I have a subject table that has subject_id column. In the table I have one row that has subject_id null other than that subject_id has a distinct value.
I am looking for single query I can fetch the data on basis of subject_id.
Select * from subject where subject_id = x;
If there is no data found w.r.t x than it should return the row with subject_id = null
In general this is a terrible pattern for tables. NULL as a primary key value is only going to cause you pain and suffering in the long run. Using a NULL-keyed row as a default for when your query matches no other rows will lead to strange behavior somewhere unexpected.
The simplest way would be to simply include the NULL row as the last row of any query and then only fetch the first row. But that only works when your query can only return at most one valid result.
select *
from subject
where subject_id = ? or subject_id is null
order by subject_id asc nulls last
Possibly the biggest problem with a NULL PK for your default/placeholder row in subject is that anywhere else you have a NULL subject_id cannot simply join to that row using x.subject_id = y.subject_id.
If you really need such a row, I suggest using -1 instead of NULL as the "not exists" value. It will make your life much easier across the board, especially if you need to join to it.
Related
This might be a basic sql questions, however I was curious to know the answer to this.
I need to fetch top one record from the db. Which query would be more efficient, one with where clause or order by?
Example:
Table
Movie
id name isPlaying endDate isDeleted
Above is a versioned table for storing records for movie.
If the endDate is not null and isDeleted = 1 then the record is old and an updated one already exist in this table.
So to fetch the movie "Gladiator" which is currently playing, I can write a query in two ways:
1.
Select m.isPlaying
From Movie m
where m.name=:name (given)
and m.endDate is null and m.isDeleted=0
2. Select TOP 1 m.isPlaying
From Movie m
where m.name=:name (given)
order by m.id desc --- This will always give me the active record (one which is not deleted)
Which query is faster and the correct way to do it?
Update:
id is the only indexed column and id is the unique key. I am expecting the queries to return me only one result.
Update:
Examples:
Movie
id name isPlaying EndDate isDeleted
3 Gladiator 1 03/1/2017 1
4 Gladiator 1 03/1/2017 1
5 Gladiator 0 null 0
I would go with the where clause:
Select m.isPlaying
From Movie m
where m.id = :id and m.endDate is null and m.isDeleted = 0;
This can take advantage of an index on (id, isDeleted, endDate).
Also, the two are not equivalent. The second might return multiple rows when the first returns 1. Or the second might return one row when the first returns none.
The first option might return more than 1 row. Maybe you know it won't because you know what data you have stored but the SQL engine doesn't, and it will affect it's execution plan.
Considering that you only have 1 index and it's on the ID column, the 2nd query should be faster in theory, since it would do an index scan from the highest ID with a predicate for the given name, stopping at the first match.
The first query will do a full table scan while comparing column name, endDate and isDeleted, since it won't stop at the first result that matches.
Posting your execution plans for both queries might enlighten a few loose cables.
I have a table, call it widgets which has columns name and created_at, among others. I want to run a query that returns the count of all the rows of widgets which share the same name and have been created within a millisecond of each other.
This is the query that I have come up with, but it returns a number greater than the total number of rows in the table, can someone point out where I am going wrong?
SELECT COUNT (DISTINCT "t1"."id")
FROM
"tasks" "t1" ,"tasks" "t2"
WHERE
"t1"."name" = "t2"."name"
AND
date_trunc('milliseconds',"t1"."created_at") = date_trunc('milliseconds',"t2"."created_at")
You should add the condition:
and "t1"."id" <> "t2"."id"
where "id" is a primary key. In the lack of a primary key you can use ctid:
and "t1".ctid <> "t2".ctid
I have a large database and i'd like to pull info from a table (Term) where the Names are not linked to a PartyId for a certain SearchId. However:
There are multiple versions of the searches (sometimes 20-40 - otherwise I think SQL - Comparing two rows and two columns would work for me)
The PartyId will almost always be NULL for the first version for the search, and if the same Name for the same SearchId has a PartyId associated in a later version the NULL row should not appear in the results of the query.
I have 8 left joins to display the information requested - 3 of them are joined on the Term table
A very simplified sample of data is below
CASE statement? Join the table with itself for comparison? A temp table or do I just return the fields I'm joining on and/or want to display?
Providing sample data that yields no expected result is not as useful as providing data that gives an expected result..
When asking a question start with defining the problem in plain English. If you can't you don't understand your problem well enough yet. Then define the tables which are involved in the problem (including the columns) and sample data; the SQL you've tried, and what you're expected result is using the data in your sample. Without this minimum information we make many guesses and even with that information we may have to make assumptions; but without a minimum verifiable example showing illustrating your question, helping is problematic.
--End soap box
I'm guessing you're after only the names for a searchID which has a NULL partyID for the highest SearchVerID
So if we eliminated ID 6 from your example data, then 'Bob' would be returned
If we added ID 9 to your sample data for name 'Harry' with a searchID of 2 and a searchVerID of 3 and a null partyID then 'Harry' too would be returned...
If my understanding is correct, then perhaps...
WITH CTE AS (
SELECT Name, Row_Number() over (partition by Name order by SearchVersID Desc)
FROM Term
WHERE SearchID = 2)
SELECT Name
FROM CTE
WHERE RN = 1
and partyID is null;
This assigns a row number (RN) to each name starting at 1 and increasing by one for each entry; for searchID's of 2. The highest searchversion will always have a RN of 1. Then we filter to include only those RN which are 1 and have a null partyID. This would result in only those names having a searchID of 2 the highest search version and a NULL partyID
Ok So I took the question a different way too..
If you simply want all the names not linked to a PartyID for a given search.
SELECT A.*
FROM TERM A
WHERE NOT EXISTS (SELECT 1
FROM TERM B
WHERE A.Name = B.Name
AND SearchID = 2) and partyID is not null)
AND searchID = 2
The above should return all term records associated to searchID 2
that have a partyId. This last method is the exists not exists and set logic I was talking about in comments.
So we had a duplicate SQL scripts running on our server and didn't realize it till just recently. Essentially I have many rows where there are 2 entries with the same column x (crn).
The initially got entered with the same column y (status) as well. Our application has users update the column y (status). However now we have 2 rows one with a status of 'S' and one with a status of something other than 'S'. My goal:
DELETE everything from the table WHERE there is a duplicate CRN and the STATUS is S. I don't want to delete rows unless there is a duplicate, but if there is, I only want to delete the row with a status of 'S'. Also, I'd rather not delete both records if both have a status of S, but if I do, that isn't such a big deal because I will get the courses again in the next download.
I have started making a select statement to query the rows I want, but don't know how to do the ONLY SELECT IF DUPLICATE EXISTS part. I feel like I need to UNION or LEFT JOIN or something to only get records if a duplicate exists.
SELECT * FROM
cas_usuECourses
WHERE
crn IN (SELECT crn FROM cas_usuECourses GROUP BY crn having count(1) > 1)
AND status = 'S'
AND termCode = 201320
EDIT: Is there a way to say... the above, but if both dups have 'S' only delete one of them?
EDIT: I "think" this looks good to me. Any thoughts?
SELECT id FROM (
SELECT id, Row_Number() Over (Partition By crn ORDER BY id DESC) as ranking
FROM cas_usuECourses
WHERE status = 'S'
AND termCode = 201320
) as ranking
WHERE ranking = 1
I think this will give me all the ids where the status is 'S', and if there are two with 'S' this will give me the one that was created second. I found out that every entry in our termCode has duplicates, so... don't need to worry about checking for the duplicates.
If you could add one column to your table and fill it with distinct values, it would be trivial - you could target each row.
Otherwise, after your initial step, I would generally open a cursor on your subselect with status S to select only crn's where both statuses are 'S', and in each loop iteration delete top 1 record with appropriate crn. That way you can get rid of duplicate crn/status pairs.
i have loanTable that contain two field loan_id and status
loan_id status
==============
1 0
2 9
1 6
5 3
4 5
1 4 <-- How do I select this??
4 6
In this Situation i need to show the last Status of loan_id 1 i.e is status 4. Can please help me in this query.
Since the 'last' row for ID 1 is neither the minimum nor the maximum, you are living in a state of mild confusion. Rows in a table have no order. So, you should be providing another column, possibly the date/time when each row is inserted, to provide the sequencing of the data. Another option could be a separate, automatically incremented column which records the sequence in which the rows are inserted. Then the query can be written.
If the extra column is called status_id, then you could write:
SELECT L1.*
FROM LoanTable AS L1
WHERE L1.Status_ID = (SELECT MAX(Status_ID)
FROM LoanTable AS L2
WHERE L2.Loan_ID = 1);
(The table aliases L1 and L2 could be omitted without confusing the DBMS or experienced SQL programmers.)
As it stands, there is no reliable way of knowing which is the last row, so your query is unanswerable.
Does your table happen to have a primary id or a timestamp? If not then what you want is not really possible.
If yes then:
SELECT TOP 1 status
FROM loanTable
WHERE loan_id = 1
ORDER BY primaryId DESC
-- or
-- ORDER BY yourTimestamp DESC
I assume that with "last status" you mean the record that was inserted most recently? AFAIK there is no way to make such a query unless you add timestamp into your table where you store the date and time when the record was added. RDBMS don't keep any internal order of the records.
But if last = last inserted, that's not possible for current schema, until a PK addition:
select top 1 status, loan_id
from loanTable
where loan_id = 1
order by id desc -- PK
Use a data reader. When it exits the while loop it will be on the last row. As the other posters stated unless you put a sort on the query, the row order could change. Even if there is a clustered index on the table it might not return the rows in that order (without a sort on the clustered index).
SqlDataReader rdr = SQLcmd.ExecuteReader();
while (rdr.Read())
{
}
string lastVal = rdr[0].ToString()
rdr.Close();
You could also use a ROW_NUMBER() but that requires a sort and you cannot use ROW_NUMBER() directly in the Where. But you can fool it by creating a derived table. The rdr solution above is faster.
In oracle database this is very simple.
select * from (select * from loanTable order by rownum desc) where rownum=1
Hi if this has not been solved yet.
To get the last record for any field from a table the easiest way would be to add an ID to each record say pID. Also say that in your table you would like to hhet the last record for each 'Name', run the simple query
SELECT Name, MAX(pID) as LastID
INTO [TableName]
FROM [YourTableName]
GROUP BY [Name]/[Any other field you would like your last records to appear by]
You should now have a table containing the Names in one column and the last available ID for that Name.
Now you can use a join to get the other details from your primary table, say this is some price or date then run the following:
SELECT a.*,b.Price/b.date/b.[Whatever other field you want]
FROM [TableName] a LEFT JOIN [YourTableName]
ON a.Name = b.Name and a.LastID = b.pID
This should then give you the last records for each Name, for the first record run the same queries as above just replace the Max by Min above.
This should be easy to follow and should run quicker as well
If you don't have any identifying columns you could use to get the insert order. You can always do it like this. But it's hacky, and not very pretty.
select
t.row1,
t.row2,
ROW_NUMBER() OVER (ORDER BY t.[count]) AS rownum from (
select
tab.row1,
tab.row2,
1 as [count]
from table tab) t
So basically you get the 'natural order' if you can call it that, and add some column with all the same data. This can be used to sort by the 'natural order', giving you an opportunity to place a row number column on the next query.
Personally, if the system you are using hasn't got a time stamp/identity column, and the current users are using the 'natural order', I would quickly add a column and use this query to create some sort of time stamp/incremental key. Rather than risking having some automation mechanism change the 'natural order', breaking the data needed.
I think this code may help you:
WITH cte_Loans
AS
(
SELECT LoanID
,[Status]
,ROW_NUMBER() OVER(ORDER BY (SELECT 1)) AS RN
FROM LoanTable
)
SELECT LoanID
,[Status]
FROM LoanTable L1
WHERE RN = ( SELECT max(RN)
FROM LoanTable L2
WHERE L2.LoanID = L1.LoanID)