這稱為觀察模式。
在此處輸入圖片說(shuō)明
以三個(gè)對(duì)象為例
Book
Title = 'Gone with the Wind'
Author = 'Margaret Mitchell'
ISBN = '978-1416548898'
Cat
Name = 'Phoebe'
Color = 'Gray'
TailLength = 9 'inch'
Beer Bottle
Volume = 500 'ml'
Color = 'Green'
這是表格的樣子：
Entity
EntityID Name Description
1 'Book' 'To read'
2 'Cat' 'Fury cat'
3 'Beer Bottle' 'To ship beer in'
。
PropertyType
PropertyTypeID Name IsTrait Description
1 'Height' 'NO' 'For anything that has height'
2 'Width' 'NO' 'For anything that has width'
3 'Volume' 'NO' 'For things that can have volume'
4 'Title' 'YES' 'Some stuff has title'
5 'Author' 'YES' 'Things can be authored'
6 'Color' 'YES' 'Color of things'
7 'ISBN' 'YES' 'Books would need this'
8 'TailLength' 'NO' 'For stuff that has long tails'
9 'Name' 'YES' 'Name of things'
。
Property
PropertyID EntityID PropertyTypeID
1 1 4 -- book, title
2 1 5 -- book, author
3 1 7 -- book, isbn
4 2 9 -- cat, name
5 2 6 -- cat, color
6 2 8 -- cat, tail length
7 3 3 -- beer bottle, volume
8 3 6 -- beer bottle, color
。
Measurement
PropertyID Unit Value
6 'inch' 9 -- cat, tail length
7 'ml' 500 -- beer bottle, volume
。
Trait
PropertyID Value
1 'Gone with the Wind' -- book, title
2 'Margaret Mitchell' -- book, author
3 '978-1416548898' -- book, isbn
4 'Phoebe' -- cat, name
5 'Gray' -- cat, color
8 'Green' -- beer bottle, color
編輯：
杰弗里提出了一個(gè)正確的觀點(diǎn)（見(jiàn)評(píng)論），所以我將擴(kuò)大答案。
該模型允許動(dòng)態(tài)（動(dòng)態(tài)）創(chuàng)建具有任何類型的屬性的任意數(shù)量的實(shí)體，而無(wú)需更改架構(gòu)。但是，這種靈活性要付出代價(jià)-與通常的桌子設(shè)計(jì)相比，存儲(chǔ)和搜索更慢，更復(fù)雜。
是一個(gè)例子了，但是首先，為了使事情變得容易，我將模型展平為一個(gè)視圖。
create view vModel as
select
e.EntityId
, x.Name as PropertyName
, m.Value as MeasurementValue
, m.Unit
, t.Value as TraitValue
from Entity as e
join Property as p on p.EntityID = p.EntityID
join PropertyType as x on x.PropertyTypeId = p.PropertyTypeId
left join Measurement as m on m.PropertyId = p.PropertyId
left join Trait as t on t.PropertyId = p.PropertyId
;
從評(píng)論中使用杰弗里的例子
with
q_00 as ( -- all books
select EntityID
from vModel
where PropertyName = 'object type'
and TraitValue = 'book'
),
q_01 as ( -- all US books
select EntityID
from vModel as a
join q_00 as b on b.EntityID = a.EntityID
where PropertyName = 'publisher country'
and TraitValue = 'US'
),
q_02 as ( -- all US books published in 2008
select EntityID
from vModel as a
join q_01 as b on b.EntityID = a.EntityID
where PropertyName = 'year published'
and MeasurementValue = 2008
),
q_03 as ( -- all US books published in 2008 not discontinued
select EntityID
from vModel as a
join q_02 as b on b.EntityID = a.EntityID
where PropertyName = 'is discontinued'
and TraitValue = 'no'
),
q_04 as ( -- all US books published in 2008 not discontinued that cost less than $50
select EntityID
from vModel as a
join q_03 as b on b.EntityID = a.EntityID
where PropertyName = 'price'
and MeasurementValue < 50
and MeasurementUnit = 'USD'
)
select
EntityID
, max(case PropertyName when 'title' than TraitValue else null end) as Title
, max(case PropertyName when 'ISBN' than TraitValue else null end) as ISBN
from vModel as a
join q_04 as b on b.EntityID = a.EntityID
group by EntityID ;
編寫(xiě)起來(lái)似乎很復(fù)雜，但是仔細(xì)檢查后，您可能會(huì)注意到CTE中的模式。
現(xiàn)在假設(shè)我們有一個(gè)標(biāo)準(zhǔn)的固定模式設(shè)計(jì)，其中每個(gè)對(duì)象屬性都有自己的列。查詢?nèi)缦滤荆?/p>
select EntityID, Title, ISBN
from vModel
WHERE ObjectType = 'book'
and PublisherCountry = 'US'
and YearPublished = 2008
and IsDiscontinued = 'no'
and Price < 50
and Currency = 'USD'
;
