SQL语法：MySQL系列之四

一、SQL语言的简介和规范

是一种特定目的程序语言，用于管理关系数据库管理系统（RDBMS），或在关系流数据管理系统（RDSMS）中进行流处理。

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语言的规范

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： #

数据库对象的命名规则

必须以字母开头
可包括数字和三个特殊字符（# _ $）
不要使用MySQL的保留字
同一database(Schema)下的对象不能同名

SQL语句的分类

DDL: Data Defination Language 数据定义语言
CREATE, DROP, ALTER
DML: Data Manipulation Language 数据操作语言
INSERT, DELETE, UPDATE
DCL：Data Control Language 数据控制语言
GRANT, REVOKE
DQL：Data Query Language 数据查询语言
SELECT

二、数据库操作

1、创建库

CREATE DATABASE [IF NOT EXISTS] db_name; 创建数据库

CHARACTER SET ‘character set name’ 设置字符集类型

COLLATE ‘collate name’ 设置排序规则

查看支持所有字符集：SHOW CHARACTER SET;

查看支持所有排序规则：SHOW COLLATION;

MariaDB [(none)]> CREATE DATABASE IF NOT EXISTS testdb;

2、删除库

我不会

3、查看数据库列表

SHOW DATABASES;

三、表操作

1、创建表

方法一：直接创建

CREATE TABLE [IF NOT EXISTS] tbl_name (create_definition,…)

MariaDB [testdb]> CREATE TABLE IF NOT EXISTS students (id TINYINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,name VARCHAR(30) NOT NULL,phone CHAR(11),gender ENUM('M','F'));

方法二：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 TABLE [IF NOT EXISTS] tbl_name select_statement

MariaDB [testdb]> CREATE TABLE user SELECT user,host,password FROM mysql.user;

如果只想模仿查询旧表创建一个无记录的表我们可以加入条件 WHERE 0=1;

MariaDB [testdb]> CREATE TABLE user2 SELECT user,host,password FROM mysql.user WHERE 0=1;

方法三：通过复制现存的表的表结构创建，但不复制数据

CREATE TABLE [IF NOT EXISTS] tbl_name LIKE old_tbl_name

MariaDB [testdb]> CREATE TABLE user3 LIKE mysql.user;

2、修改表

ALTER TABLE tbl_name [alter_specification [, alter_specification] …]

增加属性 ADD

MariaDB [testdb]> ALTER TABLE students ADD age TINYINT AFTER name;

删除属性 DROP

MariaDB [testdb]> ALTER TABLE students DROP phone;

修改属性 CHANGE, MODIFY

MariaDB [testdb]> ALTER TABLE students CHANGE age ages TINYINT(2) NOT NULL;

MariaDB [testdb]> ALTER TABLE students MODIFY gender ENUM('M','F');

3、删除表

MariaDB [testdb]> DROP TABLE user3;

4、查看表

SHOW TABLES; 列出库中所有的表

DESC [db_name.]tb_name; 查看表结构

SHOW CREATE TABLE tbl_name; 查看创建表的命令

SHOW TABLE STATUS LIKE ‘tbl_name’; 查看表状态

SHOW TABLE STATUS FROM db_name; 查看指定库中所有表状态

SHOW ENGINES; 查看所有存储引擎

四、DML: 数据操作语言

MariaDB [testdb]> DESC students;  #示例表
+--------+---------------------+------+-----+---------+----------------+
| Field  | Type                | Null | Key | Default | Extra          |
+--------+---------------------+------+-----+---------+----------------+
| id     | tinyint(3) unsigned | NO   | PRI | NULL    | auto_increment |
| name   | varchar(30)         | NO   |     | NULL    |                |
| ages   | tinyint(2)          | NO   |     | NULL    |                |
| gender | enum('M','F')       | YES  |     | NULL    |                |
+--------+---------------------+------+-----+---------+----------------+

1、INSERT 插入数据

单条记录插入

INSERT INTO tb_name (col1,col2,…) VALUES (val1,val2,…);

MariaDB [testdb]> INSERT students(id,name,ages,gender) VALUES (1,'tom',26,'M');
MariaDB [testdb]> INSERT students(name,ages,gender) VALUES ('jerry',19,'M'); 
MariaDB [testdb]> INSERT students(name,ages,gender) VALUES ('maria',19,'M');
MariaDB [testdb]> INSERT students SET name='ouyangfeng',ages=56,gender='M';

多条记录插入

INSERT INTO tb_name (col1,col2,…) VALUES (val1,val2,…)[,(val1,val2,…),…];

MariaDB [testdb]> INSERT students(name,ages,gender) VALUES ('xiaolongnv',18,'F'),('dongfangbubai',28,'F');

MariaDB [testdb]> SELECT * FROM students;
+----+---------------+------+--------+
| id | name          | ages | gender |
+----+---------------+------+--------+
|  1 | tom           |   26 | M      |
|  2 | jerry         |   19 | M      |
|  3 | maria         |   19 | M      |
|  4 | xiaolongnv    |   18 | F      |
|  5 | dongfangbubai |   28 | F      |
|  6 | ouyangfeng    |   56 | M      |
+----+---------------+------+--------+

从其他表查询数据保存到此表中

MariaDB [testdb]> ALTER TABLE students ADD address TEXT;  #加个字段做测试用

MariaDB [testdb]> INSERT students(name,address) SELECT user,host FROM mysql.user;

MariaDB [testdb]> SELECT * FROM students;
+----+---------------+------+--------+-----------+
| id | name          | ages | gender | address   |
+----+---------------+------+--------+-----------+
|  1 | tom           |   26 | M      | NULL      |
|  2 | jerry         |   19 | M      | NULL      |
|  3 | maria         |   19 | M      | NULL      |
|  4 | xiaolongnv    |   18 | F      | NULL      |
|  5 | dongfangbubai |   28 | F      | NULL      |
|  6 | ouyangfeng    |   56 | M      | NULL      |
|  7 | root          |    0 | NULL   | 127.0.0.1 |
|  8 | root          |    0 | NULL   | ::1       |
|  9 |               |    0 | NULL   | centos7   |
| 10 | root          |    0 | NULL   | centos7   |
| 11 |               |    0 | NULL   | localhost |
| 12 | root          |    0 | NULL   | localhost |
+----+---------------+------+--------+-----------+

2、UPDATE 修改数据

UPDATE tbl_name SET col1=value1,col2=value2,… WHERE col=value;

MariaDB [testdb]> UPDATE students SET gender='F' WHERE id=3;

3、DELETE 删除数据

MariaDB [testdb]> DELETE FROM students WHERE name='';  #删除名字为空的记录
MariaDB [testdb]> TRUNCATE TABLE user;  #情况表记录

注意：一定要有限制条件（WHERE | LIMIT），否则将修改所有行的指定字段

五、SELECT：数据查询

AS：别名
WHERE：指明过滤条件以实现“选择”的功能
+, -, *, /, %：算术操作符
=, !=, <>, >, <, >=, <=：比较操作符
BETWEEN min_num AND max_num：在min_num和max_mun之间
IN (element1,element2,…)：在element…中的
IS NULL：为空
IS NOT NULL：不为空
LIKE：做匹配，像。。。
- %：任意长度的任意字符
- _：单个任意字符
RLIKE：正则表达式，不建议用
REGEXP：同上
NOT, AND, OR, XOR：逻辑操作符
GROUP BY：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() 平均数
MAX() 最大数
MIN() 最小数
COUNT() 统计
SUM() 求和
HAVING ：对分组聚合运算后的结果指定过滤条件。类似WHERE的作用，但只能在分组中使用
ORDER BY：排序
ASC：正序，默认
DESC：倒序
-KEYWORD：在排序时在关键字前加-可以避免把NULL排在前边
LIMIT [[offset,]row_count]：对查询的结果进行输出行数数量限制

1、选择

MariaDB [testdb]> SELECT * FROM students WHERE name='maria';  #查询maria的信息
MariaDB [testdb]> SELECT * FROM students WHERE id BETWEEN 2 AND 5;  #查询2到5号学生的信息
MariaDB [testdb]> SELECT * FROM students WHERE name IN ('jerry','xiaolongnv');  #查询jerry和xiaolongnv的信息
MariaDB [testdb]> SELECT * FROM students WHERE gender IS NOT NULL;  #查询年龄不为空的信息
MariaDB [testdb]> SELECT * FROM students WHERE name LIKE '%o%';  #查询姓名中包含'o'的信息

2、投影

MariaDB [testdb]> SELECT user AS 用户,host AS 主机,password AS 密码 FROM mysql.user;

3、分组

MariaDB [testdb]> SELECT gender,AVG(ages) FROM students GROUP BY gender;  #查询男生、女生年龄的平均值
MariaDB [testdb]> SELECT gender,AVG(ages) FROM students GROUP BY gender HAVING gender='M';  #只显示男生的平均年龄信息

4、排序

MariaDB [testdb]> SELECT * FROM students ORDER BY ages DESC;  #按年龄排序，倒序显示
MariaDB [testdb]> SELECT * FROM students WHERE ages > 0 ORDER BY ages LIMIT 3;  #按年龄排序，过滤年龄大于0的，正序排序，取前三条记录

六、多表查询

为了练习，我们将表在扩展一下

MariaDB [testdb]> DELETE FROM students WHERE id BETWEEN 7 AND 12;
MariaDB [testdb]> CREATE TABLE score (id TINYINT(2) UNSIGNED AUTO_INCREMENT PRIMARY KEY,score TINYINT(3));
MariaDB [testdb]> ALTER TABLE students ADD sid TINYINT(2); 
MariaDB [testdb]> UPDATE students SET sid=6 WHERE id=6;
MariaDB [testdb]> INSERT score SET score=87;
MariaDB [testdb]> SELECT * FROM students;
+----+---------------+------+--------+---------+------+
| id | name          | ages | gender | address | sid  |
+----+---------------+------+--------+---------+------+
|  1 | tom           |   26 | M      | NULL    |    1 |
|  2 | jerry         |   19 | M      | NULL    |    2 |
|  3 | maria         |   19 | F      | NULL    |    3 |
|  4 | xiaolongnv    |   18 | F      | NULL    |    4 |
|  5 | dongfangbubai |   28 | F      | NULL    |    5 |
|  6 | ouyangfeng    |   56 | M      | NULL    |    6 |
+----+---------------+------+--------+---------+------+
MariaDB [testdb]> SELECT * FROM score;   
+----+-------+
| id | score |
+----+-------+
|  1 |    99 |
|  2 |    98 |
|  3 |    88 |
|  4 |    68 |
|  5 |    78 |
|  6 |    87 |
+----+-------+

JOIN ON：交叉连接

INNER JOIN ON：内连接

LEFT OUTER JOIN ON：左外连接

RIGHT OUTER JOIN ON：右外连接

UNION ON：完全外连接

MariaDB [testdb]> SELECT * FROM students AS s,score AS o WHERE s.sid=o.id;  #俩张表取交集

1、交叉连接

MariaDB [testdb]> SELECT * FROM students JOIN score;

2、内连接

MariaDB [testdb]> SELECT t.name,s.score FROM students AS t INNER JOIN score AS s ON t.sid=s.id;
+---------------+-------+
| name          | score |
+---------------+-------+
| tom           |    99 |
| jerry         |    98 |
| maria         |    88 |
| xiaolongnv    |    68 |
| dongfangbubai |    78 |
| ouyangfeng    |    87 |
+---------------+-------+

3、外连接

MariaDB [testdb]> SELECT t.name,s.score FROM students AS t LEFT JOIN score AS s ON t.sid=s.id;  #左外连接
+---------------+-------+
| name          | score |
+---------------+-------+
| tom           |    99 |
| jerry         |    98 |
| maria         |    88 |
| xiaolongnv    |    68 |
| dongfangbubai |    78 |
| ouyangfeng    |    87 |
+---------------+-------+

MariaDB [testdb]> SELECT * FROM students AS t RIGHT JOIN score AS s ON t.sid=s.id;  #右外连接

6、完全外连接

MariaDB [testdb]> SELECT name,address FROM students
    -> UNION
    -> SELECT user,host FROM mysql.user;
+---------------+-----------+
| name          | address   |
+---------------+-----------+
| tom           | NULL      |
| jerry         | NULL      |
| maria         | NULL      |
| xiaolongnv    | NULL      |
| dongfangbubai | NULL      |
| ouyangfeng    | NULL      |
| root          | 127.0.0.1 |
| root          | ::1       |
|               | centos7   |
| root          | centos7   |
|               | localhost |
| root          | localhost |
+---------------+-----------+

5、自连接

MariaDB [testdb]> ALTER TABLE students ADD tid TINYINT(2);  #再加一个tid字段
MariaDB [testdb]> SELECT * FROM students;
+----+---------------+------+--------+---------+------+------+
| id | name          | ages | gender | address | sid  | tid  |
+----+---------------+------+--------+---------+------+------+
|  1 | tom           |   26 | M      | NULL    |    1 |    2 |
|  2 | jerry         |   19 | M      | NULL    |    2 |    1 |
|  3 | maria         |   19 | F      | NULL    |    3 |    4 |
|  4 | xiaolongnv    |   18 | F      | NULL    |    4 |    5 |
|  5 | dongfangbubai |   28 | F      | NULL    |    5 |    4 |
|  6 | ouyangfeng    |   56 | M      | NULL    |    6 |    4 |
+----+---------------+------+--------+---------+------+------+

MariaDB [testdb]> SELECT s1.name AS studentname,s2.name AS teachername FROM students AS s1 INNER JOIN students AS s2 ON s1.id=s2.tid;
+---------------+---------------+
| studentname   | teachername   |
+---------------+---------------+
| jerry         | tom           |
| tom           | jerry         |
| xiaolongnv    | maria         |
| dongfangbubai | xiaolongnv    |
| xiaolongnv    | dongfangbubai |
| xiaolongnv    | ouyangfeng    |
+---------------+---------------+

七、子查询

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

1、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

MariaDB [testdb]> SELECT name,ages FROM students WHERE ages > (SELECT AVG(ages) FROM students);    #查询大于平均年龄的同学

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

2、用于FROM子句中的子查询

SELECT tb_alias.col1,… FROM (SELECT clause) AS tb_alias WHERE Clause;

@^_^@ 2018.06.07 13:37

本文链接：https://www.cnblogs.com/L-dongf/p/9150167.html