New to SQL here - I am trying to get 1 row from a table matching to a particular criteria
Typically this would look like
SELECT TOP 1 *
FROM myTable
WHERE id = 'abc'
The output may look like
value id
--------------
1 abc
The table has many entries for an 'id', and I am trying to get one entry per 'id'. Now I have list of 'id's. How would I execute something like
SELECT TOP 1 *
FROM myTable
FOR EACH id
WHERE id IN ('abc', 'edf', 'fgh')
Expecting result like
value id
--------------
1 abc
10 edf
12 fgh
I do not know if it is some sort union or concat operation, but would like to learn. I am working on Azure SQL Server
The table has many entries for an 'id', and I am trying to get one entry per 'id'. Now I have list of 'id's.
A typical method is row_number():
select t.*
from (select t.*,
row_number() over (partition by id order by id) as seqnum
from mytable t
) t
where seqnum = 1;
Note: you can filter on particular ids, if you want. It is unclear if that is really required for your question.
If you happen to be using SQL Server (as select top suggests), you can use the more concise, but somewhat less performant:
select top (1) with ties t.*
from mytable t
order by row_number() over (order by id order by (select null));
Related
My query returns a result like shown in the table. I would like to randomly pick an ID from the ID column and get all the rows having that ID. How can I do that in SnowFlake or SQL:
ID
Postalcode
Value
...
1e3d
NK25F4
3214
...
1e3d
NK25F4
3258
...
1e3d
NK25F4
3354
...
1f74
NG2LK8
5524
1f74
NG2LK8
5548
3e9a
N6B7H4
3694
3e9a
N6B7H4
3325
38e4
N6C7H2
3654
...
There is a Snowflake function to return a fix number of "random" rows SAMPLE, so using that will reduce the need to read all rows.
SELECT t.*
FROM your_table as t
JOIN (SELECT ID FROM your_table SAMPLE (1 ROWS)) as r
ON t.id = r.id
thus using your data above:
with your_table(id, postalcode, value) as (
select * from values
('1e3d', 'NK25F4', 3214),
('1e3d', 'NK25F4', 3258),
('1e3d', 'NK25F4', 3354),
('1f74', 'NG2LK8', 5524),
('1f74', 'NG2LK8', 5548),
('3e9a', 'N6B7H4', 3694),
('3e9a', 'N6B7H4', 3325),
('38e4', 'N6C7H2', 3654)
)
I get (random set) but one looks like:
ID
POSTALCODE
VALUE
1f74
NG2LK8
5,524
1f74
NG2LK8
5,548
You could also use a NATURAL JOIN like:
SELECT *
FROM your_table
NATURAL JOIN (SELECT ID FROM your_table SAMPLE (1 ROWS))
You could put your existing query in a common table expression, then pick a random ID from it, and use it to filter the dataset:
with
dat as ( ... your query ...),
tid as (select id from dat order by random() fetch first 1 row)
select d.*
from dat d
inner join tid t on t.id = d.id
The second CTE, tid picks the random id; it does that by randomly ordering the dataset, then getting the id of the top row.
Something like
SELECT *
FROM Table_NAME
WHERE ID IN (SELECT ID FROM Table_Name ORDER BY RAND() LIMIT 1);
Should work. Though it's not particularly efficient and in many application scenarios it would arguably be more reasonable overall to compute the random ID in your application (e.g. keeping the set of all ids cached, periodically pulling it separately if need be etc).
(Note: The query assumes MYSQL, other variants may have slightly different keywords/structure, e.g. for the random function).
WITH DATA AS (
select '1e3d' id,'NK25F4' postalcode,3214 some_value union all
select '1e3d' id,'NK25F4' postalcode,3258 some_value union all
select '1e3d' id,'NK25F4' postalcode,3354 some_value union all
select '1f74' id,'NG2LK8' postalcode,5524 some_value union all
select '1f74' id,'NG2LK8' postalcode,5548 some_value union all
select '3e9a' id,'N6B7H4' postalcode,3694 some_value union all
select '3e9a' id,'N6B7H4' postalcode,3325 some_value union all
select '38e4' id,'N6C7H2' postalcode,3654 some_value )
SELECT * FROM DATA ,LATERAL (SELECT ID FROM DATA SAMPLE(2 ROWS)) I WHERE I.ID = DATA.ID
You can also play with the window frame a little and let qualify do the work
select *
from your_table
qualify id=first_value(id) over (order by random() rows between unbounded preceding and unbounded following)
Snowflake deviates from ANSI standard on the default window frames for rank-related functions (first_value, last_value, nth_value), so that makes the above equivalent to :
select *
from your_table
qualify id=first_value(id) over (order by random())
for example i have data like this(sql server)
id name
4 anu
3 lohi
1 pras
2 chand
i want 2nd record in a table (means 3 lohi)
if i use row_number() function its changes the order and i get (2 chand)
i want 2nd record from table data
can anyonr please give me the query fro above scenario
There is no such thing as the nth row in a table. And for a simple reason: SQL tables represent unordered sets (technically multi-sets because they allow duplicates).
You can do what you want use offset/fetch:
select t.*
from t
order by id desc
offset 1 fetch first 1 row only;
This assumes that the descending ordering on id is what you want, based on your example data.
You can also do this using row_number():
select t.*
from (select t.*,
row_number() over (order by id desc) as seqnum
from t
) t
where seqnum = 2;
I should note that that SQL Server allows you to assign row_number() without having an effective sort using something like this:
select t.*
from (select t.*,
row_number() over (order by (select NULL)) as seqnum
from t
) t
where seqnum = 2;
However, this returns an arbitrary row. There is no guarantee it returns the same row each time it runs, nor that the row is "second" in any meaningful use of the term.
Let's say I have a table with two columns:
id | value
----------
1 | 101
2 | 356
3 | 28
I need to randomly permute the value column so that each id is randomly assigned a new value from the existing set {101,356,28}. How could I do this in Oracle SQL?
It may sound odd but this is a real problem, just with more columns.
You can do this by using row_number() with a random number generator and then joining back to the original rows:
with cte as (
select id, value,
row_number() over (order by id) as i,
row_number() over (order by dbms_random.random) as rand_i
from table t
)
select cte.id, cte1.value
from cte join
cte cte1
on cte.i = cte.rand_i;
This guarantees a permutation (i.e. no original row has its value used twice).
EDIT:
By the way, if the original ids are sequential from 1 and have no gaps, you could just do:
select row_number() over (order by dbms.random) as id, value
from table t;
An Option : select * from x_table where id = round(dbms_random.value() * 3) + 1; [Here 3 is the number of rows in your random data table and I am assuming that id is incremental and unique?]
I'll think of other options.
I'm not sure whether this is the right task for SQL database. Maybe you should implement something like this:
Factoradic permutation - in PL/SQL and then return a cursor via PIPE ROW construct. Ordering by dbms.random might be slow for large data sets.
I am working on SQL Server. I have a table, that contains around 75000 records. Among them there are several duplicate records. So i wrote a query to know which record repeated how many times like,
SELECT [RETAILERNAME],COUNT([RETAILERNAME]) as Repeated FROM [Stores] GROUP BY [RETAILERNAME]
It gives me result like,
---------------------------
RETAILERNAME | Repeated
---------------------------
X | 4
---------------------------
Y | 6
---------------------------
Z | 10
---------------------------
Among 4 record(s) of X record, i need take only first record of X.
so here i want to retrieve all fields from first row of duplicate records. i.e. Take all records whose RETAILERNAME='X' we will get some no. of duplicate records, we need to get only first row from them.
Please guide me.
You could try using ROW_NUMBER.
Something like
;WITH Vals AS (
SELECT [RETAILERNAME],
ROW_NUMBER() OVER(PARTITION BY [RETAILERNAME] ORDER BY [RETAILERNAME]) RowID
FROM [Stores ]
)
SELECT *
FROm Vals
WHERE RowID = 1
SQL Fiddle DEMO
You can then also remove the duplicates if need be (BUT BE CAREFUL THIS IS PERMANENT)
;WITH Vals AS (
SELECT [RETAILERNAME],
ROW_NUMBER() OVER(PARTITION BY [RETAILERNAME] ORDER BY [RETAILERNAME]) RowID
FROM Stores
)
DELETE
FROM Vals
WHERE RowID > 1;
You Can write query as under
SELECT TOP 1 * FROM [Stores] GROUP BY [RETAILERNAME]
HAVING your condition
WITH cte
AS (SELECT [retailername],
Row_number()
OVER(
partition BY [retailername]
ORDER BY [retailername])'RowRank'
FROM [retailername])
SELECT *
FROM cte
Assuming the following table:
ID Name Revision
--- ----- --------
1 blah 0
2 yada 1
3 blah 1
4 yada 0
5 blah 2
6 blah 3
How do I get the two records, one for "blah" and one for "yada" with highest revision number (3 for blah and 1 for yada)? Something like:
ID Name Revision
--- ----- --------
6 blah 3
2 yada 1
Also, once these records are retrieved, how do I get the rest, ordered by name and revision?
I am trying to create a master-detail view where master records are latest revisions and details include the previous revisions.
Basically, with the aggregate function MAX():
SELECT "Name", MAX("Revision") AS max_revison
FROM tbl
WHERE "Name" IN ('blah', 'yada');
GROUP BY "Name"
ORDER BY "Name"; -- ordering by revision would be pointless;
If you need more columns from the row, there are several ways. One would be to join the above subquery back to the base table:
SELECT t.*
FROM (
SELECT "Name", max("Revision") AS max_revison
FROM tbl
WHERE "Name" IN ('blah', 'yada');
GROUP BY "Name"
) AS sub
JOIN tbl AS t ON t."Revision" = sub.max_revison
AND t."Name" = sub."Name"
ORDER BY "Name";
Generally, this has the potential to yield more than one row per "Name" - if "Revision" is not unique (per "Name"). You would have to define how to pick one from a group of peers sharing the same maximum "Revision" - a tiebreaker.
Another way would be with NOT EXISTS, excluding rows that have greater peers, possibly faster:
SELECT t.*
FROM tbl AS t
WHERE "Name" IN ('blah', 'yada')
AND NOT EXISTS (
SELECT 1
FROM tbl AS t1
WHERE t1."Name" = t."Name"
AND t1."Revision" > t."Revision"
)
ORDER BY "Name";
Or you could use a CTE with an analytic function (window function):
WITH cte AS (
SELECT *, ROW_NUMBER() OVER(PARTITION BY "Name" ORDER BY "Revision" DESC) AS rn
FROM tbl
WHERE "Name" IN ('blah', 'yada')
)
SELECT *
FROM cte
WHERE rn = 1;
The last one is slightly different: one row per "Name" is guaranteed. If you don't use more ORDER BY items, an arbitrary row will be picked in case of a tie. If you want all peers use RANK() instead.
This approach will select the rows for each Name with the maximum revision number for that Name. The result will be the exact output you were looking for in your post.
SELECT *
FROM tbl a
WHERE a.revision = (select max(revision) from tbl where name = a.name)
ORDER BY a.name
In Oracle, you can use LAST function to simplify this.
select max(id) keep (dense_rank last order by revision),
name,
max(revision)
from table
group by name;
Demo