Why we need refresh view after change table

最新推荐文章于 2024-02-04 22:04:59 发布

转载最新推荐文章于 2024-02-04 22:04:59 发布 · 382 阅读

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#table #sql server #books #insert #null #date

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本文介绍了当基础表结构发生变化时，如何更新SQL视图以反映这些变化。通过使用`sp_refreshview`等方法，可以避免因视图元数据未同步而导致的问题。

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Problem
I recently added a column to one of my core system tables referenced by a reporting view. When I run the view, the added column is not appearing in my result set! What can I do?

Solution
When a view is created in SQL Server, metadata for the referenced table columns (column name and ordinal position) is persisted in the databas e . Any change to the referenced base table(s) (column re-ordering, new column addition, etc) will not be reflected in the view until the view is either:

Altered with an ALTER VIEW statement
Recreated with DROP VIEW/CREATE VIEW statements

Refreshed using system stored procedure sp_refreshview

The following example creates a table and an associated view

create table dbo.Customer
(
customer_id int identity(1,1) not null primary key,
firstname nvarchar(40) not null,
lastname nvarchar(40) not null,
birthdate datetime null
)
go

insert into dbo.Customer (firstname, lastname, birthdate)
select 'George', 'Washington', '1950-07-01'

insert into dbo.Customer (firstname, lastname, birthdate)
select 'James', 'Madison', '1948-11-09'

insert into dbo.Customer (firstname, lastname, birthdate)
select 'Alexander', 'Hamilton', '1970-03-02'
go

create view dbo.v_Customer
as
select * from dbo.Customer
go

Running the view using select * from dbo.v_Customer, we see the following output:

A new business requirement has now been identified; we need to add a last order date to keep track of the last time this customer ordered stock from our warehouse:

alter table dbo.Customer
add last_order_date datetime null
go
exec sp_help [dbo.Customer]
go

As we can see, the last_order_date has been added to the table. However, running our view again we get the same output as before the column change!

This occurs because the view's metadata information about the table needs to be updated; it's not maintained automatically. This can be corrected using one of the methods mentioned at the beginning of this tip. My preference is to use sp_refreshview since I don't have to hunt down the view and issue a DROP VIEW/CREATE VIEW and re-grant permissions or issue an ALTER VIEW statement:

exec sp_refreshview [dbo.v_customer]
go
select * from dbo.v_customer
go

As you can see, the view now returns all columns including the newly added one.

I find that this can always be avoided by not issuing "SELECT *" in your view definitions. By explicitly defining a column list, you're forced to change any views that reference tables that may require a change. Another approach is to issue your CREATE VIEW statements with a SCHEMABINDING option. Views defined with this option do not allow SELECT * syntax (you'll receive an error if you try) and forces you to enter an explicit column list. This way, you can prevent a less experienced developer from changing a view to use this syntax

If you're curious about how your view metadata is being stored, you can take a peek at the INFORMATION_SCHEMA.COLUMNS view (view column information is kept there as well as table column information).

Next Steps

Examine your views and consider changing SELECT * syntax to explicit column lists, if possible
Read more about sp_refreshview in greater detail in the SQL Server 2000 and 2005 Books Online
Read more about SCHEMABINDING option in greater detail in the SQL Server 2000 and 2005 Books Online
Read more about INFORMATION_SCHEMA.COLUMNS view in greater detail in the SQL Server 2000 and 2005 Books Online