I have a table:
abc_test with columns n_num, k_str.
This query doesnt work:
select distinct(n_num) from abc_test order by(k_str)
But this one works:
select n_num from abc_test order by(k_str)
How do DISTINCT and ORDER BY keywords work internally that output of both the queries is changed?
As far as i understood from your question .
distinct :- means select a distinct(all selected values should be unique).
order By :- simply means to order the selected rows as per your requirement .
The problem in your first query is
For example :
I have a table
ID name
01 a
02 b
03 c
04 d
04 a
now the query select distinct(ID) from table order by (name) is confused which record it should take for ID - 04 (since two values are there,d and a in Name column). So the problem for the DB engine is here when you say
order by (name).........
You might think about using group by instead:
select n_num
from abc_test
group by n_num
order by min(k_str)
The first query is impossible.
Lets explain this by example. we have this test:
n_num k_str
2 a
2 c
1 b
select distinct (n_num) from abc_test is
2
1
Select n_num from abc_test order by k_str is
2
1
2
What do you want to return
select distinct (n_num) from abc_test order by k_str?
it should return only 1 and 2, but how to order them?
How do extended sort key columns
The logical order of operations in SQL for your first query, is (simplified):
FROM abc_test
SELECT n_num, k_str i.e. add a so called extended sort key column
ORDER BY k_str DESC
SELECT n_num i.e. remove the extended sort key column again from the result.
Thanks to the SQL standard extended sort key column feature, it is possible to order by something that is not in the SELECT clause, because it is being temporarily added to it behind the scenes prior to ordering, and then removed again after ordering.
So, why doesn't this work with DISTINCT?
If we add the DISTINCT operation, it would need to be added between SELECT and ORDER BY:
FROM abc_test
SELECT n_num, k_str i.e. add a so called extended sort key column
DISTINCT
ORDER BY k_str DESC
SELECT n_num i.e. remove the extended sort key column again from the result.
But now, with the extended sort key column k_str, the semantics of the DISTINCT operation has been changed, so the result will no longer be the same. This is not what we want, so both the SQL standard, and all reasonable databases forbid this usage.
Workarounds
PostgreSQL has the DISTINCT ON syntax, which can be used here for precisely this job:
SELECT DISTINCT ON (k_str) n_num
FROM abc_test
ORDER BY k_str DESC
It can be emulated with standard syntax as follows, if you're not using PostgreSQL
SELECT n_num
FROM (
SELECT n_num, MIN(k_str) AS k_str
FROM abc_test
GROUP BY n_num
) t
ORDER BY k_str
Or, just simply (in this case)
SELECT n_num, MIN(k_str) AS k_str
FROM abc_test
GROUP BY n_num
ORDER BY k_str
I have blogged about SQL DISTINCT and ORDER BY more in detail here.
You are selecting the collection distinct(n_num) from the resultset from your query. So there is no actual relation with the column k_str anymore. A n_num can be from two rows each having a different value for k_str. So you can't order the collection distinct(n_num) by k_str.
According to SQL Standards, a SELECT clause may refer either to as clauses ("aliases") in the top level SELECT clause or columns of the resultset by ordinal position, and therefore nether of your queries would be compliant.
It seems Oracle, in common with other SQL implemetations, allows you to refer to columns that existed (logically) immediately prior to being projected away in the SELECT clause. I'm not sure whether such flexibility is such a good thing: IMO it is good practice to expose the sort order to the calling application by including the column/expressions etc in the SELECT clause.
As ever, you need to apply dsicpline to get meaningful results. For your first query, the definition of order is potentially entirely arbitrary.You should be grateful for the error ;)
This approach is available in SQL server 2000, you can select distinct values from a table and order by different column which is not included in Distinct.
But in SQL 2012 this will through you an error
"ORDER BY items must appear in the select list if SELECT DISTINCT is specified."
So, still if you want to use the same feature as of SQL 2000 you can use the column number for ordering(its not recommended in best practice).
select distinct(n_num) from abc_test order by 1
This will order the first column after fetching the result. If you want the ordering should be done based on different column other than distinct then you have to add that column also in select statement and use column number to order by.
select distinct(n_num), k_str from abc_test order by 2
When I got same error, I got it resolved by changing it as
SELECT n_num
FROM(
SELECT DISTINCT(n_num) AS n_num, k_str
FROM abc_test
) as tbl
ORDER BY tbl.k_str
My query doesn't match yours exactly, but it's pretty close.
select distinct a.character_01 , (select top 1 b.sort_order from LookupData b where a.character_01 = b.character_01 )
from LookupData a
where
Dataset_Name = 'Sample' and status = 200
order by 2, 1
did you try this?
SELECT DISTINCT n_num as iResult
FROM abc_test
ORDER BY iResult
you can do
select distinct top 10000 (n_num) --assuming you won't have more than 10,000 rows
from abc_test order by(k_str)
Related
If I have a table like this:
Id Rnk
1 1
1 1
1 2
1 2
and I want to arrange the table like that:
Id Rnk
1 1
1 2
1 1
1 2
And I want it to be fixed so when ever I'll select the table the order will be like this. Any Help on how can I do it?
And I want it to be fixed so when ever I'll select the table the order
will be like this.
Quick answer: it cannot be done. You have to always use ORDER BY clause in the query if you want to get rows in desired order.
A few related questions and answers on this topis:
What is the default order of records for a SELECT statement in MySQL?
Default row ordering for select query in oracle
SQL: What is the default Order By of queries?
MySQL, ORDER BY insertion order, no sorting columns
Quote from the Wikipedia: Order by
ORDER BY is the only way to sort the rows in the result set. Without
this clause, the relational database system may return the rows in any
order. If an ordering is required, the ORDER BY must be provided in
the SELECT statement sent by the application.
Another quote from the Wikipedia: Relational database
The relational model specifies that the tuples of a relation have no
specific order and that the tuples, in turn, impose no order on the
attributes.
In order to get this concrete order you can use row_number analytic functions in this way:
SELECT "Id", "Rnk"
FROM (
SELECT t.*,
row_number() over (partition by "Id", "Rnk" order by "Id", "Rnk") as rn
FROM Table1 t
) x
ORDER BY "Id", rn
A demo for PostgreSQL: http://dbfiddle.uk/?rdbms=postgres_10&fiddle=0b86522f37e927a86701e778006e8cad
row_number is now supported by most databases, even MySql will have this feature in the upcoming version
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
What could be the meaning of this sql statement ?
select * from tab1 order by (select count(*) from tab2) desc
The below line just returns the number of rows in tab2, which is some constant number
select count(*) from tab2
Consider the columns numbered 1 through n where n is the last column.
select * from tab1 order by 1
would order by the first column
select * from tab1 order by 2
would order by the second column and etc.
If n is larger than the number of columns then you'll run into a problemEDIT
You are using a subquery however and having
select * from tbl1 order by (select 1000)
does not cause a problem if you have <1000 columns, it seems to do nothing; the query may be missing some information
The result is to order by the column whose index is the count returned by the inner query in the ORDER BY clause. Whoever wrote this, especially without a comment, should be hanged by body parts important for reproduction.
The answer is based in Microsoft SQL functionality [edit:] where subquery in ORDER BY (subquery) expression indicates sort value.
Here's how I see it: since tab2 is not linked to tab1 in a subquery, the SQL can be reduced to:
select * from tab1 order by (SELECT <CONSTANT>) desc
therefore it's equivalent to:
select * from tab1
Quite frankly all that query is going to do is return all records from tab1 in some unknown order.
The order by clause is a bit asinine because the value returned will always be the count of all records in tab2.
I suspect it's missing a where clause on the (select count(*) from tab2) part. Something along the lines of (select count(*) from tab2 t where t.tab1id = tab1.id) Although it's hard to say without knowing the structure of those two tables.
The ORDER BY is equivalent to ORDER BY 'X'; that is, it has no effect. It does not order by the column number referenced by the count(*) in the second query -- it is not equivalent to order by 3 if the second table has three rows.
See fiddles for Oracle, MySQL, and SQL Server. If the ORDER BY was based on the count(*), the result should then be sorted by the third column. None of them are. Also, a count(*)+100 has no effect.
I have a page to show 10 messages by each user (don't ask me why)
I have the following code:
SELECT *, row_number() over(partition by user_id) as row_num
FROM "posts"
WHERE row_num <= 10
It doesn't work.
When I do this:
SELECT *
FROM (
SELECT *, row_number() over(partition by user_id) as row_num FROM "posts") as T
WHERE row_num <= 10
It does work.
Why do I need nested query to see row_num column? Btw, in first request I actually see it in results but can't use where keyword for this column.
It seems to be the same "rule" as any query, column aliases aren't visible to the WHERE clause;
This will also fail;
SELECT id AS newid
FROM test
WHERE newid=1; -- must use "id" in WHERE clause
SQL Query like:
SELECT *
FROM table
WHERE <condition>
will execute in next order:
3.SELECT *
1.FROM table
2.WHERE <condition>
so, as Joachim Isaksson say, columns in SELECt clause are not visible in WHERE clause, because of processing order.
In your second query, column row_num are fetched in FROM clause first, so it will be visible in WHERE clause.
Here is simple list of steps in order they executes.
There is a good reason for this rule in standard SQL.
Consider the statement:
SELECT *, row_number() over (partition by user_id) as row_num
FROM "posts"
WHERE row_num <= 10 and p.type = 'xxx';
When does the p.type = 'xxx' get evaluated relative to the row number? In other words, would this return the first ten rows of "xxx"? Or would it return the "xxx"s in the first ten rows?
The designers of the SQL language recognize that this is a hard problem to resolve. Only allowing them in the select clause resolves the issue.
You can check this topic and this one on dba.stockexchange.com about order in which SQL executes SELECT clause. I think it aplies not only for PostgreSQL, but for all RDBMS.
I have a table with n number of records
How can i retrieve the nth record and (n-1)th record from my table in SQL without using derived table ?
I have tried using ROWID as
select * from table where rowid in (select max(rowid) from table);
It is giving the nth record but i want the (n-1)th record also .
And is there any other method other than using max,derived table and pseudo columns
Thanks
You cannot depend on rowid to get you to the last row in the table. You need an auto-incrementing id or creation time to have the proper ordering.
You can use, for instance:
select *
from (select t.*, row_number() over (order by <id> desc) as seqnum
from t
) t
where seqnum <= 2
Although allowed in the syntax, the order by clause in a subquery is ignored (for instance http://docs.oracle.com/javadb/10.8.2.2/ref/rrefsqlj13658.html).
Just to be clear, rowids have nothing to do with the ordering of rows in a table. The Oracle documentation is quite clear that they specify a physical access path for the data (http://docs.oracle.com/cd/B28359_01/server.111/b28318/datatype.htm#i6732). It is true that in an empty database, inserting records into a newtable will probably create a monotonically increasing sequence of row ids. But you cannot depend on this. The only guarantees with rowids are that they are unique within a table and are the fastest way to access a particular row.
I have to admit that I cannot find good documentation on Oracle handling or not handling order by's in subqueries in its most recent versions. ANSI SQL does not require compliant databases to support order by in subqueries. Oracle syntax allows it, and it seems to work in some cases, at least. My best guess is that it would probably work on a single processor, single threaded instance of Oracle, or if the data access is through an index. Once parallelism is introduced, the results would probably not be ordered. Since I started using Oracle (in the mid-1990s), I have been under the impression that order bys in subqueries are generally ignored. My advice would be to not depend on the functionality, until Oracle clearly states that it is supported.
select * from (select * from my_table order by rowid) where rownum <= 2
and for rows between N and M:
select * from (
select * from (
select * from my_table order by rowid
) where rownum <= M
) where rownum >= N
Try this
select top 2 * from table order by rowid desc
Assuming rowid as column in your table:
SELECT * FROM table ORDER BY rowid DESC LIMIT 2