In sql server we can Update data view.I think the concept of view is a read only table.
Why we can edit view in sql.is there possible in oracle?
To answer your question of why can we create an editable view, it is so that you can limit access to fields that you do not want updated (or viewed). Then you can give a user access to the view, but not to the underlying tables
For a simple example, you could have a personnel table. You could create an view allowing some users to update a field like emergency contact details, but not see or update bank details or salary
There are lots of criteria to meet to make a view updatable, and you can indeed use INSTEAD OF triggers for extended functionality http://msdn.microsoft.com/en-us/library/ms187956.aspx
I think the concept of view is a read only table
No, it's more of a virtual table - anywhere you have a real table, you ought to be able to replace it with a view, and the users should be none the wiser.
According to Codd:
Rule 6: The view updating rule:
All views that are theoretically updatable must be updatable by the system.
However, in practicality, this ideal has not been achievable.
In addition to what #JamieA wrote, views can not only limit access to fields, but also limit access to data in the table.
Look at simple SQL-Fiddle example and experiment with it.
The view in the example restrict access only to columns id,val1 of the table, but also restrics access to rows (only id = 2..10). You can update and delete only rows 2..10 throught the view.
However the view does not prevent insertion of a row with id = 20
Here is another example - a view with check option - it this case the view prevents not only deletes and updates, but prevent also inserting rows that do not match a where clause of the view.
#yogi wrote that we can't update a view if the view joins two tables -> here is a third demo that shows a simple view that joins two tables, and how an update of this view works.
These simple examples are for Oracle, but after small modifications should also work in MS-SQL (must change datatypes in create tables), since when i looked througs MSDN documentation (section: updatable views -> http://msdn.microsoft.com/en-us/library/ms187956.aspx), I didn't find any significant differences between ms-sql and oracle, it seems that views work similary on both databases.
Yes it is possible in Oracle, the other answers already explained why views are updatable and had shed some light on that question, they are also allowed in Oracle but have some restrictions/limitations here is the Oracle documentation
Like, the view select cant have: aggregate functions, distinct clause, group by... read the link for more info
Since views are read only tables and its doesn't support DML statements you can't perform update on view.
An interesting factor is there you can write update statemnt over view and write a instead of trigger for that hence you can perform multiple update statements on tables which are in the view.
According to Pinal Dev Views having following limitations
ORDER BY Does Not Work.
Adding Column is Expensive by Joining Table Outside View
Index Created on View not Used Often
SELECT * and Adding Column Issue in View
COUNT(*) Not Allowed but COUNT_BIG(*) Allowed
UNION Not Allowed but OR Allowed in Index View
Cross Database Queries Not Allowed in Indexed View
Outer Join Not Allowed in Indexed Views
SELF JOIN Not Allowed in Indexed View
Keywords View Definition Must Not Contain for Indexed View
View Over the View Not Possible with Index View
Related
I'm interested in getting the structure of each table in my DB.
Currently I'm using: DESCRIBE TABLE table1.
However, this means I have to do a separate query for each table. Was wondering whether there is a query I can get the structure of several tables at once (and therefore saving me some queries)?
Thanks,
Nir.
You can use Account Usage/Information Schema view COLUMNS
https://docs.snowflake.com/en/sql-reference/account-usage/columns.html
Following article have a slight difference example of using COLUMNS view to create a select statement but it should give you an idea
https://community.snowflake.com/s/article/Select-columns-based-on-condition-in-Snowflake-using-Information-Schema-and-Stored-Procedure
You have a couple options:
you can use the COLUMNS view in the information schema
https://docs.snowflake.com/en/sql-reference/info-schema/columns.html
Note: The view only displays objects for which the current role for the session has been granted access privileges.
you can use the COLUMNS view in the account_usage share schema:
https://docs.snowflake.com/en/sql-reference/account-usage/columns.html
Note: this will show all the columns in all tables, will show deleted objects and such as well.
Also note, there is a delay in the data (latency could be as much as 90 minutes, typically isn't though)
I know in SQL Server you can create indexes on a view, then the view saves the data from the underlying table. Then you can query the view. But, why I need to use view instead of table?
You may want to use a view to simplify on queries. In our projects, the consensus is on using views for interfaces, and especially "report interfaces".
Imagine you've got a client table, and the manager wants a report every morning with the client's name, and their account balance (or whatever). If you code your report against the table, you're creating a strong link between your report and your table, making later changes difficult.
On the other hand if your report hits a view, you can twist the database freely; as long as the view is the same the report works, the manager is happy and you're free to experiment with the database. You want to separate client metadata from the main client table? go for it, and join the two tables in the view. You want to denormalize the cart info for the client? no problem, the view can adapt...
To be honest, it's my view as a programmer but other uses will certainly be found by db gurus :)
One advantage of using an indexed view is for ordering results of 2 or more columns, where the columns are in different tables. ie, have a view which is the result of table1 and table2 sorted by table1.column1, table2.column2. You could then create an index on column1, column2 to optimise that query
A table is where the data is physically stored.
A view is where tables are summarized or grouped to make groups of tables easier to use.
An indexed view allows a query to use a view, and not need to get data from the underlying table, as the view already has the data, thus increasing performance.
You could not achieve the same result with just tables, without denormalizing your database, and thus potentially creating other issues.
Basically, use a view:
When you use the same complex query on many tables, multiple times.
When new system need to read old table data, but doesn't watch to change their perceived schema.
Indexed Views can improve performance by creating more specific index without increasing redundancy.
A view is simply a SELECT statement that has been given a name and stored in a database. The main advantage of a view is that once it's created, it acts like a table for any other SELECT statements that you want to write.
The select statement for the view can reference tables, other views and functions.
You can create an index on the view (indexed view) to improve performance. An indexed view is self-updating, immediately reflecting changes to the underlying tables.
If your indexed view only selects columns from one table, you could just as well place the index on that table and query that table directly, the view would only cause overhead for your database. However, if your SELECT statement covers multiple tables with joins etc. than you could gain a performance boost by placing an index on the view.
I have a SQL Server as backend and use ms access as frontend.
I have two tables (persons and managers), manager is derived from persons (a 1:1 relation), thus i created a view managersFull which is basically a:
SELECT *
FROM `managers` `m`
INNER JOIN `persons` `p`
ON `m`.`id` = `p`.`id`
id in persons is autoincrementing and the primary key, id in managers is the primary key and a foreign key, referencing persons.id
now i want to be able to insert a new dataset with a form in ms access, but i can’t get it to work. no error message, no status line, nothing. the new rows aren’t inserted, and i have to press escape to cancel my changes to get back to design view in ms access.
i’m talking about a managers form and i want to be able to enter manager AND person information at the same time in a single form
my question is now: is it possible what i want to do here? if not, is there a “simple” workaround using after insert triggers or some lines of vba code?
thanks in advance
The problem is that your view is across several tables. If you access multiple tables you could update or insert in only one of them.
Please also check the MSDN for more detailed information on restrictions and on proper strategies for view updates
Assuming ODBC, some things to consider:
make sure you have a timestamp field in the person table, and that it is returned in your managers view. You also probably need the real PK of the person table in the manager view (I'm assuming your view takes the FK used for the self-join and aliases it as the ID field -- I wouldn't do that myself, as it is confusing. Instead, I'd use the real foreign key name in the managers view, and let the PK stand on its own with its real name).
try the Jet/ACE-specific DISTINCTROW predicate in your recordsource. With Jet/ACE back ends, this often makes it possible to insert into both tables when it's otherwise impossible. I don't know for certain if Jet will be smart enough to tell SQL Server to do the right thing, though.
if neither of those things works, change your form to use a recordsource based on your person table, and use a combo box based on the managers view as the control with which you edit the record to relate the person to a manager.
Ilya Kochetov pointed out that you can only update one table, but the work-around would be to apply the updates to the fields on one table and then the other. This solution assumes that the only access you have to these two tables is through this view and that you are not allowed to create a stored procedure to take care of this.
To model and maintain two related tables in access you don’t use a query or view that is a join of both tables. What you do is use a main form, and drop in a sub-form that is based on the child table. If the link master and child setting in the sub-form is set correctly, then you not need to write any code and access will insert the person’s id in the link field.
So, don’t use a joined table here. Simply use a form + sub-form setup and you be able to edit and maintain the data and the data in the related child table.
This means you base the form on the table, and not a view. And you base the sub-form on the child table. So, don't use a view here.
Huge database in mssql2005 with big codebase depending on the structure of this database.
I have about 10 similar tables they all contain either the file name or the full path to the file. The full path is always dependent on the item id so it doesn't make sense to store it in the database. Getting useful data out of these tables goes a little like this:
SELECT a.item_id
, a.filename
FROM (
SELECT id_item AS item_id
, path AS filename
FROM xMedia
UNION ALL
-- media_path has a different collation
SELECT item_id AS item_id
, (media_path COLLATE SQL_Latin1_General_CP1_CI_AS) AS filename
FROM yMedia
UNION ALL
-- fullPath contains more than just the filename
SELECT itemId AS item_id
, RIGHT(fullPath, CHARINDEX('/', REVERSE(fullPath))-1) AS filename
FROM zMedia
-- real database has over 10 of these tables
) a
I'd like to create a single view of all these tables so that new code using this data-disaster doesn't need to know about all the different media tables. I'd also like use this view for insert and update statements. Obviously old code would still rely on the tables to be up to date.
After reading the msdn page about creating views in mssql2005 I don't think a view with SCHEMABINDING would be enough.
How would I create such an updateable view?
Is this the right way to go?
Scroll down on the page you linked and you'll see a paragraph about updatable views. You can not update a view based on unions, amongst other limitations. The logic behind this is probably simple, how should Sql Server decide on what source table/view should receive the update/insert?
You can modify partitioned views, provided they satisfy certain conditions.
These conditions include having a partitioning column as a part of the primary key on each table, and having a set on non-overlapping check constraints for the partitioning column.
This seems to be not your case.
In your case, you may do either of the following:
Recreate you tables as views (with computed columns) for your legacy soft to work, and refer to the whole table from the new soft
Use INSTEAD OF triggers to update the tables.
If a view is based on multiple base tables, UPDATE statement on the view may or may not work depending on the UPDATE statement. If the UPDATE statement affects multiple base tables, SQL server throws an error. Whereas, if the UPDATE affects only one base table in the view then the UPDATE will work (Not correctly always). The insert and delete statements will always fail.
INSTEAD OF Triggers, are used to correctly UPDATE, INSERT and DELETE from a view that is based on multiple base tables. The following links has examples along with a video tutorial on the same.
INSTEAD OF INSERT Trigger
INSTEAD OF UPDATE Trigger
INSTEAD OF DELETE Trigger
What is a view in Oracle?
A View in Oracle and in other database systems is simply the representation of a SQL statement that is stored in memory so that it can easily be re-used. For example, if we frequently issue the following query
SELECT customerid, customername FROM customers WHERE countryid='US';
To create a view use the CREATE VIEW command as seen in this example
CREATE VIEW view_uscustomers
AS
SELECT customerid, customername FROM customers WHERE countryid='US';
This command creates a new view called view_uscustomers. Note that this command does not result in anything being actually stored in the database at all except for a data dictionary entry that defines this view. This means that every time you query this view, Oracle has to go out and execute the view and query the database data. We can query the view like this:
SELECT * FROM view_uscustomers WHERE customerid BETWEEN 100 AND 200;
And Oracle will transform the query into this:
SELECT *
FROM (select customerid, customername from customers WHERE countryid='US')
WHERE customerid BETWEEN 100 AND 200
Benefits of using Views
Commonality of code being used. Since a view is based on one common set of SQL, this means that when it is called it’s less likely to require parsing.
Security. Views have long been used to hide the tables that actually contain the data you are querying. Also, views can be used to restrict the columns that a given user has access to.
Predicate pushing
You can find advanced topics in this article about "How to Create and Manage Views in Oracle."
If you like the idea of Views, but are worried about performance you can get Oracle to create a cached table representing the view which oracle keeps up to date.
See materialized views
regular view----->short name for a query,no additional space is used here
Materialised view---->similar to creating table whose data will refresh periodically based on data query used for creating the view
A view is a virtual table, which provides access to a subset of column from one or more table. A view can derive its data from one or more table. An output of query can be stored as a view. View act like small a table but it does not physically take any space. View is good way to present data in particular users from accessing the table directly. A view in oracle is nothing but a stored sql scripts. Views itself contain no data.
A view is simply any SELECT query that has been given a name and saved in the database. For this reason, a view is sometimes called a named query or a stored query. To create a view, you use the SQL syntax:
CREATE OR REPLACE VIEW <view_name> AS
SELECT <any valid select query>;