1 type1 修饰符, col2 type2 修饰符, ...) #[IF NOT EXISTS]字段在脚本中一般要加
字段信息
•col type1
•PRIMARY KEY(col1,...)
•INDEX(col1, ...)
•UNIQUE KEY(col1, ...)
表选项：
•ENGINE [=] engine_name
SHOW ENGINES;查看支持的engine类型
•ROW_FORMAT [=] {DEFAULT|DYNAMIC|FIXED|COMPRESSED|REDUNDANT|COMPACT}
获取帮助：mysql> HELP CREATE TABLE;
表操作
查看所有的引擎：SHOW ENGINES
查看表：SHOW TABLES [FROM db_name]
查看表结构：DESC [db_name.]tb_name
删除表：DROP TABLE [IF EXISTS] tb_name
查看表创建命令：SHOW CREATE TABLE tbl_name
查看表状态：SHOW TABLE STATUS LIKE 'tbl_name'
查看库中所有表状态：SHOW TABLE STATUS FROM db_name
数据类型
数据类型：
数据长什么样？
数据需要多少空间来存放？
系统内置数据类型和用户定义数据类型
MySql支持多种列类型：
数值类型
日期/时间类型
字符串(字符)类型
https://dev.mysql.com/doc/refman/5.5/en/data-types.html
选择正确的数据类型对于获得高性能至关重要，三大原则：
更小的通常更好，尽量使用可正确存储数据的最小数据类型
简单就好，简单数据类型的操作通常需要更少的CPU周期
尽量避免NULL，包含为NULL的列，对MySQL更难优化（可填个默认值）
数据类型
数据类型
1、整型
tinyint(m) 1个字节 范围(-128~127)
smallint(m) 2个字节 范围(-32768~32767)
mediumint(m) 3个字节 范围(-8388608~8388607)
int(m) 4个字节 范围(-2147483648~2147483647)
bigint(m) 8个字节 范围(+-9.22*10的18次方)
取值范围如果加了unsigned，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、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
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
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<m 是小数位
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：小数点前的数字用4个字节，小数点后的数字用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
4、字符串(char,varchar,_text)
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
char和varchar：
•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
varchar和text：
•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
5.二进制数据：BLOB
•BLOB和text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是以二进制方式存储，不分大小写
•BLOB存储的数据只能整体读出
•TEXT可以指定字符集，BLOB不用指定字符集
6.日期时间类型
•date 日期 '2008-12-2'
•time 时间 '12:25:36'
•datetime 日期时间 '2008-12-2 22:06:44'
•timestamp 自动存储记录修改时间
•YEAR(2), YEAR(4)：年份
timestamp字段里的时间数据会随其他字段修改的时候自动刷新，这个数据类型的字段可以存放这条记录最后被修改的时间
修饰符
所有类型：
•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
数值型
•AUTO_INCREMENT 自动递增，适用于整数类型
•UNSIGNED 无符号
示例
CREATE TABLE students (id int UNSIGNED NOT NULL PRIMARY KEY,name VARCHAR（20）NOT NULL,age tinyint UNSIGNED);
DESC students;
CREATE TABLE students2 (id int UNSIGNED NOT NULL ,name VARCHAR(20) NOT NULL,age tinyint UNSIGNED,PRIMARY KEY(id,name));
表操作
DROP TABLE [IF EXISTS] 'tbl_name'; [CASCADE](级联删除)
ALTER TABLE 'tbl_name'
字段：
添加字段：add
ADD col1 data_type [FIRST|AFTER col_name]
删除字段：drop
修改字段：
alter（默认值）, change（字段名）, modify（字段属性）
索引:
添加索引：add index
删除索引: drop index
表选项
修改:
查看表上的索引：SHOW INDEXES FROM [db_name.]tbl_name;
查看帮助：Help ALTER TABLE
修改表示例
ALTER TABLE students RENAME s1;
ALTER TABLE s1 ADD phone varchar(11) AFTER name;
ALTER TABLE s1 MODIFY phone int;
ALTER TABLE s1 CHANGE COLUMN phone mobile char(11);
ALTER TABLE s1 DROP COLUMN mobile;
Help ALTER TABLE 查看帮助
修改表示例
ALTER TABLE students ADD gender ENUM('m','f')（enum单选/枚举，set多选）
ALETR TABLE students CHANGE id sid int UNSIGNED NOT NULL PRIMARY KEY;
ALTER TABLE students ADD UNIQUE KEY(name);
ALTER TABLE students ADD INDEX(age);
DESC students;
SHOW INDEXES FROM students;
ALTER TABLE students DROP age;
DML语句
DML:INSERT, DELETE, UPDATE, SELECT
INSERT：
一次插入一行或多行数据
语法
INSERT [L OW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name [(col_name,...)]
{VALUES | VALUE} ({expr | DEFAULT},...),(...),...
[ ON DUPLICATE KEY UPDATE 如果重复更新之
col_name=expr
[, col_name=expr] ... ]
简化写法：
INSERT tbl_name [(col1,...)] VALUES (val1,...), (val21,...)
DML语句
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name
SET col_name={expr | DEFAULT}, ...
[ ON DUPLICATE KEY UPDATE
col_name=expr
[, col_name=expr] ... ]
INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name [(col_name,...)]
SELECT ...
[ ON DUPLICATE KEY UPDATE
col_name=expr
[, col_name=expr] ... ]
DML语句
UPDATE：
UPDATE [LOW_PRIORITY] [IGNORE] table_reference
SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
[WHERE where_condition]
[ORDER BY ...]
[LIMIT row_count]
注意：一定要有限制条件，否则将修改所有行的指定字段
限制条件：
WHERE
LIMIT
Mysql 选项：--safe-updates| --i-am-a-dummy|-U
DML语句
DELETE:
DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM tbl_name
[WHERE where_condition]
[ORDER BY ...]
[LIMIT row_count]
可先排序再指定删除的行数
注意：一定要有限制条件，否则将清空表中的所有数据
限制条件：
WHERE
LIMIT
TRUNCATE TABLE tbl_name; 清空表
DQL语句
SELECT
[ALL | DISTINCT | DISTINCTROW ]
[SQL_CACHE | SQL_NO_CACHE]
select_expr [, select_expr ...]
[FROM table_references
[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}]
[FOR UPDATE | LOCK IN SHARE MODE]
SELECT
字段显示可以使用别名：
col1 AS alias1, col2 AS alias2, ...（AS可不写）
WHERE子句：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：=, !=, <>, <=, >, >=, <, <=
BETWEEN min_num AND max_num
IN (element1, element2, ...)
IS NULL
IS NOT NULL
SELECT
DISTINCT 去除重复列
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
SELECT
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
avg(), max(), min(), count(), sum()
建议：一旦用分组，前边select只需要写用以分组的字段，和统计的函数，写别的字段没有意义
HAVING: 对分组聚合运算后的结果指定过滤条件（在分组前做过滤用where）
ORDER BY: 根据指定的字段对查询结果进行排序
升序：ASC
降序：DESC
示例：
MariaDB [db1]> select * from students order by score; 顺序，NULL在前
MariaDB [db1]> select * from students order by -score desc; 顺序，但NULL在后
LIMIT [[offset,]row_count]：对查询的结果进行输出行数数量限制
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
LOCK IN SHARE MODE: 读锁，共享锁，同时多个读（但只能有一个写）
示例
DESC students;
INSERT INTO students VALUES(1,'tom'，'m'),(2,'alice','f');
INSERT INTO students(id,name) VALUES(3,'jack'),(4,'allen');
SELECT * FROM students WHERE id < 3;
SELECT * FROM students WHERE gender='m';
SELECT * FROM students WHERE gender IS NULL;
SELECT * FROM students WHERE gender IS NOT NULL;
SELECT * FROM students ORDER BY name DESC LIMIT 2;
SELECT * FROM students ORDER BY name DESC LIMIT 1,2;
SELECT * FROM students WHERE id >=2 and id <=4
SELECT * FROM students WHERE BETWEEN 2 AND 4
SELECT * FROM students WHERE name LIKE 't%'
SELECT * FROM students WHERE name RLIKE '.*[lo].*';
SELECT id stuid,name as stuname FROM students
练习
练习数据库下载链接：https://pan.baidu.com/s/11Qk9yI9lx6oZVCGXhvQ6PQ
下载其中的hellodb_innodb.sql，输命令mysql < hellodb_innodb.sql生成一个hellodb数据库：
[root@centos7 ~]#mysql #连接mariadb
MariaDB [(none)]> use hellodb #使用hellodb数据库
MariaDB [hellodb]> show tables; #共有7张表
+-------------------+
| Tables_in_hellodb |
+-------------------+
| classes |
| coc |
| courses |
| scores |
| students |
| teachers |
| toc |
+-------------------+
MariaDB [hellodb]> select * from students; #学生表
+-------+---------------+-----+--------+---------+-----------+
| 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 | 2 | 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 | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
MariaDB [hellodb]> select * from teachers; #老师表
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
MariaDB [hellodb]> select * from classes; #班级表
+---------+----------------+----------+
| ClassID | Class | NumOfStu |
+---------+----------------+----------+
| 1 | Shaolin Pai | 10 |
| 2 | Emei Pai | 7 |
| 3 | QingCheng Pai | 11 |
| 4 | Wudang Pai | 12 |
| 5 | Riyue Shenjiao | 31 |
| 6 | Lianshan Pai | 27 |
| 7 | Ming Jiao | 27 |
| 8 | Xiaoyao Pai | 15 |
+---------+----------------+----------+
MariaDB [hellodb]> select * from courses; #课程表
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 3 | Jinshe Jianfa |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
+----------+----------------+
MariaDB [hellodb]> select * from coc; #班级选课表
+----+---------+----------+
| ID | ClassID | CourseID |
+----+---------+----------+
| 1 | 1 | 2 |
| 2 | 1 | 5 |
| 3 | 2 | 2 |
| 4 | 2 | 6 |
| 5 | 3 | 1 |
| 6 | 3 | 7 |
| 7 | 4 | 5 |
| 8 | 4 | 2 |
| 9 | 5 | 1 |
| 10 | 5 | 9 |
| 11 | 6 | 3 |
| 12 | 6 | 4 |
| 13 | 7 | 4 |
| 14 | 7 | 3 |
+----+---------+----------+
MariaDB [hellodb]> select * from scores; #分数表
+----+-------+----------+-------+
| ID | StuID | CourseID | Score |
+----+-------+----------+-------+
| 1 | 1 | 2 | 77 |
| 2 | 1 | 6 | 93 |
| 3 | 2 | 2 | 47 |
| 4 | 2 | 5 | 97 |
| 5 | 3 | 2 | 88 |
| 6 | 3 | 6 | 75 |
| 7 | 4 | 5 | 71 |
| 8 | 4 | 2 | 89 |
| 9 | 5 | 1 | 39 |
| 10 | 5 | 7 | 63 |
| 11 | 6 | 1 | 96 |
| 12 | 7 | 1 | 86 |
| 13 | 7 | 7 | 83 |
| 14 | 8 | 4 | 57 |
| 15 | 8 | 3 | 93 |
+----+-------+----------+-------+
MariaDB [hellodb]> select * from toc; #空表
Empty set (0.04 sec)
以下是练习题：
(1) 在students表中，查询年龄大于25岁，且为男性的同学的名字和年龄
MariaDB [hellodb]> SELECT name,age FROM students WHERE age>25 AND gender='M';
+--------------+-----+
| name | age |
+--------------+-----+
| Xie Yanke | 53 |
| Ding Dian | 32 |
| Yu Yutong | 26 |
| Shi Qing | 46 |
| Tian Boguang | 33 |
| Xu Xian | 27 |
| Sun Dasheng | 100 |
+--------------+-----+
(2) 以ClassID为分组依据，显示每组的平均年龄
MariaDB [hellodb]> SELECT classid,avg(age) FROM students GROUP BY classid;
+---------+----------+
| classid | avg(age) |
+---------+----------+
| NULL | 63.5000 |
| 1 | 20.5000 |
| 2 | 36.0000 |
| 3 | 20.2500 |
| 4 | 24.7500 |
| 5 | 46.0000 |
| 6 | 20.7500 |
| 7 | 19.6667 |
+---------+----------+
(3) 显示第2题中平均年龄大于30的分组及平均年龄
MariaDB [hellodb]> SELECT classid,avg(age) FROM students GROUP BY classid HAVING avg(age)>30;
+---------+----------+
| classid | avg(age) |
+---------+----------+
| NULL | 63.5000 |
| 2 | 36.0000 |
| 5 | 46.0000 |
+---------+----------+
(4) 显示以L开头的名字的同学的信息
MariaDB [hellodb]> SELECT * FROM students WHERE name like 'L%';
MariaDB [hellodb]> SELECT * FROM students WHERE name like 'L%';
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
+-------+-------------+-----+--------+---------+-----------+
(5) 显示TeacherID非空的同学的相关信息
MariaDB [hellodb]> 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 | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
+-------+-------------+-----+--------+---------+-----------+
(6) 以年龄排序后，显示年龄最大的前10位同学的信息
MariaDB [hellodb]> SELECT * FROM students ORDER BY -age LIMIT 10;
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
+-------+--------------+-----+--------+---------+-----------+
(7) 查询年龄大于等于20岁，小于等于25岁的同学的信息
MariaDB [hellodb]> SELECT * FROM students WHERE age >=20 and age <=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 |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
+-------+---------------+-----+--------+---------+-----------+
SQL JOINS
表连接图示
多表查询
交叉连接：笛卡尔乘积
内连接：
等值连接：让表之间的字段以“等值”建立连接关系；
不等值连接
自然连接:去掉重复列的等值连接
自连接
外连接：
左外连接：
FROM tb1 LEFT JOIN tb2 ON tb1.col=tb2.col
右外连接
FROM tb1 RIGHT JOIN tb2 ON tb1.col=tb2.col
#要求掌握内连接，左外连接，右外连接
示例：
由于输出结果太长，就不贴上来了，感兴趣的可以试试体会一下
#内连接INNER JOIN
MariaDB [hellodb]> select s.name as studentname,t.name as teachername from teachers as t inner join students as s on s.teacherid=t.tid;
#交叉连接，笛卡尔乘积CROSS JOIN
MariaDB [hellodb]> select * from students cross join teachers;
#左外连接LEFT OUTER JOIN
MariaDB [hellodb]> select s.name as studentname,t.name as teachername from students as s left outer join teachers as t on s.teacherid=t.tid;
#对调过来左外连接
MariaDB [hellodb]> select s.name as studentname,t.name as teachername from teachers as t right outer join students as s on s.teacherid=t.tid;
#右外连接
MariaDB [hellodb]> select s.name as studentname,t.name as teachername from students as s right outer join teachers as t on s.teacherid=t.tid;
#左外连接去除交集
MariaDB [hellodb]> select s.name as studentname,t.name as teachername from students as s left outer join teachers as t on s.teacherid=t.tid and t.name is null;
#因MySQL不支持FULL OUTER JOIN，这里用UNION联合查询
MariaDB [hellodb]> select stuid,name from students union select tid,name from teachers;
#完全外连接，左外连接union右外连接即可
MariaDB [hellodb]> select s.name as studentname,t.name as teachername from students as s left outer join teachers as t on s.teacherid=t.tid UNION select s.name as studentname,t.name as teachername from students as s right outer join teachers as t on s.teacherid=t.tid;
#完全外连接去除交集
MariaDB [hellodb]> SELECT * FROM (select s.name as studentname,t.name as teachername from students as s left outer join teachers as t on s.teacherid=t.tid UNION select s.name as studentname,t.name as teachername from students as s right outer join teachers as t on s.teacherid=t.tid) as aWHERE studentname is null or teachername is null;
#自内连接
MariaDB [hellodb]> select s1.name as emp, s2.name as leader from students as s1 inner join students as s2 on s1.teacherid=s2.stuid;
#自左外连接
MariaDB [hellodb]> select s1.name as emp, s2.name as leader from students as s1 left outer join students as s2 on s1.teacherid=s2.stuid;
多表查询
子查询：在查询语句嵌套着查询语句，性能较差
基于某语句的查询结果再次进行的查询
例：MariaDB [hellodb]> select st.name,sc.score from students as st inner join scores as sc on st.stuid=sc.stuid and score > ( select avg(score) from scores); #成绩大于平均成绩的学生的姓名和分数
用在WHERE子句中的子查询：
用于比较表达式中的子查询；子查询仅能返回单个值
SELECT Name,Age FROM students WHERE Age>(SELECT avg(Age) FROM students);
用于IN中的子查询：子查询应该单键查询并返回一个或多个值从构成列表
SELECT Name,Age FROM students WHERE Age IN (SELECT Age FROM teachers);
用于EXISTS
多表查询
用于FROM子句中的子查询
使用格式：SELECT tb_alias.col1,... FROM (SELECT clause) AS tb_alias WHERE Clause;
示例：
SELECT s.aage,s.ClassID FROM (SELECT avg(Age) AS aage,ClassID FROM students WHERE ClassID IS NOT NULL GROUP BY ClassID) AS s WHERE s.aage>30;
联合查询：UNION
SELECT Name,Age FROM students UNION SELECT Name,Age FROM teachers;
#联合查询的字段数和字段类型必须一致
SELECT语句执行流程
练习
导入hellodb_innodb.sql，以下操作在students表上执行
1、以ClassID分组，显示每班的同学的人数
MariaDB [hellodb]> SELECT classid,count(stuid) FROM students GROUP BY classid;
+---------+--------------+
| classid | count(stuid) |
+---------+--------------+
| NULL | 2 |
| 1 | 4 |
| 2 | 3 |
| 3 | 4 |
| 4 | 4 |
| 5 | 1 |
| 6 | 4 |
| 7 | 3 |
+---------+--------------+
2、以Gender分组，显示其年龄之和
MariaDB [hellodb]> SELECT gender,sum(age) FROM students GROUP BY gender;
+--------+----------+
| gender | sum(age) |
+--------+----------+
| F | 190 |
| M | 495 |
+--------+----------+
3、以ClassID分组，显示其平均年龄大于25的班级
MariaDB [hellodb]> SELECT classid,sum(age) FROM students GROUP BY classid HAVING avg(age)>25;
+---------+----------+
| classid | sum(age) |
+---------+----------+
| NULL | 127 |
| 2 | 108 |
| 5 | 46 |
+---------+----------+
4、以Gender分组，显示各组中年龄大于25的学员的年龄之和
MariaDB [hellodb]> SELECT gender,sum(age) FROM students WHERE age>25 GROUP BY gender;
+--------+----------+
| gender | sum(age) |
+--------+----------+
| M | 317 |
+--------+----------+
5、显示前5位同学的姓名、课程及成绩
MariaDB [hellodb]> SELECT st.name,co.course,sc.score FROM (SELECT * FROM students WHERE stuid <=5) st INNER JOIN courses co INNER JOIN scores sc ON st.stuid=sc.stuid AND sc.courseid=co.courseid;
+-------------+----------------+-------+
| Name | course | score |
+-------------+----------------+-------+
| Shi Zhongyu | Kuihua Baodian | 77 |
| Shi Zhongyu | Weituo Zhang | 93 |
| Shi Potian | Kuihua Baodian | 47 |
| Shi Potian | Daiyu Zanghua | 97 |
| Xie Yanke | Kuihua Baodian | 88 |
| Xie Yanke | Weituo Zhang | 75 |
| Ding Dian | Daiyu Zanghua | 71 |
| Ding Dian | Kuihua Baodian | 89 |
| Yu Yutong | Hamo Gong | 39 |
| Yu Yutong | Dagou Bangfa | 63 |
+-------------+----------------+-------+
6、显示其成绩高于80的同学的名称及课程；
MariaDB [hellodb]> SELECT st.name,co.course FROM students st INNER JOIN scores sc INNER JOIN courses co ON st.stuid=sc.stuid AND sc.courseid=co.courseid AND sc.score>80;
+-------------+----------------+
| name | course |
+-------------+----------------+
| Shi Zhongyu | Weituo Zhang |
| Shi Potian | Daiyu Zanghua |
| Xie Yanke | Kuihua Baodian |
| Ding Dian | Kuihua Baodian |
| Shi Qing | Hamo Gong |
| Xi Ren | Hamo Gong |
| Xi Ren | Dagou Bangfa |
| Lin Daiyu | Jinshe Jianfa |
+-------------+----------------+
7、求前8位同学每位同学自己两门课的平均成绩，并按降序排列
MariaDB [hellodb]> SELECT st.name,avg(sc.score) as avg_score FROM students st INNER JOIN scores sc ON st.stuid=sc.stuid GROUP BY name ORDER BY avg_score desc;
+-------------+-----------+
| name | avg_score |
+-------------+-----------+
| Shi Qing | 96.0000 |
| Shi Zhongyu | 85.0000 |
| Xi Ren | 84.5000 |
| Xie Yanke | 81.5000 |
| Ding Dian | 80.0000 |
| Lin Daiyu | 75.0000 |
| Shi Potian | 72.0000 |
| Yu Yutong | 51.0000 |
+-------------+-----------+
8、显示每门课程课程名称及学习了这门课的同学的个数
MariaDB [hellodb]> SELECT co.course,count(stuid) FROM students st INNER JOIN courses co INNER JOIN coc ON st.classid=coc.classid AND coc.courseid=co.courseid GROUP BY co.course;
+----------------+--------------+
| course | count(stuid) |
+----------------+--------------+
| Dagou Bangfa | 4 |
| Daiyu Zanghua | 8 |
| Hamo Gong | 5 |
| Jinshe Jianfa | 7 |
| Kuihua Baodian | 11 |
| Taiji Quan | 7 |
| Weituo Zhang | 3 |
+----------------+--------------+
9、如何显示其年龄大于平均年龄的同学的名字
MariaDB [hellodb]> SELECT name FROM students WHERE age>(SELECT avg(age) FROM students);
+--------------+
| name |
+--------------+
| Xie Yanke |
| Ding Dian |
| Shi Qing |
| Tian Boguang |
| Sun Dasheng |
+--------------+
10、如何显示其学习的课程为第1、2，4或第7门课的同学的名字
MariaDB [hellodb]> SELECT s.name FROM students AS s INNER JOIN (SELECT * FROM coc WHERE courseid IN (1,2,4,7)) AS co ON s.classid=co.classid GROUP BY s.name;
+---------------+
| name |
+---------------+
| Diao Chan |
| Ding Dian |
| Duan Yu |
| Hua Rong |
| Huang Yueying |
| Lin Chong |
| Lin Daiyu |
| Lu Wushuang |
| Ma Chao |
| Ren Yingying |
| Shi Potian |
| Shi Qing |
| Shi Zhongyu |
| Tian Boguang |
| Wen Qingqing |
| Xi Ren |
| Xiao Qiao |
| Xie Yanke |
| Xu Zhu |
| Xue Baochai |
| Yu Yutong |
| Yuan Chengzhi |
| Yue Lingshan |
+---------------+
23 rows in set (0.01 sec)
11、如何显示其成员数最少为3个的班级的同学中年龄大于同班同学平均年龄的同学
MariaDB [hellodb]> SELECT s.name,s.age,s.classid FROM students s INNER JOIN (SELECT avg(age) as avg_age,classid FROM students GROUP BY classid HAVING count(stuid)>=3) AS a ON s.classid=a.classid WHERE age>avg_age;
+---------------+-----+---------+
| name | age | classid |
+---------------+-----+---------+
| Shi Potian | 22 | 1 |
| Xie Yanke | 53 | 2 |
| Ding Dian | 32 | 4 |
| Yu Yutong | 26 | 3 |
| Yuan Chengzhi | 23 | 6 |
| Xu Zhu | 21 | 1 |
| Lin Chong | 25 | 4 |
| Hua Rong | 23 | 7 |
| Huang Yueying | 22 | 6 |
+---------------+-----+---------+
12、统计各班级中年龄大于全校同学平均年龄的同学
MariaDB [hellodb]> SELECT name,age,classid FROM students WHERE age >(SELECT avg(age) FROM students);
+--------------+-----+---------+
| name | age | classid |
+--------------+-----+---------+
| Xie Yanke | 53 | 2 |
| Ding Dian | 32 | 4 |
| Shi Qing | 46 | 5 |
| Tian Boguang | 33 | 2 |
| Sun Dasheng | 100 | NULL |
+--------------+-----+---------+
视图
视图：VIEW，虚表，保存有实表的查询结果
创建方法：
CREATE VIEW view_name [(column_list)]
AS select_statement
[WITH [CASCADED | LOCAL] CHECK OPTION]
查看视图定义：SHOW CREATE VIEW view_name
删除视图：
DROP VIEW [IF EXISTS]
view_name [, view_name] ...
[RESTRICT | CASCADE]
视图中的数据事实上存储于“基表”中，因此，其修改操作也会针对基表实现；其修改操作受基表限制
视图是虚拟的，相当于一个别名
物化视图：把视图生成为一个实体表，MySQL不支持，某些别的数据库支持