Related
Is there a way to setup something like a <SelectInput> filter on a column of the list to get only distinct values of this column ?
Something like the <ReferenceInput> but on the same table and with unique values ...
No, but for good reason. Say you have data with billions of distinct records. You don't want your frontend determining what is unique. Instead you want an API that can support that data specifically, and hopefully quickly.
So long story short, you'll need an API for that.
Along the lines of what Shawn K says, perhaps create a View on your backend that represents the state of what is currently 'distinct', acknowledging that it might be stale/non-realtime. Then you could use the contents of that View to represent the choices available to the user. If generating the distinct set of values is non-performant, then if you're in a DB like Postgres (et al), create a Materialized View, refreshed on a timer.
The binding of the view data to the becomes the trick at that point, but there are probably clues to doing that here of SO and you could piece these two together.
BTW, I use Views regularly to handle edge certain edge cases like this. Beats caching data in a middle tier for sure.
I know this might probably sound like a stupid question, but please bear with me.
In SQL-server we have
SELECT TOP N ...
now in that we can get the first n rows in ascending order (by default), cool. If we want records to be sorted on any other column, we just specify that in the order by clause, something like this...
SELECT TOP N ... ORDER BY [ColumnName]
Even more cool. But what if I want the last row? I just write something like this...
SELECT TOP N ... ORDER BY [ColumnName] DESC
But there is a slight concern with that. I said concern and not issue because it isn't actually an issue. By this way, I could get the last row based on that column, but what if I want the last row that was inserted. I know about SCOPE_IDENTITY, IDENT_CURRENT and ##IDENTITY, but consider a heap (a table without a clustered index) without any identity column, and multiple accesses from many places (please don't go into this too much as to how and when these multiple operation are happening, this doesn't concern the main thing). So in this case there doesn't seems to be an easy way to find which row was actually inserted last. Some might answer this as
If you do a select * from [table] the last row shown in the sql result window will be the last one inserted.
To anything thinking about this, this is not actually the case, at least not always and one that you can always rely on (msdn, please read the Advanced Scanning section).
So the question boils down to this, as in the title itself. Why doesn't SQL Server have a
SELECT LAST
or say
SELECT BOTTOM
or something like that, where we don't have to specify the Order By and then it would give the last record inserted in the table at the time of executing the query (again I am not going into details about how would this result in case of uncommitted reads or phantom reads).
But if still, someone argues that we can't talk about this without talking about these read levels, then, for them, we could make it behave as the same way as TOP work but just the opposite. But if your argument is then we don't need it as we can always do
SELECT TOP N ... ORDER BY [ColumnName] DESC
then I really don't know what to say here. I know we can do that, but are there any relation based reason, or some semantics based reason, or some other reason due to which we don't have or can't have this SELECT LAST/BOTTOM. I am not looking for way to does Order By, I am looking for reason as to why do don't have it or can't have it.
Extra
I don't know much about how NOSQL works, but I've worked (just a little bit) with mongodb and elastic search, and there too doesn't seems to be anything like this. Is the reason they don't have it is because no one ever had it before, or is it for some reason not plausible?
UPDATE
I don't need to know that I need to specify order by descending or not. Please read the question and understand my concern before answering or commenting. I know how will I get the last row. That's not even the question, the main question boils down to why no select last/bottom like it's counterpart.
UPDATE 2
After the answers from Vladimir and Pieter, I just wanted to update that I know the the order is not guaranteed if I do a SELECT TOP without ORDER BY. I know from what I wrote earlier in the question might make an impression that I don't know that's the case, but if you just look a further down, I have given a link to msdn and have mentioned that the SELECT TOP without ORDER BY doesn't guarantees any ordering. So please don't add this to your answer that my statement in wrong, as I have already clarified that myself after a couple of lines (where I provided the link to msdn).
You can think of it like this.
SELECT TOP N without ORDER BY returns some N rows, neither first, nor last, just some. Which rows it returns is not defined. You can run the same statement 10 times and get 10 different sets of rows each time.
So, if the server had a syntax SELECT LAST N, then result of this statement without ORDER BY would again be undefined, which is exactly what you get with existing SELECT TOP N without ORDER BY.
You have stressed in your question that you know and understand what I've written below, but I'll still keep it to make it clear for everyone reading this later.
Your first phrase in the question
In SQL-server we have SELECT TOP N ... now in that we can get the
first n rows in ascending order (by default), cool.
is not correct. With SELECT TOP N without ORDER BY you get N "random" rows. Well, not really random, the server doesn't jump randomly from row to row on purpose. It chooses some deterministic way to scan through the table, but there could be many different ways to scan the table and server is free to change the chosen path when it wants. This is what is meant by "undefined".
The server doesn't track the order in which rows were inserted into the table, so again your assumption that results of SELECT TOP N without ORDER BY are determined by the order in which rows were inserted in the table is not correct.
So, the answer to your final question
why no select last/bottom like it's counterpart.
is:
without ORDER BY results of SELECT LAST N would be exactly the same as results of SELECT TOP N - undefined.
with ORDER BY result of SELECT LAST N ... ORDER BY X ASC is exactly the same as result of SELECT TOP N ... ORDER BY X DESC.
So, there is no point to have two key words that do the same thing.
There is a good point in the Pieter's answer: the word TOP is somewhat misleading. It really means LIMIT result set to some number of rows.
By the way, since SQL Server 2012 they added support for ANSI standard OFFSET:
OFFSET { integer_constant | offset_row_count_expression } { ROW | ROWS }
[
FETCH { FIRST | NEXT } {integer_constant | fetch_row_count_expression } { ROW | ROWS } ONLY
]
Here adding another key word was justified that it is ANSI standard AND it adds important functionality - pagination, which didn't exist before.
I would like to thank #Razort4x here for providing a very good link to MSDN in his question. The "Advanced Scanning" section there has an excellent example of mechanism called "merry-go-round scanning", which demonstrates why the order of the results returned from a SELECT statement cannot be guaranteed without an ORDER BY clause.
This concept is often misunderstood and I've seen many question here on SO that would greatly benefit if they had a quote from that link.
The answer to your question
Why doesn't SQL Server have a SELECT LAST or say SELECT BOTTOM or
something like that, where we don't have to specify the ORDER BY and
then it would give the last record inserted in the table at the time
of executing the query (again I am not going into details about how
would this result in case of uncommitted reads or phantom reads).
is:
The devil is in the details that you want to omit. To know which record was the "last inserted in the table at the time of executing the query" (and to know this in a somewhat consistent/non-random manner) the server would need to keep track of this information somehow. Even if it is possible in all scenarios of multiple simultaneously running transactions, it is most likely costly from the performance point of view. Not every SELECT would request this information (in fact very few or none at all), but the overhead of tracking this information would always be there.
So, you can think of it like this: by default the server doesn't do anything specific to know/keep track of the order in which the rows were inserted, because it affects performance, but if you need to know that you can use, for example, IDENTITY column. Microsoft could have designed the server engine in such a way that it required an IDENTITY column in every table, but they made it optional, which is good in my opinion. I know better than the server which of my tables need IDENTITY column and which do not.
Summary
I'd like to summarise that you can look at SELECT LAST without ORDER BY in two different ways.
1) When you expect SELECT LAST to behave in line with existing SELECT TOP. In this case result is undefined for both LAST and TOP, i.e. result is effectively the same. In this case it boils down to (not) having another keyword. Language developers (T-SQL language in this case) are always reluctant to add keywords, unless there are good reasons for it. In this case it is clearly avoidable.
2) When you expect SELECT LAST to behave as SELECT LAST INSERTED ROW. Which should, by the way, extend the same expectations to SELECT TOP to behave as SELECT FIRST INSERTED ROW or add new keywords LAST_INSERTED, FIRST_INSERTED to keep existing keyword TOP intact. In this case it boils down to the performance and added overhead of such behaviour. At the moment the server allows you to avoid this performance penalty if you don't need this information. If you do need it IDENTITY is a pretty good solution if you use it carefully.
There is no select last because there is no need for it. Consider a "select top 1 * from table" . Top 1 would get you the first row that is returned. And then the process stops.
But there is no guarantees about ordering if you don't specify an order by. So it may as well be any row in the dataset you get back.
Now do a "select last 1 * from table". Now the database will have to process all the rows in order to get you the last one.
And because ordering is non-deterministic, it may as well be the same result as from the select "top 1".
See now where the problem comes? Without an order by top and last are actually the same, only "last" will take more time. And with an order by, there's really only a need for top.
SELECT TOP N ...
now in that we can get the first n rows in ascending order (by
default), cool. If we want records to be sorted on any other column,
we just specify that in the order by clause, something like this...
What you say here is totally wrong and absolutely NOT how it works. There is no guarantee on what order you get. Ascending order on what ?
create table mytest(id int, id2 int)
insert into mytest(id,id2)values(1,5),(2,4),(3,3),(4,2),(5,1)
select top 1 * from mytest
select * from mytest
create clustered index myindex on mytest(id2)
select top 1 * from mytest
select * from mytest
insert into mytest(id,id2)values(6,0)
select top 1 * from mytest
Try this code line by line and see what you get with the last "select top 1".....you get in this case the last inserted record.
update
I think you understand that "select top 1 * from table" basically means: "Select a random row from the table".
So what would last mean? "Select the last random row from the table?" Wouldn't the last random row from a table be conceptually the same as saying any 1 random row from the table? And if that's true, top and last are the same, so there is no need for last.
Update 2
In hindsight I was happier with the syntax mysql uses : LIMIT.
Top doesn't say anything about ordering it is only there to specify the number of rows to be returned.
Limits the rows returned in a query result set to a specified number of rows or percentage of rows in SQL Server 2014.
The reasons why SELECT LAST_INSERTED does not make sense.
It cannot be easily applied to non-heap tables.
Heap data can be freely moved by DBMS so those "natural" order is subject to change. To keep it the system needs some additional mechanism which seems to be a useless waste.
If really desired it can be simulated by adding some 'auto-increment' column.
SQL Server ordering is arbitrary unless otherwise stated. It's set based, therefore you must define what your set is. Correct SCOPE_IDENTITY() is the correct way to capture the last inserted record, or the OUTPUT clause. Why would you do inserts on a heap that you need to reference chronologically anyway?? That's super bad database design.
Browsing through the more dubious parts of the web, I happened to come across this particular SQL injection:
http://server/path/page.php?id=1+union+select+0,1,concat_ws(user(),0x3a,database(),0x3a,version()),3,4,5,6--
My knowledge of SQL - which I thought was half decent - seems very limiting as I read this.
Since I develop extensively for the web, I was curious to see what this code actually does and more importantly how it works.
It replaces an improperly written parametrized query like this:
$sql = '
SELECT *
FROM products
WHERE id = ' . $_GET['id'];
with this query:
SELECT *
FROM products
WHERE id = 1
UNION ALL
select 0,1,concat_ws(user(),0x3A,database(),0x3A,version()),3,4,5,6
, which gives you information about the database name, version and username connected.
The injection result relies on some assumptions about the underlying query syntax.
What is being assumed here is that there is a query somewhere in the code which will take the "id" parameter and substitute it directly into the query, without bothering to sanitize it.
It's assuming a naive query syntax of something like:
select * from records where id = {id param}
What this does is result in a substituted query (in your above example) of:
select * from records where id = 1 union select 0, 1 , concat_ws(user(),0x3a,database(),0x3a,version()), 3, 4, 5, 6 --
Now, what this does that is useful is that it manages to grab not only the record that the program was interested in, but also it UNIONs it with a bogus dataset that tells the attacker (these values appear separated by colons in the third column):
the username with which we are
connected to the database
the name of the database
the version of the db software
You could get the same information by simply running:
select concat_ws(user(),0x3a,database(),0x3a,version())
Directly at a sql prompt, and you'll get something like:
joe:production_db:mysql v. whatever
Additionally, since UNION does an implicit sort, and the first column in the bogus data set starts with a 0, chances are pretty good that your bogus result will be at the top of the list. This is important because the program is probably only using the first result, or there is an additional little bit of SQL in the basic expression I gave you above that limits the result set to one record.
The reason that there is the above noise (e.g. the select 0,1,...etc) is that in order for this to work, the statement you are calling the UNION with must have the same number of columns as the first result set. As a consequence, the above injection attack only works if the corresponding record table has 7 columns. Otherwise you'll get a syntax error and this attack won't really give you what you want. The double dashes (--) are just to make sure anything that might happen afterwords in the substitution is ignored, and I get the results I want. The 0x3a garbage is just saying "separate my values by colons".
Now, what makes this query useful as an attack vector is that it is easily re-written by hand if the table has more or less than 7 columns.
For example if the above query didn't work, and the table in question has 5 columns, after some experimentation I would hit upon the following query url to use as an injection vector:
http://server/path/page.php?id=1+union+select+0,1,concat_ws(user(),0x3a,database(),0x3a,version()),3,4--
The number of columns the attacker is guessing is probably based on an educated look at the page. For example if you're looking at a page listing all the Doodads in a store, and it looks like:
Name | Type | Manufacturer
Doodad Foo Shiny Shiny Co.
Doodad Bar Flat Simple Doodads, Inc.
It's a pretty good guess that the table you're looking at has 4 columns (remember there's most likely a primary key hiding somewhere if we're searching by an 'id' parameter).
Sorry for the wall of text, but hopefully that answers your question.
this code adds an additional union query to the select statement that is being executed on page.php. The injector has determined that the original query has 6 fields, thus the selection of the numeric values (column counts must match with a union). the concat_ws just makes one field with the values for the database user , the database, and the version, separated by colons.
It seems to retrieve the user used to connect to the database, the database adress and port, the version of it. And it will be put by the error message.
we have a view in our database which has an ORDER BY in it.
Now, I realize views generally don't order, because different people may use it for different things, and want it differently ordered. This view however is used for a VERY SPECIFIC use-case which demands a certain order. (It is team standings for a soccer league.)
The database is Sql Server 2008 Express, v.10.0.1763.0 on a Windows Server 2003 R2 box.
The view is defined as such:
CREATE VIEW season.CurrentStandingsOrdered
AS
SELECT TOP 100 PERCENT *, season.GetRanking(TEAMID) RANKING
FROM season.CurrentStandings
ORDER BY
GENDER, TEAMYEAR, CODE, POINTS DESC,
FORFEITS, GOALS_AGAINST, GOALS_FOR DESC,
DIFFERENTIAL, RANKING
It returns:
GENDER, TEAMYEAR, CODE, TEAMID, CLUB, NAME,
WINS, LOSSES, TIES, GOALS_FOR, GOALS_AGAINST,
DIFFERENTIAL, POINTS, FORFEITS, RANKING
Now, when I run a SELECT against the view, it orders the results by GENDER, TEAMYEAR, CODE, TEAMID. Notice that it is ordering by TEAMID instead of POINTS as the order by clause specifies.
However, if I copy the SQL statement and run it exactly as is in a new query window, it orders correctly as specified by the ORDER BY clause.
The order of rows returned by a view with an ORDER BY clause is never guaranteed. If you need a specific row order, you must specify where you select from the view.
See this the note at the top of this Book On-Line entry.
SQL Server 2005 ignores TOP 100 PERCENT by design.
Try TOP 2000000000 instead.
Now, I'll try and find a reference... I was at a seminar presented by Itzak Ben-Gan who mentioned it
Found some...
Kimberly L. Tripp
"TOP 100 Percent ORDER BY Considered Harmful"
In this particular case, the optimizer
recognizes that TOP 100 PERCENT
qualifies all rows and does not need
to be computed at all.
Just use :
"Top (99) Percent "
or
"Top (a number 1000s times more than your data rows like 24682468123)"
it works! just try it.
In SQL server 2008, ORDER BY is ignored in views that use TOP 100 PERCENT. In prior versions of SQL server, ORDER BY was only allowed if TOP 100 PERCENT was used, but a perfect order was never guaranteed. However, many assumed a perfect order was guaranteed. I infer that Microsoft does not want to mislead programmers and DBAs into believing there is a guaranteed order using this technique.
An excellent comparative demonstration of this inaccuracy, can be found here...
http://blog.sqlauthority.com/2009/11/24/sql-server-interesting-observation-top-100-percent-and-order-by
Oops, I just noticed that this was already answered. But checking out the comparative demonstration is worth a look anyway.
Microsoft has fixed this. You have patch your sql server
http://support.microsoft.com/kb/926292
I found an alternative solution.
My initial plan was to create a 'sort_order' column that would prevent users from having to perform a complex sort.
I used a windowed function ROW_NUMBER. In the ORDER BY clause, I specified the default sort order that I needed (just as it would have been in the ORDER BY of a SELECT statement).
I get several positive outcomes:
By default, the data is getting returned in the default sort order I originally intended (this is probably due to the windowed function having to sort the data prior to assigning the sort_order value)
Other users can sort the data in alternative ways if they choose to
The sort_order column is there for a very specific sort need, making it easier for users to sort the data should whatever tool they use rearranges the rowset.
Note: In my specific application, users are accessing the view via Excel 2010, and by default the data is presented to the user as I had hoped without further sorting needed.
Hope this helps those with a similar problem.
Cheers,
Ryan
run a profiler trace on your database and see the query that's actually being run when you query your view.
You also might want to consider using a stored procedure to return the data from your view, ordered correctly for your specific use case.
I have a PHP application that displays a list of options to a user. The list is generated from a simple query against SQL 2000. What I would like to do is have a specific option at the top of the list, and then have the remaining options sorted alphabetically.
For example, here's the options if sorted alphabetically:
Calgary
Edmonton
Halifax
Montreal
Toronto
What I would like the list to be is more like this:
**Montreal**
Calgary
Edmonton
Halifax
Toronto
Is there a way that I can do this using a single query? Or am I stuck running the query twice and appending the results?
SELECT name
FROM locations
ORDER BY
CASE
WHEN name = 'Montreal'
THEN 0
ELSE 1
END, name
SELECT name FROM options ORDER BY name = "Montreal", name;
Note: This works with MySQL, not SQL 2000 like the OP requested.
create table Places (
add Name varchar(30),
add Priority bit
)
select Name
from Places
order by Priority desc,
Name
I had a similar problem on a website I built full of case reports. I wanted the case reports where the victim name is known to sort to the top, because they are more compelling. Conversely I wanted all the John Doe cases to be at the bottom. Since this also involved people's names, I had the firstname/lastname sorting problem as well. I didn't want to split it into two name fields because some cases aren't people at all.
My solution:
I have a "Name" field which is what is displayed. I also have a "NameSorted" field that is used in all queries but is never displayed. My input UI takes care of converting "LAST, FIRST" entered into the sorting field into the display version automatically.
Finally, to "rig" the sorting I simply put appropriate characters at the beginning of the sort field. Since I want stuff to come out at the end, I put "zzz" at the beginning. To sort at the top you could put "!" at the beginning. Again your editing UI can take care of this for you.
Yes, I admit its a bit cheezy, but it works. One advantage for me is I have to do more complex queries with joins in different places to generate pages versus RSS etc, and I don't have to keep remembering a complex expression to get the sorting right, its always just sort by the "NameSorted" field.
Click my profile to see the resulting website.
I ended up with this
SELECT name
FROM locations
LEFT JOIN (VALUES ('Toronto', 1), ('Montreal', 2)) city (name, rank)
ON locations.name = city.name
ORDER BY city.rank, locations.name;
Which may be overkill for this example but can be extended for more complex needs.