Difference between views and SELECT queries - sql

Question

If views are used to show selected columns to the user, and same can be done by using
SELECT col1, col2
FROM xyz
, what is the point of using views?

Answer 1

Using a view saves you copying and pasting your queries and adds code reusability, so you can change a single view instead of 10 queries in the different places of your code.
Different permissions can be granted on views and tables, so that you can show only a portion of data to a user
A view can be materialized, which means caching the results of the underlying query

Answer 2

As Quassnoi said, it's useful for granting permission to certain rows in a table.
For example, let's say a lecturer at a university needs access to information on their students. The lecturer shouldn't have access to the "students" table because they could look up or modify information for any student in the whole university. The database admin makes a view that only shows students from the lecturers classes, and gives the lecturer the appropriate permissions for the view. Now the lecturer has access to their own students' data but not the whole "students" table.

Answer 3

A view can be described as a virtual table, created from a SQL query stored in the database.
Therefore, the following are aspects to consider in using VIEWS
Performance: it can improve data access performance as queries involving several tables generate transactions already exchanged to generate the view.
Simplicity: most of the views I work with are data arrangements of columns from 4+ tables, a bunch of inner joins. Once the view is created, your application developers will have to deal with the SELECT statements using column in the same view, hence the term virtual table.
Security: or just called it access control. Most relational database management system allow properties in the view object that control the type of access. For instance, one can allow users to update a view but only the DBA can make modifications to the tables that compose the view.

Answer 4

A view can be more complicated than just showing certain columns. It is a stored query. Wikipedia has much more detail.

Answer 5

Views make SQL easier to write (and read).
You can also use views to control access permissions.

Answer 6

Views:
1. View will not store any data
2. Used for Security purpose
3. When the base table is dropped, then the view is no longer accessible
4. One can perform DML operations directly on the view
Materialized view:
1. Materialized view does not store data
2. It is used for better performance
3. When the base table is dropped, a materialized view is still accessible
4. One cannot perform DML operations on materialized view.
Reference: https://www.youtube.com/watch?v=8ySsyZlixuE

Answer 7

All answers above provide an excellent explanation for the difference between a view and a query.
The query in the question is simple to an extreme degree, and creating a view for it might be overkill.
However, most queries are more complex, for example:
;with Orders2016 as (
select Customers.CustomerID
, Customers.CompanyName
, TotalOrderAmount = sum(OD.Quantity * OD.UnitPrice)
from Customers
join Orders O on Customers.CustomerID = O.CustomerID
join OrderDetails OD on O.OrderID = OD.OrderID
where OrderDate >= '2016-01-01'
and OrderDate < '2017-01-01'
group by Customers.CustomerID, Customers.CompanyName
)
, CustomersGroups as
(
select CustomerID
, CompanyName
, TotalOrderAmount
, CustomerGroup =
(
case
when
TotalOrderAmount >= 0 and TotalOrderAmount < 1000
then 'low'
when TotalOrderAmount >= 1000 and TotalOrderAmount < 5000
then 'Medium'
when TotalOrderAmount >= 5000 and TotalOrderAmount < 10000
then 'High'
when TotalOrderAmount >= 10000 then 'VeryHigh'
end
)
from Orders2016
)
select CustomerGroup
, TotalInGroup = Count(*)
, PercentageInGroup = Count(*) * 1.0 / (select count(*) from CustomersGroups)
from CustomersGroups
group by CustomerGroup
order by TotalInGroup desc;
Imagine rewriting it each time you want to access data. (Or even searching through files, copy & paste). Poor time management.
Views also save us tons of time and are a way to be DRY.

Answer 8

Views are very useful for access permissions. There are multiple other advantages (as stated in those links provided above), but for me the main advantage is the reusability. as Quassnoi writes, you can have a single point where you can edit your query, instead of editing a list of methods.
Perfect!