How to get random output of multiple rows from any table in Oracle? Also what is the meaning of (dbms_random.value)
select *
from (select *
from emp
order by dbms_random.value)
where rownum=1;
It gives only one random output. How do I get multiple?
try
select * from (select * from emp order by dbms_random.value) where rownum<10
10 can be replaced by any other count of your choice
Related
I am writing a very simple query for an Oracle DB (version 9).
Somehow I can get first 5 rows:
select * from cities where rownum <= 5
But skipping 5 rows returns an empty result:
select * from cities where rownum >= 5
Using:
Oracle SQL Developer
Oracle DB version 9
Why is the second query returning an empty result?
In Oracle Database 12c (release 1) and above, you can do this very simple, for skip 5 rows:
SELECT * FROM T OFFSET 5 ROWS
and for skip 5 rows and take 15 rows:
SELECT * FROM T OFFSET 5 ROWS FETCH NEXT 15 ROWS ONLY
You can use the following query to skip the first not n of rows.
select * from (
select rslts.*, rownum as rec_no from (
<<Query with proper order by (If you don't have proper order by you will see weird results)>>
) rslts
) where rec_no > <<startRowNum - n>>
The above query is similar to pagination query below.
select * from (
select rslts.*, rownum as rec_no from (
<<Query with proper order by (If you don't have proper order by you will see weird results)>>
) rslts where rownum <= <<endRowNum>>
) where rec_no > <<startRowNum>>
Your cities query:
select * from (
select rslts.*, rownum as rec_no from (
select * from cities order by 1
) rslts
) where rec_no > 5 <<startRowNum>>
Note: Assume first column in cities table is unique key
Oracle increments rownum each time it adds a row to the result set. So saying rownum < 5 is fine; as it adds each of the first 5 rows it increments rownum, but then once ruwnum = 5 the WHERE clause stops matching, no more rows are added to the result, and though you don't notice this rownum stops incrementing.
But if you say WHERE rownum > 5 then right off the bat, the WHERE clause doesn't match; and since, say, the first row isn't added to the result set, rownum isn't incremented... so rownum can never reach a value greater than 5 and the WHERE clause can never match.
To get the result you want, you can use row_number() over() in a subquery, like
select *
from (select row_number() over() rn, -- other values
from table
where -- ...)
where rn > 5
Update - As noted by others, this kind of query only makes sense if you can
control the order of the row numbering, so you should really use row_number() over(order bysomething) where something is a useful ordering key in deciding which records are "the first 5 records".
rownum is being increased only when a row is being output, so this type of condition won't work.
In any case, you are not ordering your rows, so what's the point?
Used row_number() over (order by id):
select * from
(select row_number() over (order by id) rn, c.* from countries c)
where rn > 5
Used ROWNUM:
select * from
(select rownum rn, c.* from countries c)
where rn > 5
Important note:
Using alias as countries c instead of countries is required! Without, it gives an error "missing expression"
Even better would be:
select * from mytab sample(5) fetch next 1 rows only;
Sample clause indicates the probability of each row getting picked up in the sampling process. FETCH NEXT clause indicates the number of rows you want to select.
With this code, you can query your table with skip and take.
select * from (
select a.*, rownum rnum from (
select * from cities
) a
) WHERE rnum >= :skip + 1 AND rnum <= :skip + :take
This code works with Oracle 11g. With Oracle 12, there is already a better way to perform this queries with offset and fetch
I am working on python sqlite3.
This statement gets records 5 - 14;
SELECT * FROM something LIMIT 5, 10;
But how do I get, lets say the first five and last five records through a single statement?
You can combine output of two select statement like this:
(SELECT * FROM `something` order by some_column_name
limit 0,5)
union
(SELECT * FROM `something` order by some_column_name desc
limit 0,5
)
Specify some ordering of rows, so that it will select rows accordingly.
Maybe the better way is using rowid in your order by clause to get the first and last rows based on inserting the rows:
select test from
(select test,rowid from table1 order by rowid asc limit 0,5)t1
union all
select test from
(select test,rowid from table1 order by rowid desc limit 0,5)t2;
Here is a sample in SQL Fiddle
I observed that rownum can be used as follows:
1) select * from emp where rownum<5;
2) select * from emp where rownum<=5;
3) select * from emp where rownum=1;
All this queries return expected output. Say table emp has 7 rows then 1st query returns 4 rows, 2nd returns 5 rows ,3rd returns 1 row.
But when I try to use something like:
4) select * from emp where rownum=5;
5) select * from emp where rownum>5;
6) select * from emp where rownum>=5;
7) select * from emp where rownum between 5 and 10;
In all this cases, it returns 0 rows. Why this is so? Is there any specific reason for this?
This is because rownum is a pseudo column and it counts rows in the result set after you apply the where clause.
From the Oracle docs:-
Conditions testing for ROWNUM values greater than a positive integer
are always false. For example, this query returns no rows:
SELECT *
FROM employees
WHERE ROWNUM > 1;
The first row fetched is assigned a ROWNUM of 1 and makes the condition false. The second row to be fetched is now the first row and is also assigned a ROWNUM of 1 and makes the condition false. All rows subsequently fail to satisfy the condition, so no rows are returned.
You can also use ROWNUM to assign unique values to each row of a table, as in this example:
UPDATE my_table
SET column1 = ROWNUM;
You may find these references helpfull:-
Using ROWNUM in Oracle
SQL Query With Row_Number, order by and where clause
You can do something similar like this:
select * from (
select
emp.*,
row_number() over (order by fieldname) as rnum
from emp
) tmp
where
rnum between 5 and 10;
How can i find the fifth record in a table using sql query?
If you are feeling argumentative, consider using "SELECT * FROM table LIMIT 1" and arguing that since SQL does not promise to return results in any particular order, the row returned is spiritually equivalent to the fifth, then show your tattoo: "The nth element of an unordered set is meaningless!"
SELECT TOP 1 * FROM (SELECT TOP 5 * FROM Table T ORDER BY Column ASC) ORDER BY Column Desc
If you are using SqlServer you could use the TOP keyword to achieve this.
select top 1 * from(
select top 5 * from myTable order by orderingColumn) as A
order by orderingColumn desc
If you are using Oracle this should work (however i am not able to test this now)
select *
from (
select *, row_number() over (order by orderingColumn) r
from items
)
where r = 5;
SELECT * FROM table LIMIT 1 OFFSET 4;
Fifth record only in MySQL
SELECT * FROM anytable LIMIT ORDER BY id LIMIT 4,1
For SQL Server (recent-ish incarnations, anyway) something like this should work:
SELECT
*
FROM
(
SELECT
*,
ROW_NUMBER() OVER (ORDER BY the_table.the_column) 'row_num'
FROM
the_table
) numbered_rows
WHERE
row_num = 5
However, I'd actually put my vote with Thomas L Holaday's answer :)
I want to select the last three rows of a table in ascending order. What is the query for that?
SELECT * FROM table ORDER BY DESC LIMIT 3
SELECT *
FROM (
SELECT *
FROM tblname
ORDER BY rowid DESC
) WHERE rownum < 4;
Rowid is the physical address of every row.
Rownum is the psuedo column which is used to generate sequence number of rows.