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HomeDatabaseMysql TutorialMySQL入门教程(五)之表的创建、修改和删除_MySQL

MySQL 为关系型数据库(Relational Database Management System), 这种所谓的"关系型"可以理解为"表格"的概念, 一个关系型数据库由一个或数个表格组成。

表是DB存储数据的基本单位,一个表包含若干个字段或记录。表的操作包括创建、修改和删除。

  1、创建表

  创建表即在已存在的数据库中创立新表。在使用USE语句选择具体数据库后,可以使用它SQL语句创建表

  其中,属性名指表中字段的名称,数据类型即对应字段的数据类型,完整性约束条件指对应字段的某些特殊约束条件。

  MySQL中基本的完整性约束条件有

  1.1 设置表的主键

  主键是表的一个特殊字段,该字段能唯一地表示该表中的每条信息,即主键如同表中记录的身份证。主键的主要目的在于帮助MySQL以最快的速度查找表中的某一条信息。主键必须按满足唯一性,表中任意提两条记录的主键字段的值不可相同,主键的值为非空值。主键可以是单一的字段也可以是多个字段的组合。

  对于单字段主键只要在创建表时使用下属语句即可

  属性名 数据类型 PRIMARY KEY

  对于多字段主键,要在属性定义完后统一设置主键,如

  即用stu_id和course_id两个字段组合来唯一确定一条记录。

  1.2 设置表的外键

  如果字段a是表A的属性,且依赖于表B的主键,那么,称表B为父表,表A为子表,a为表A的外键。通过字段a将父表B和子表A建立了关联关系,即外键的作用在于建立该表与其父表的关联关系。当父表中删除某条信息时,子表中与之对应的信息也必须有相应的改变,来保证信息的完整性。

  在创建表时的设置外键的基本语法为

  其中,外键别名指外键的代号,属性一列表指子表中设置的外键,属性二列表指父表的主键,而表名指父表的名称。如


 注意,子表的外键关联的必须是父表的主键,且数据类型必须一致。

  1.3 设置表的非空约束

  非空性指字段的值不能为空值NULL。基本语法为

  属性名 数据类型 NOT NULL

  当在有非空约束的字段上插入的记录对应该字段的值为空时,系统将保存并不接受此次插入。

  1.4 设置表的唯一性约束

  唯一性是指所有记录中该字段的值不可重复出现。基本语法为

  属性名 数据类型 UNIQUE

  1.5 设置表的属性值自动增加

  AUTO_INCREMENT是MySQL数据库中的一个特殊约束条件,主要用于为表中插入的新纪录自动生成唯一的ID。基本语法为

  属性名 数据类型 AUTO_INCREMENT

  一个表只能有一个字段使用该约束,且该字段必须为主键的一部分,该字段可以是任何整数类型。默认情况下,该字段的值从1开始自增。如果第一条记录设置了该字段的初值,则后续新增加的记录就从初值开始自增。

  1.6 设置表的属性的默认值

  对于设定了默认值的字段,在新插入记录时如果没有为这个字段赋值,则系统自动为其赋予此默认值。基本语法为

  属性名 数据类型 DEFAULT 默认值

  2、查看表结构

  查看表结构指查看DB中已存在的表的定义,包括DESCRIBE语句和SHOW CREATE TABLE语句。

  2.1 DESCRIBE语句

  可查看表的基本定义,包括字段名、字段数据类型、是否为主外键、默认值和额外信息。

  基本语法为

  DESCRIBE 表名;

  2.2 SHOW CREATE TABLE语句

  可查看表的详细定义,包括字段名、字段数据类型、完整性约束条件等定义时的信息,以及默认的存储引擎和字符编码。同样可用\G结尾代替;使得显示结果更美观。

  3、修改表

  修改表是指修改数据库中已存在的表的定义。

  通过Alter Table语句来修改表,包括修改表名、修改字段数据类型、修改字段名、增加字段、删除字段、修改字段的排列位置、更改默认存储引擎和删除表的外键约束等。

  3.1 修改表名

  语法为

  ALTER TABLE 旧表名 RENAME [TO] 新表名;

  其中To是可选参数,其是否在语句中出现不会影响语句的执行。

  3.2 修改字段名

  基本语法为

  ALTER TABLE 表名 Change 旧属性名 新属性名 新数据类型;

  如果只修改字段名不修改字段数据类型,则保持新数据类型与原来一致即可。

  3.3 修改字段的数据类型

  基本语法为

  ALTER TABLE 表名 MODIFY 属性名 新数据类型;

  3.4 增加字段

  基本语法为

  ALTER TABLE 表名 ADD 属性名1 数据类型 [完整性约束条件] [FIRST | AFTER 属性名2];

  其中FIRST为可选参数,用于将新增字段设置为表的第一个字段,AFTER属性名2也为可选参数,用户将新增字段放到属性名2后面,如果这两个可选参数都不选,新增字段将默认放到表的最后一个字段。

  实际上,对于一个数据表而言,其中字段的排列顺序对表不会有什么影响,只是对于创建的人来说,将有某种直接或间接联系的字段放在一起,便于理解这个表的结构。

  3.5 增加字段

  基本语法为

  ALTER TABLE 表名 DROP 属性名;

  3.6 修改字段的排列位置

  基本语法为

  ALTER TABLE 表名 MODIFY 属性名1 数据类型 FIRST | AFTER 属性名2;

  3.7 更改表的存储引擎

  基本语法为

  ALTER TABLE 表名 ENGINE=新存储引擎名;

  如果一个表中已经存在了很多数据,最好不要轻易更改其存储引擎。

  3.8 删除表的外键约束

  即断开表与其父表之间的关联关系基本语法为

  ALTER TABLE 表名 DROP FOREIGN KEY 外键别名;

  4、删除表

  删除表会删除表中的所有数据,由于创建表时可能存在外键约束,一些表成为了与之关联的表的父表,要删除这些父表,情况比较复杂。

  4.1 删除没有被关联的普通表

  基本语法为

  DROP TABLE 表名;

  4.2 删除被关联的父表

  此时由于有外键依赖于该表,用4.1的删除语法会报错。

  删除此类父表一般有两种方法:直接先将与之关联的子表删除再删父表,或者先删除子表的外键约束再删父表,一般我们采取第二种方法。

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