从相关子表创建视图 [英] Creating View from Related Child Tables

问题描述

我具有以下常规表结构（在我的人为例子中原谅以美国为中心的汽车制造商）：

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)
)

我想在上方创建一个视图这些表，以便我可以检索所有Fords和Chevys，而视图中只有一个生成的列可以告诉我品牌。我的第一个刺是这样的：

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)

但这会令我口中难闻，我担心自己的表现。我会更好地以单独的CTE值检索所有福特和雪佛兰，然后对它们进行联合吗？我完全走错了轨道吗？我还将需要包括Model列（以及两个子表共有的其他一些列），这显然会使我的观点变成一系列巨大的case语句。处理这种情况的正确方法是什么？

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?

编辑：以为我应该添加此模式，因此请更改基础

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

推荐答案

首先，让我们尝试看看两种方法的优缺点：

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;

在联接中使用它时，第一个更好，特别是如果您不使用全部您的专栏。例如，假设您在使用 vw_Car 时拥有视图：

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;

现在，如果要进行简单选择，则：

Now, if you want to do simple select:

select Name from vw_people1;

select Name from vw_people2;

第一个简单的选择是个人 （根本不会查询 vw_Car1 ）。第二个会更复杂-将同时查询 Ford 和。
您可能会认为第一种方法更好，但是让我们尝试另一个查询：

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';

这里第二个会更快，特别是如果您在 Model 列。

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

=> sql fiddle演示 -查看这些查询的查询计划。

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

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