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
Related
Recently i was using the rownum in the sql query to fetch the data from oracle db in batch of 1000 records. for example 1 to 1000, 1001 to 2000 etc.
Note:
I have about a million records in the table
I am querying the table to get records 1000 at a time
I used the below query
SELECT NAME FROM (
SELECT NAME, ROWNUM RN
FROM employee
) WHERE RN >= ? AND RN <= ?
to fetch the data but stuck in strange issue.
The records in the db are unique but after complete execution I end up getting some duplicate record.
Is there any issue with the query? Is rownum causing issue? Is it possible that the record fetched in first batch of 1000 is also coming in the next subsequent batch?
You can't trust the order of records returned from a query without an explicit order by clause. If the inner query doesn't return values in the same order in each execution, you may get the same row in two separate executions, and thus duplicates in the overall execution.
Is there any issue with the query? Is rownum causing issue?
Yes, Oracle does not guarantee you that a common ordering will be used each time you see the output from a query. So ROWNUM, without an ORDER BY is not useful.Try these options.
In 11g use row_number with an order by, in Oracle 12c use FETCH..FIRST syntax, you should have a primary key / unique key column for this to work.
SELECT NAME
FROM (SELECT NAME,
row_number()
OVER(
ORDER BY primary_key_col ) rn
FROM employee)
WHERE rn >=?
AND rn <=? ;
SELECT NAME
FROM employee
ORDER BY primary_key_col offset ? FETCH first ? rows only;
In one of the technical interviews I was asked to find the last row of a Table without using where clause or any other defined sql functions. It was given that the table has integer ids as Primary key in the ascending order. I was only able to give an answer by order by desc of the table and later stuck on how to get the top row without where or any function. Can someone please help.
EDIT : There was no database constraint in the question. Assume any database which solves the query
In Oracle 11g and below, this is not possible as it does not support the TOP/LIMIT/FETCH syntax and you would normally do:
SELECT *
FROM (
SELECT *
FROM table_name
ORDER BY id DESC
)
WHERE ROWNUM = 1;
In PostgreSQL, Oracle 12 and SQL Server 2012 you can do:
SELECT *
FROM table_name
ORDER BY id DESC
OFFSET 0 ROWS -- This line is optional in Oracle 12, PostgreSQL
FETCH FIRST 1 ROW ONLY;
In SQL Server, you can use TOP:
SELECT TOP 1
*
FROM table_name
ORDER BY id DESC
In MySQL, you can use LIMIT:
SELECT *
FROM table_name
ORDER BY id DESC
LIMIT 1;
TRY ROWCOUNT for SQL-SERVER similarly you can use
ROWNUM or FETCH FIRST for ORACLE -- Do search for this, it quite easy to use
LIMIT for MYSQL -- Do search for this, it quite easy to use
SET ROWCOUNT 1
SELECT * FROM <table name> ORDER BY [id] DESC
Assuming tableID is the surrogate key implemented as an autonumber/autoincrement
SELECT TOP 1 FROM tTable Order by tableID DESC
There is no way to do this. Relational databases are not required to keep their rows in any particular order... So, there's no way of knowing which row is physically stored last. You'll need to use the ORDER BY.
You might be able to improve performance with some appropriate indexes. This sort of thing should be pretty fast with an index.
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 want to run a SQL query in Postgres that is exactly the reverse of the one that you'd get by just running the initial query without an order by clause.
So if your query was:
SELECT * FROM users
Then
SELECT * FROM users ORDER BY <something here to make it exactly the reverse of before>
Would it just be this?
ORDER BY Desc
You are building on the incorrect assumption that you would get rows in a deterministic order with:
SELECT * FROM users;
What you get is really arbitrary. Postgres returns rows in any way it sees fit. For simple queries typically in order of their physical storage, which typically is the order in which rows were entered. But there are no guarantees, and the order may change any time between two calls. For instance after any UPDATE (writing a new physical row version), or when any background process reorders rows - like VACUUM. Or a more complex query might return rows according to an index or a join. Long story short: there is no reliable order for table rows in a relational database unless you specify it with ORDER BY.
That said, assuming you get rows from the above simple query in the order of physical storage, this would get you the reverse order:
SELECT * FROM users
ORDER BY ctid DESC;
ctid is the internal tuple ID signifying physical order. Related:
In-order sequence generation
How list all tables with data changes in the last 24 hours?
here is a tsql solution, thid might give you an idea how to do it in postgres
select * from (
SELECT *, row_number() over( order by (select 1)) rowid
FROM users
) x
order by rowid desc
I want to implement paging in a gridview or in an html table which I will fill using ajax. How should I write queries to support paging? For example if pagesize is 20 and when the user clicks page 3, rows between 41 and 60 must be shown on table. At first I can get all records and put them into cache but I think this is the wrong way. Because data can be very huge and data can be change from other sessions. so how can I implement this? Is there any generic way ( for all databases ) ?
As others have suggested, you can use rownum in Oracle. It's a little tricky though and you have to nest your query twice.
For example, to paginate the query
select first_name from some_table order by first_name
you need to nest it like this
select first_name from
(select rownum as rn, first_name from
(select first_name from some_table order by first_name)
) where rn > 100 and rn <= 200
The reason for this is that rownum is determined after the where clause and before the order by clause. To see what I mean, you can query
select rownum,first_name from some_table order by first_name
and you might get
4 Diane
2 Norm
3 Sam
1 Woody
That's because oracle evaluates the where clause (none in this case), then assigns rownums, then sorts the results by first_name. You have to nest the query so it uses the rownum assigned after the rows have been sorted.
The second nesting has to do with how rownum is treated in a where condition. Basically, if you query "where rownum > 100" then you get no results. It's a chicken and egg thing where it can't return any rows until it finds rownum > 100, but since it's not returning any rows it never increments rownum, so it never counts to 100. Ugh. The second level of nesting solves this. Note it must alias the rownum column at this point.
Lastly, your order by clause must make the query deterministic. For example, if you have John Doe and John Smith, and you order by first name only, then the two can switch places from one execution of the query to the next.
There are articles here http://www.oracle.com/technology/oramag/oracle/06-sep/o56asktom.html
and here http://www.oracle.com/technology/oramag/oracle/07-jan/o17asktom.html. Now that I see how long my post is, I probably should have just posted those links...
Unfortunately, the methods for restricting the range of rows returned by a query vary from one DBMS to another: Oracle uses ROWNUM (see ocdecio's answer), but ROWNUM won't work in SQL Server.
Perhaps you can encapsulate these differences with a function that takes a given SQL statement and first and last row numbers and generates the appropriate paginatd SQL for the target DBMS - i.e. something like:
sql = paginated ('select empno, ename from emp where job = ?', 101, 150)
which would return
'select * from (select v.*, ROWNUM rn from ('
+ theSql
+ ') v where rownum < 150) where rn >= 101'
for Oracle and something else for SQL Server.
However, note that the Oracle solution is adding a new column RN to the results that you'll need to deal with.
I believe that both have a ROWNUM analytic Function. Use that and you'll be identical.
In Oracle it is here
ROW_NUMBER
Yep, just verified that ROW_NUMBER is the same function in both.
"Because...data can be change from other sessions."
What do you want to happen for this ?
For example, user gets the 'latest' ten rows at 10:30.
At 10:31, 3 new rows are added (so those ten being view by the user are no longer the latest).
At 10:32, the user requests then 'next' ten entries.
Do you want that new set to include those three that have been bumped from 8/9/10 down to 11/12/13 ?
If not, in Oracle you can select the data as it was at 10:30
SELECT * FROM table_1 as of timestamp (timestamp '2009-01-29 10:30:00');
You still need the row_number logic, eg
select * from
(SELECT a.*, row_number() over (order by hire_date) rn
FROM hr.employees as of timestamp (timestamp '2009-01-29 10:30:00') a)
where rn between 10 and 19
select *
from ( select /*+ FIRST_ROWS(n) */ a.*,
ROWNUM rnum
from ( your_query_goes_here,
with order by ) a
where ROWNUM <=
:MAX_ROW_TO_FETCH )
where rnum >= :MIN_ROW_TO_FETCH;
Step 1: your query with order by
Step 2: select a.*, ROWNUM rnum from ()a where ROWNUM <=:MAX_ROW_TO_FETCH
Step 3: select * from ( ) where rnum >= :MIN_ROW_TO_FETCH;
put 1 in 2 and 2 in 3
If the expected data set is huge, I'd recommend to create a temp table, a view or a snapshot (materialized view) to store the query results + a row number retrieved either using ROWNUM or ROW_NUMBER analytic function. After that you can simply query this temp storage using row number ranges.
Basically, you need to separate the actual data fetch from the paging.
There is no uniform way to ensure paging across various RDBMS products. Oracle gives you rownum which you can use in where clause like:
where rownum < 1000
SQL Server gives you row_id( ) function which can be used similar to Oracle's rownum. However, row_id( ) isn't available before SQL Server 2005.