Creating View from Related Child Tables

问题

I have the following general table structure (forgive the United States-centric carmakers in my contrived example):

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)
)

I am wanting to create a view on top of these tables so that I can retrieve all Fords and Chevys and just have a generated column in the view that tells me the make. My first stab was this:

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)

But that leaves a bad taste in my mouth and I am concerned about performance. Would I be better off retrieving all the Fords and Chevys in separate CTE values and just performing a union on them? Am I on the wrong track entirely? I will also be needing to include the Model column (as well as a few other columns common to the two child tables), which would obviously make my view turn into a giant series of case statements. What is the "proper" way to handle such a situation?

EDIT: Thought I should add that this schema already exists so changing the underlying tables is not possible.

回答1:

First of all, let's try to see pros and cons of each of 2 approaches:

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;

The first one is better when you use it in joins, especially if you'll not using all of your columns. For example, let's say you have a view when you're using your vw_Car:

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;

Now, if you want to do simple select:

select Name from vw_people1;

select Name from vw_people2;

First one would be simple select from people (vw_Car1 will not be queried at all). Second one will be more complex - Ford and Chevy will be both queried. You could think that first approach is better, but let's try another query:

select *
from vw_people1
where Maker = 'Ford' and Model = 'Fiesta';

select *
from vw_people2
where Maker = 'Ford' and Model = 'Fiesta';

Here second one will be faster, especially if you have index on Model column.

=> sql fiddle demo - see query plans of these queries.

来源：https://stackoverflow.com/questions/18662247/creating-view-from-related-child-tables