>“员工”总是描述人.
>有些客户是人.
>有些客户是企业
(组织).
>“供应商”通常(总是？)
组织.
>员工也可以是客户.
>供应商也可以是客户.

使用单独的员工电话号码,供应商电话号码和客户电话号码表存在严重问题.

>员工可以成为客户.如果是员工
电话号码变更,做客户
电话号码还需要更新吗？你怎么知道更新哪一个？
>供应商可以是客户.如果一个
供应商的电话号码更改,是否为客户
电话号码还需要更新吗？你怎么知道更新哪一个？
>您必须复制并维护没有错误的约束
对于每个表中的电话号码
存储电话号码.
>当a时出现同样的问题
客户的电话号码更改.现在
你必须检查是否
员工和供应商电话号码
还需要更新.
>回答“谁的电话
号码是123-456-7890？“,你必须
看看’n’个不同的表,在哪里
‘n’是不同的数量
您处理的各方“种类”.在
除了员工,客户和
供应商,认为“承包商
手机“,”潜在客户的电话“等

您需要实现超类型/子类型架构. (Postgresql代码,未经过严格测试.)

create table parties (
    party_id integer not null unique,party_type char(1) check (party_type in ('I','O')),party_name varchar(10) not null unique,primary key (party_id,party_type)
);

insert into parties values (1,'I','Mike');
insert into parties values (2,'Sherry');
insert into parties values (3,'O','Vandelay');

-- For "persons",a subtype of "parties"
create table person_st (
    party_id integer not null unique,party_type char(1) not null default 'I' check (party_type = 'I'),height_inches integer not null check (height_inches between 24 and 108),primary key (party_id),foreign key (party_id,party_type) references parties (party_id,party_type) on delete cascade
);

insert into person_st values (1,72);
insert into person_st values (2,60);

-- For "organizations",a subtype of "parties"
create table organization_st (
    party_id integer not null unique,party_type CHAR(1) not null default 'O' check (party_type = 'O'),ein CHAR(10),-- In US,federal Employer Identification Number
    primary key (party_id),party_type) on delete cascade
);

insert into organization_st values (3,'00-0000000');

create table phones (
    party_id integer references parties (party_id) on delete cascade,-- Whatever you prefer to distinguish one kind of phone usage from another.
    -- I'll just use a simple 'phone_type' here,for work,home,emergency,-- business,and mobile.
    phone_type char(1) not null default 'w' check 
        (phone_type in ('w','h','e','b','m')),-- Phone numbers in the USA are 10 chars. YMMV.
    phone_number char(10) not null check (phone_number ~ '[0-9]{10}'),phone_type)
);

insert into phones values (1,'0000000000');
insert into phones values (1,'m','0000000001');
insert into phones values (3,'0000000002');

-- Do what you need to do on your platform--triggers,rules,whatever--to make 
-- these views updatable. Client code uses the views,not the base tables.
-- In current versions of Postgresql,I think you'd create some "instead
-- of" rules.
--
create view people as
select t1.party_id,t1.party_name,t2.height_inches
from parties t1
inner join person_st t2 on (t1.party_id = t2.party_id);

create view organizations as 
select t1.party_id,t2.ein
from parties t1
inner join organization_st t2 on (t1.party_id = t2.party_id);

create view phone_book as
select t1.party_id,t2.phone_type,t2.phone_number
from parties t1
inner join phones t2 on (t1.party_id = t2.party_id);

为了进一步扩展这一点,实现“工作人员”的表需要引用人员子类型,而不是派对超类型.组织不能是员工.

create table staff (
    party_id integer primary key references person_st (party_id) on delete cascade,employee_number char(10) not null unique,first_hire_date date not null default CURRENT_DATE
);

如果供应商只能是组织而不是个人,那么实施供应商的表格将以类似的方式引用组织子类型.

对于大多数公司而言,客户可以是个人或组织,因此实现客户的表应该引用超类型.

create table customers (
    party_id integer primary key references parties (party_id) on delete cascade
    -- Other attributes of customers
);