I am facing a strange error in SQL Server and I want some explanation of it.
When I write ORDER BY in a subquery, for instance
SELECT a FROM (SELECT * FROM A ORDER BY a) T
it throws the following error
The ORDER BY clause is invalid in views, inline functions, derived
tables, subqueries, and common table expressions, unless TOP or FOR
XML is also specified.
But when I use TOP in subquery it works normally
SELECT a
FROM
(SELECT TOP 1000000 * FROM A ORDER BY a) T
So, does it mean that I can select top row count of A, instead of
SELECT a FROM (SELECT * FROM A ORDER BY a) T
In that case. what is the reason of error?
There is no much sense to sort the subquery and after that select something from it - it is not guaranteed that top-level select will be ordered, so - there is no sense to order the inner query
But if you order inner query with TOP statement - it also not guaranteed that top level select will be ordered in such a way, but it will contain only top X rows from the inner query - that is already makes sense.
Related
I am trying to select the last 20 rows of my SQL Database, but I get this error:
[Microsoft][ODBC Driver 17 for SQL Server][SQL Server]The ORDER BY clause is invalid in views, inline functions, derived tables, subqueries, and common table expressions, unless TOP, OFFSET or FOR XML is also specified.
My query is:
SELECT TOP 20 * FROM (SELECT * FROM TBArticles ORDER BY id_art DESC)
I think it's because I am using ORDER BY in this second expression... but what can I do to select the 20 last rows fixing this error?
You don't need a subquery for this:
SELECT TOP 20 *
FROM TBArticles
ORDER BY id_art DESC
The documentation is quite clear on the use of ORDER BY in subqueries:
The ORDER BY clause is not valid in views, inline functions, derived tables, and subqueries, unless either the TOP or OFFSET and FETCH clauses are also specified. When ORDER BY is used in these objects, the clause is used only to determine the rows returned by the TOP clause or OFFSET and FETCH clauses. The ORDER BY clause does not guarantee ordered results when these constructs are queried, unless ORDER BY is also specified in the query itself.
Gordon's answer is probably the most direct way to handle your requirement. However, if you wanted to use a query along the same lines as the pattern you were already using, you could use ROW_NUMBER here:
SELECT *
FROM
(
SELECT *, ROW_NUMBER() OVER (ORDER BY id_art DESC) rn
FROM TBArticles
) t
WHERE rn <= 20;
By computing a row number in the derived table, the ordering "sticks" in the same way your original query was expecting.
My situation is that a SQL statement which is not predictable, is given to the program and I need to do pagination on top of it. The final SQL statement would be similar to the following one:
SELECT * FROM (*Given SQL Statement*) b
OFFSET 0 ROWS FETCH NEXT 50 ROWS ONLY;
The problem here is that the *Given SQL Statement* is unpredictable. It may or may not contain order by clause. I am not able to change the query result of this SQL Statement and I need to do pagination on it.
I searched for solution on the Internet, but all of them suggested to use an arbitrary column, like primary key, in order by clause. But it will change the original order.
The short answer is that it can't be done, or at least can't be done properly.
The problem is that SQL Server (or any RDBMS) does not and can not guarantee the order of the records returned from a query without an order by clause.
This means that you can't use paging on such queries.
Further more, if you use an order by clause on a column that appears multiple times in your resultset, the order of the result set is still not guaranteed inside groups of values in said column - quick example:
;WITH cte (a, b)
AS
(
SELECT 1, 'a'
UNION ALL
SELECT 1, 'b'
UNION ALL
SELECT 2, 'a'
UNION ALL
SELECT 2, 'b'
)
SELECT *
FROM cte
ORDER BY a
Both result sets are valid, and you can't know in advance what will you get:
a b
-----
1 b
1 a
2 b
2 a
a b
-----
1 a
1 b
2 a
2 b
(and of course, you might get other sorts)
The problem here is that the *Given SQL Statement" is unpredictable. It may or may not contain order by clause.
your inner query(unpredictable sql statement) should not contain order by,even if it contains,order is not guaranteed.
To get guaranteed order,you have to order by some column.for the results to be deterministic,the ordered column/columns should be unique
Please note: what I'm about to suggest is probably horribly inefficient and should really only be used to help you go back to the project leader and tell them that pagination of an unordered query should not be done. Having said that...
From your comments you say you are able to change the SQL statement before it is executed.
You could write the results of the original query to a temporary table, adding row count field to be used for subsequent pagination ordering.
Therefore any original ordering is preserved and you can now paginate.
But of course the reason for needing pagination in the first place is to avoid sending large amounts of data to the client application. Although this does prevent that, you will still be copying data to a temp table which, depending on the row size and count, could be very slow.
You also have the problem that the page size is coming from the client as part of the SQL statement. Parsing the statement to pick that out could be tricky.
As other notified using anyway without using a sorted query will not be safe, But as you know about it and search about it, I can suggest using a query like this (But not recommended as a good way)
;with cte as (
select *,
row_number() over (order by (select 0)) rn
from (
-- Your query
) t
)
select *
from cte
where rn between (#pageNumber-1)*#pageSize+1 and #pageNumber*#pageSize
[SQL Fiddle Demo]
I finally found a simple way to do it without any order by on a specific column:
declare #start AS INTEGER = 1, #count AS INTEGER = 5;
select * from (SELECT *,ROW_NUMBER() OVER (ORDER BY (SELECT 1)) AS fakeCounter
FROM (select * from mytable) AS t) AS t2 order by fakeCounter OFFSET #start ROWS
FETCH NEXT #count ROWS ONLY
where select * from mytable can be any query
I found that, when using order-by directly, it is ok.
SELECT t0."D" AS fd,
SUM(t0."SD") AS top
FROM "mock_table_1" AS t0
GROUP BY t0."D"
ORDER BY top ASC
LIMIT 10
but when using it in a subquery, an syntax error is reported.
SELECT * FROM (
SELECT t0."D" AS fd,
SUM(t0."SD") AS top
FROM "mock_table_1" AS t0
GROUP BY t0."D"
ORDER BY top ASC
LIMIT 10
)
here is the error message.
syntax error, unexpected ORDER, expecting UNION or EXCEPT or INTERSECT or ')' in: "select t0."A" as d0,
So, I wonder if monetdb is designed to be like this, or it is a bug?
that is the expected behavior. offset, limit, and order by are not allowed in subqueries
https://www.monetdb.org/pipermail/users-list/2013-October/006856.html
SQL-conforming DBMSes are not supposed to allow ORDER BY in subqueries, because it contradicts the conceptual model of a relational DBMS. See:
Is order by clause allowed in a subquery
for details. A way around that, however, is to use Window Functions, which MonetDB does support. Specifically, in your subquery, instead of, say,
SELECT c1 FROM t1;
you can
SELECT c1, ROW_NUMBER() OVER () as rownum from t1;
and now you have the relative order of the inner query result available to the outer query.
I want to fetch the last n rows from a table in a Postgres database. I don't want to use an ORDER BY clause as I want to have a generic query. Anyone has any suggestions?
A single query will be appreciated as I don't want to use FETCH cursor of Postgres.
That you get what you expect with Lukas' solution (as of Nov. 1st, 2011) is pure luck. There is no "natural order" in an RDBMS by definition. You depend on implementation details that could change with a new release without notice. Or a dump / restore could change that order. It can even change out of the blue when db statistics change and the query planner chooses a different plan that leads to a different order of rows.
The proper way to get the "last n" rows is to have a timestamp or sequence column and ORDER BY that column. Every RDBMS you can think of has ORDER BY, so this is as 'generic' as it gets.
The manual:
If ORDER BY is not given, the rows are returned in whatever order the
system finds fastest to produce.
Lukas' solution is fine to avoid LIMIT, which is implemented differently in various RDBMS (for instance, SQL Server uses TOP n instead of LIMIT), but you need ORDER BY in any case.
Use window functions!
select t2.* from (
select t1.*, row_number() over() as r, count(*) over() as c
from (
-- your generic select here
) t1
) t2
where t2.r + :n > t2.c
In the above example, t2.r is the row number of every row, t2.c is the total records in your generic select. And :n will be the n last rows that you want to fetch. This also works when you sort your generic select.
EDIT: A bit less generic from my previous example:
select * from (
select my_table.*, row_number() over() as r, count(*) over() as c
from my_table
-- optionally, you could sort your select here
-- order by my_table.a, my_table.b
) t
where t.r + :n > t.c
I have the following query which does what I want, but I suspect it is possible to do this without a subquery:
SELECT *
FROM (SELECT *
FROM 'versions'
ORDER BY 'ID' DESC) AS X
GROUP BY 'program'
What I need is to group by program, but returning the results for the objects in versions with the highest value of "ID".
In my past experience, a query like this should work in MySQL, but for some reason, it's not:
SELECT *
FROM 'versions'
GROUP BY 'program'
ORDER BY MAX('ID') DESC
What I want to do is have MySQL do the ORDER BY first and then the GROUP BY, but it insists on doing the GROUP BY first followed by the ORDER BY. i.e. it is sorting the results of the grouping instead of grouping the results of the ordering.
Of course it is not possible to write
SELECT * FROM 'versions' ORDER BY 'ID' DESC GROUP BY 'program'
Thanks.
By definition, ORDER BY is processed after grouping with GROUP BY. By definition, the conceptual way any SELECT statement is processed is:
Compute the cartesian product of all tables referenced in the FROM clause
Apply the join criteria from the FROM clause to filter the results
Apply the filter criteria in the WHERE clause to further filter the results
Group the results into subsets based on the GROUP BY clause, collapsing the results to a single row for each such subset and computing the values of any aggregate functions -- SUM(), MAX(), AVG(), etc. -- for each such subset. Note that if no GROUP BY clause is specified, the results are treated as if there is a single subset and any aggregate functions apply to the entire results set, collapsing it to a single row.
Filter the now-grouped results based on the HAVING clause.
Sort the results based on the ORDER BY clause.
The only columns allowed in the results set of a SELECT with a GROUP BY clause are, of course,
The columns referenced in the GROUP BY clause
Aggregate functions (such as MAX())
literal/constants
expresssions derived from any of the above.
Only broken SQL implementations allow things like select xxx,yyy,a,b,c FROM foo GROUP BY xxx,yyy — the references to colulmsn a, b and c are meaningless/undefined, given that the individual groups have been collapsed to a single row,
This should do it and work pretty well as long as there is a composite index on (program,id). The subquery should only inspect the very first id for each program branch, and quickly retrieve the required record from the outer query.
select v.*
from
(
select program, MAX(id) id
from versions
group by program
) m
inner join versions v on m.program=v.program and m.id=v.id
SELECT v.*
FROM (
SELECT DISTINCT program
FROM versions
) vd
JOIN versions v
ON v.id =
(
SELECT vi.id
FROM versions vi
WHERE vi.program = vd.program
ORDER BY
vi.program DESC, vi.id DESC
LIMIT 1
)
Create an index on (program, id) for this to work fast.
Regarding your original query:
SELECT * FROM 'versions' GROUP BY 'program' ORDER BY MAX('ID') DESC
This query would not parse in any SQL dialect except MySQL.
It abuses MySQL's ability to return ungrouped and unaggregated expressions from a GROUP BY statement.