SQL基础操作语句

企业开发 2023-08-22 18:22:03 阅读次数: 0

SQL数据库基本操作

mysql -uroot -p            # 在终端打开 mysql 
exit；                    #退出MySQL数据库服务器
create database 数据库名;   # 创建数据库
drop database 数据库名;     # 删除数据库
show databases;            # 查看已经创建的数据库
use 数据库名;               # 使用某个数据库


修改密码语句
 alter user 'root'@'localhost' identified by '123456';


显示所有数据库
show databases;
-- 创建数据库
CREATE DATABASE test;
-- 切换数据库
use test;
-- 显示数据库中的所有表
show tables;
-- 创建数据表
CREATE TABLE pet (
    name VARCHAR(20),
    owner VARCHAR(20),
    species VARCHAR(20),
    sex CHAR(1),
    birth DATE,
    death DATE
);
-- 查看数据表结构
-- describe pet;
desc pet;
-- 查询表
SELECT * from pet;
-- 插入数据
INSERT INTO pet VALUES ('puffball', 'Diane', 'hamster', 'f', '1990-03-30', NULL);
-- 修改数据
UPDATE pet SET name = 'squirrel' where owner = 'Diane';
-- 删除数据
DELETE FROM pet where name = 'squirrel';
-- 删除表
DROP TABLE myorder;


 如何查看数据表的架构?   
describe tableName;
 说明:
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
 Field    :    字段的名称
 Type     :    字段的类型,可以有int    char    varchar    
 Key      :    是否是关键字 如可以定义为:  primary key 或者 unique key   ...
Default: :    若是该字段没有主动设置值的时候,该字段的默认值是什么?


如何插入数据?
 INSERT INTO pet VALUES('kk','cc','dog','1','1998-8-2',null);
           +------+-------+---------+------+------------+-------+
           | name | owner | specise | sex  | brith      | death |
           +------+-------+---------+------+------------+-------+
           | kk   | cc    | dog     |      | 1998-08-02 | NULL  |
           +------+-------+---------+------+------------+-------+
       注意:
           NULL:代表的是空,表示该字段还没有数据.千万不要主动填写'NULL',这代表你的字段有一个值叫做'null'.

其实还有一种写法:
                 
INSERT INTO pet(name,owner) VALUES ('xx','cc');
代表我只在name和owner字段上面插入的一条,其他皆为NULL/默认值的数据

mysql建表约束

  • 主键约束
    它能够唯一确定一张表中的一条记录,增加主键约束之后,就可以使得字段不重复而且不为空。

     create table user(
    id int PRIMARY KEY,
    name VARCHAR(20)
 );
 INSERT INTO user VALUES (1,'张三');
 	
 +----+------+
 | id | name |
 +----+------+
 |  1 | 张三 |
 +----+------+
 
 运行DESCRIBE user;
 +-------+-------------+------+-----+---------+-------+
 | Field | Type        | Null | Key | Default | Extra |
 +-------+-------------+------+-----+---------+-------+
 | id    | int(11)     | NO   | PRI | NULL    |       |
 | name  | varchar(20) | YES  |     | NULL    |       |
 +-------+-------------+------+-----+---------+-------+
 发现 id是不可以为null 而且 key的值 也变为:PRI(primary)

  • 联合主键（复合主键）
    只要联合起来的主键值加起来不重复就可以。

     CREATE TABLE user2(
     id INT,
     name VARCHAR(20),
     password VARCHAR(20),
     PRIMARY key(id,name)
 );
 
 运行DESCRIBE user2;
 +----------+-------------+------+-----+---------+-------+
 | Field    | Type        | Null | Key | Default | Extra |
 +----------+-------------+------+-----+---------+-------+
 | id       | int(11)     | NO   | PRI | NULL    |       |
 | name     | varchar(20) | NO   | PRI | NULL    |       |
 | password | varchar(20) | YES  |     | NULL    |       |
 +----------+-------------+------+-----+---------+-------+
  select * from user2;
 +----+------+----------+
 | id | name | password |
 +----+------+----------+
 |  1 | zcc  | 345      |
 |  2 | zc2  | 345      |
 +----+------+----------+
  select * from user2;
 +----+------+----------+
 | id | name | password |
 +----+------+----------+
 |  1 | zcc  | 345      |
 |  2 | zc2  | 345      |
 |  2 | zcc  | 345      |
 +----+------+----------+

  • 自增约束

       CREATE TABLE user3(
     id INT PRIMARY KEY AUTO_INCREMENT,
     name VARCHAR(20)
 );
 
 运行DESCRIBE user3;
 +-------+-------------+------+-----+---------+----------------+
 | Field | Type        | Null | Key | Default | Extra          |
 +-------+-------------+------+-----+---------+----------------+
 | id    | int(11)     | NO   | PRI | NULL    | auto_increment |
 | name  | varchar(20) | YES  |     | NULL    |                |
 +-------+-------------+------+-----+---------+----------------+
 
 INSERT INTO user3(name) VALUES('zcc');
 INSERT INTO user3(name) VALUES('zc');
 +----+------+
 | id | name |
 +----+------+
 |  1 | zcc |
 |  2 | zc |
 +----+------+
 没有自定义id值 但是自动生成了id

    • 如果创建表的时候忘记创建主键约束了？该怎们办？

         CREATE TABLE user4(
   		    id INT ,
   		    name VARCHAR(20)
   		);
   desc user4;
   +-------+-------------+------+-----+---------+-------+
   | Field | Type        | Null | Key | Default | Extra |
   +-------+-------------+------+-----+---------+-------+
   | id    | int         | YES  |     | NULL    |       |
   | name  | varchar(20) | YES  |     | NULL    |       |
   +-------+-------------+------+-----+---------+-------+
   
   alter table user4 add primary key(id);
   desc user4;
   +-------+-------------+------+-----+---------+-------+
   | Field | Type        | Null | Key | Default | Extra |
   +-------+-------------+------+-----+---------+-------+
   | id    | int         | NO   | PRI | NULL    |       |
   | name  | varchar(20) | YES  |     | NULL    |       |
   +-------+-------------+------+-----+---------+-------+
   如何删除主键约束？
   alter table user4 drop primary key;
   使用modify修改字段添加约束。
   alter table user4 modify id int primary key;

  • 唯一约束
    约束修饰字段的值不能重复

     CREATE TABLE user5(
     id INT PRIMARY KEY AUTO_INCREMENT,
     name VARCHAR(20)
 );
 运行 DESCRIBE user5;
 +-------+-------------+------+-----+---------+----------------+
 | Field | Type        | Null | Key | Default | Extra          |
 +-------+-------------+------+-----+---------+----------------+
 | id    | int(11)     | NO   | PRI | NULL    | auto_increment |
 | name  | varchar(20) | YES  |     | NULL    |                |
 +-------+-------------+------+-----+---------+----------------+
 
 新增name为唯一约束:
 ALTER TABLE user5 ADD UNIQUE(name);
 运行 DESCRIBE user5;
 +-------+-------------+------+-----+---------+----------------+
 | Field | Type        | Null | Key | Default | Extra          |
 +-------+-------------+------+-----+---------+----------------+
 | id    | int(11)     | NO   | PRI | NULL    | auto_increment |
 | name  | varchar(20) | YES  | UNI | NULL    |                |
 +-------+-------------+------+-----+---------+----------------+
 测试:插入数据
 INSERT INTO user5(name) VALUES ('cc');
 运行 SELECT * FROM user5; 查看结果:
 +----+------+
 | id | name |
 +----+------+
 |  1 | cc   |
 +----+------+
 再次插入INSERT INTO user5(name) VALUES ('cc');
 出现:ERROR 1062 (23000): Duplicate entry 'cc' for key 'name'
 
 换个试试 INSERT INTO user5(name) VALUES ('aa');
 运行 SELECT * FROM user5; 查看结果:
 +----+------+
 | id | name |
 +----+------+
 |  3 | aa   |
 |  1 | cc   |
 +----+------+
 总结一下:
     主键约束(primary key)中包含了唯一约束
 场景:业务需求:设计一张用户注册表,用户姓名必须要用手机号来注册,而且手机号和用户名称都不能为空,那么:
 CREATE TABLE user_test(
     id INT PRIMARY KEY AUTO_INCREMENT COMMENT'主键id',
     name VARCHAR(20) NOT NULL COMMENT'用户姓名,不能为空',
     phone_number VARCHAR(20) UNIQUE NOT NULL COMMENT'用户手机,不能重复且不能为空'
 );
 运行 DESCRIBE user_test;
 +--------------+-------------+------+-----+---------+----------------+
 | Field        | Type        | Null | Key | Default | Extra          |
 +--------------+-------------+------+-----+---------+----------------+
 | id           | int(11)     | NO   | PRI | NULL    | auto_increment |
 | name         | varchar(20) | NO   |     | NULL    |                |
 | phone_number | int(11)     | NO   | UNI | NULL    |                |
 +--------------+-------------+------+-----+---------+----------------+
 这样的话就达到了每一个手机号都只能出现一次,达到了每个手机号只能被注册一次.
 用户姓名可以重复,但是手机号码却不能重复,符合正常的逻辑需求

  • 添加约束总结：
    1.建表的时候添加约束；
    2.可以使用alter … add …
    3.alter … modify …
    4.删除： alter … drop …

  • 非空约束
    修饰的字段不能为空

      create table user6(id int, name varchar(20) not null);
  desc user6;
 +-------+-------------+------+-----+---------+-------+
 | Field | Type        | Null | Key | Default | Extra |
 +-------+-------------+------+-----+---------+-------+
 | id    | int         | YES  |     | NULL    |       |
 | name  | varchar(20) | NO   |     | NULL    |       |
 +-------+-------------+------+-----+---------+-------+
 
 insert into user6 values(1,'张三');
  insert into user6 (name) values('三');
 
  select * from user6;
 +------+--------+
 | id   | name   |
 +------+--------+
 |    1 | 张三   |
 | NULL | 三     |
 +------+--------+

  • 默认约束
    当我们插入字段值的时候如果没有传值，就会使用默认值；

     create table user7(id int, name varchar(20),age int default 10);
 Query OK, 0 rows affected (0.03 sec)
 
 mysql> desc user7;
 +-------+-------------+------+-----+---------+-------+
 | Field | Type        | Null | Key | Default | Extra |
 +-------+-------------+------+-----+---------+-------+
 | id    | int         | YES  |     | NULL    |       |
 | name  | varchar(20) | YES  |     | NULL    |       |
 | age   | int         | YES  |     | 10      |       |
 +-------+-------------+------+-----+---------+-------+
 
 mysql> insert into user7(id,name) values(1,'dsd');
 Query OK, 1 row affected (0.01 sec)
 
 mysql> insert into user7(id,name,age) values(1,'dsd',20);
 Query OK, 1 row affected (0.01 sec)
 
 mysql> select * from user7;
 +------+------+------+
 | id   | name | age  |
 +------+------+------+
 |    1 | dsd  |   10 |
 |    1 | dsd  |   20 |
 +------+------+------+
 应用场景:
 业务需求:找正常的用户,对这些正常用户进行发放优惠卷或者积分之类的东西,而被禁封的用户我们不让其参加多动.
 我们想要封用户只要将status的值从0改为1就行了,当然我们取用户的时候必须要先判断status是否是0.若是1.说明该用户已经被禁封.
 先封手机号为'1234'的用户:
 UPDATE user6 SET status = 1 WHERE phone_number= '1234';
 SELECT * FROM user6;
 +----+------+--------------+--------+
 | id | name | phone_number | status |
 +----+------+--------------+--------+
 |  1 | aa   | 123          |      0 |
 |  2 | bb   | 1234         |      1 |
 |  3 | cc   | 1263456      |      0 |
 +----+------+--------------+--------+
 status为1,说明用户已经被封,该用户不可以参加活动
 
 我们取用户的时候加上status的判断,如:
 SELECT * FROM user6 WHERE status = 0;
 +----+------+--------------+--------+
 | id | name | phone_number | status |
 +----+------+--------------+--------+
 |  1 | aa   | 123          |      0 |
 |  3 | cc   | 1263456      |      0 |
 +----+------+--------------+--------+

  • 外键约束：
    涉及两个表：主表，副表（父表，子表）；
    1.主表（父表）classes 中没有的数据值，在副表（子表）中，是不可以使用的。
    2.主表中记录被副表引用，是不可以被删除的。

     mysql> create table classes(id int primary key,name varchar(20));
 Query OK, 0 rows affected (0.04 sec)
 
 mysql> create table students(id int primary key,name varchar(20),class_id int, foreign key(class_id) references classes(id));
 Query OK, 0 rows affected (0.04 sec)
 
 mysql> desc classes;
 +-------+-------------+------+-----+---------+-------+
 | Field | Type        | Null | Key | Default | Extra |
 +-------+-------------+------+-----+---------+-------+
 | id    | int         | NO   | PRI | NULL    |       |
 | name  | varchar(20) | YES  |     | NULL    |       |
 +-------+-------------+------+-----+---------+-------+
 2 rows in set (0.00 sec)
 
 mysql> desc students;
 +----------+-------------+------+-----+---------+-------+
 | Field    | Type        | Null | Key | Default | Extra |
 +----------+-------------+------+-----+---------+-------+
 | id       | int         | NO   | PRI | NULL    |       |
 | name     | varchar(20) | YES  |     | NULL    |       |
 | class_id | int         | YES  | MUL | NULL    |       |
 +----------+-------------+------+-----+---------+-------+
 3 rows in set (0.00 sec)
 
 mysql> insert into classes values(1,'一班');
 Query OK, 1 row affected (0.01 sec)
 
 mysql> insert into classes values(2,'二班');
 Query OK, 1 row affected (0.01 sec)
 
 mysql> insert into classes values(3,'三班');
 Query OK, 1 row affected (0.01 sec)
 
 mysql> insert into classes values(4,'四班');
 Query OK, 1 row affected (0.01 sec)
 
 mysql> select * from classes;
 +----+--------+
 | id | name   |
 +----+--------+
 |  1 | 一班   |
 |  2 | 二班   |
 |  3 | 三班   |
 |  4 | 四班   |
 +----+--------+
 4 rows in set (0.00 sec)
 
 mysql> insert into students values(1001,'张三',1);
 Query OK, 1 row affected (0.01 sec)
 
 mysql> insert into students values(1002,'张三',2);
 Query OK, 1 row affected (0.01 sec)
 
 mysql> insert into students values(1003,'张三',3);
 Query OK, 1 row affected (0.01 sec)
 
 mysql> insert into students values(1004,'张三',4);
 Query OK, 1 row affected (0.01 sec)
 
 
 mysql> select * from students;
 +------+--------+----------+
 | id   | name   | class_id |
 +------+--------+----------+
 | 1001 | 张三   |        1 |
 | 1002 | 张三   |        2 |
 | 1003 | 张三   |        3 |
 | 1004 | 张三   |        4 |
 +------+--------+----------+
 4 rows in set (0.00 sec)
 
 mysql> insert into students values(1005,'张三',5);
 ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`students`, CONSTRAINT `students_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `classes` (`id`))
 mysql> delete from classes where id=4;
 ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`test`.`students`, CONSTRAINT `students_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `classes` (`id`))

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