[英]Creating View from Related Child Tables
我具有以下常規表結構(在我的作過的示例中原諒了以美國為中心的汽車制造商):
CREATE TABLE Car (
[Id] int PRIMARY KEY
)
CREATE TABLE Ford (
[FordId] int PRIMARY KEY, --also a foreign key on Car
[Model] nvarchar(max)
)
CREATE TABLE Chevy (
[ChevyId] int PRIMARY KEY, --also a foreign key on Car
[Model] nvarchar(max)
)
我想在這些表的頂部創建一個視圖,以便可以檢索所有Fords和Chevys,並在視圖中僅包含一個生成的列來告訴我。 我的第一個刺是:
SELECT
c.CarId,
case when f.FordId is not null then 'Ford' else 'Chevy' end
FROM Car as c
LEFT JOIN Ford as f on c.Id = f.FordId
LEFT JOIN Chevy as ch on c.Id = ch.ChevyId
WHERE (f.FordId is not null or ch.ChevyId is not null)
但這會使我口中難聞,我擔心表現。 我會更好地以單獨的CTE值檢索所有的福特和雪佛蘭,然后對它們進行聯合嗎? 我完全走錯了軌道嗎? 我還將需要包括Model列(以及兩個子表共有的其他一些列),這顯然會使我的觀點變成一系列case語句。 處理這種情況的“正確”方法是什么?
編輯 :以為我應該補充一點,該架構已經存在,因此無法更改基礎表。
首先,讓我們嘗試看看兩種方法各自的優缺點:
create view vw_Car1
as
SELECT
c.Id,
case when f.FordId is not null then 'Ford' else 'Chevy' end as Maker,
coalesce(f.Model, ch.Model) as Model
FROM Car as c
LEFT JOIN Ford as f on c.Id = f.FordId
LEFT JOIN Chevy as ch on c.Id = ch.ChevyId
WHERE (f.FordId is not null or ch.ChevyId is not null);
create view vw_Car2
as
select FordId as id, 'Ford' as Maker, Model from Ford
union all
select ChevyId as id, 'Chevy' as Maker, Model from Chevy;
當在聯接中使用它時,第一個更好,特別是如果您不使用所有列。 例如,假設您在使用vw_Car
時有一個視圖:
create table people (name nvarchar(128), Carid int);
insert into people
select 'John', 1 union all
select 'Paul', 2;
create view vw_people1
as
select
p.Name, c.Maker, c.Model
from people as p
left outer join vw_Car1 as c on c.ID = p.CarID;
create view vw_people2
as
select
p.Name, c.Maker, c.Model
from people as p
left outer join vw_Car2 as c on c.ID = p.CarID;
現在,如果要簡單選擇:
select Name from vw_people1;
select Name from vw_people2;
第一個方法是簡單地從people
進行選擇( vw_Car1
不會查詢vw_Car1
)。 第二個將更復雜- Ford
和Chevy
將被同時查詢。 您可能認為第一種方法更好,但讓我們嘗試另一個查詢:
select *
from vw_people1
where Maker = 'Ford' and Model = 'Fiesta';
select *
from vw_people2
where Maker = 'Ford' and Model = 'Fiesta';
這里第二個會更快,特別是如果您在“ Model
列上有索引。
=> sql fiddle演示 -請參閱這些查詢的查詢計划。
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