Here is a snippet of the table I am working with:
My goal is to only select one of these highlighted rows (doesn't matter which). They have duplicate approval, amount, and type columns but unique numbers in the "test" column. I would like to select all of these with the exception of one of the highlighted rows.
Any help would be greatly appreciated.
This is a "greatest-n-per-group" problem. If you really don't care which row you get, you can use ##SPID:
;WITH cte AS
(
SELECT APPROVAL, amount, type, test,
rn = ROW_NUMBER() OVER
(
PARTITION BY APPROVAL, amount, type -- this defines grouping
ORDER BY ##SPID -- this says "I don't care what order"
)
FROM dbo.source_table
)
SELECT APPROVAL, amount, type, test
FROM cte
WHERE rn = 1;
If you care, you can add different criteria, e.g.
ORDER BY test DESC -- for the biggest value of test
Or
ORDER BY test -- for the smallest value of test
If you don't care which test you get, why does it even get returned?
Related
I'm working with a table that has multiple rows for each order id (e.g. variations in spelling for addresses and different last_updated dates), that in theory shouldn't be there (not my doing). I want to select just 1 row for each id and so far I figured I can do that using partitioning like so:
SELECT dp.order_id,
MAX(cr.updated_at) OVER(PARTITION BY dp.order_id) AS updated_at
but I have seen other queries which only use MAX and list every other column like so
SELECT dp.order_id,
MAX(dp.ship_address) as address,
MAX(cr.updated_at) as updated_at
etc...
this solution looks more neat but I can't get it to work (still returns multiple rows per single order_id). What am I doing wrong?
If you want one row per order_id, then window functions are not sufficient. They don't filter the data. You seem to want the most recent row. A typical method uses row_number():
select t.*
from (select t.*,
row_number() over (partition by order_id order by created_at desc) as seqnum
from t
) t
where seqnum = 1;
You can also use aggregation:
select order_id, max(ship_address), max(created_at)
from t
group by order_id;
However, the ship_address may not be from the most recent row and that is usually not desirable. You can tweak this using keep syntax:
select order_id,
max(ship_address) keep (dense_rank first order by created_at desc),
max(created_at)
from t
group by order_id;
However, this gets cumbersome for a lot of columns.
The 2nd "solution" doesn't care about values in other columns - it selects their MAX values. It means that you'd get ORDER_ID and - possibly - "mixed" values for other columns, i.e. those ADDRESS and UPDATED_AT might belong to different rows.
If that's OK with you, then go for it. Otherwise, you'll have to select one MAX row (using e.g. row_number analytic function), and fetch data that is related only to it (i.e. doesn't "mix" values from different rows).
Also, saying that you
can't get it to work (still returns multiple rows per single order_id)
is kind of difficult to believe. The way you put it, it can't be true.
I have a table containing 2 columns for example. First column has unique values and the second column duplicates. Is there any way for me to select the first unique value only from the first column in relation to the second column?
For example: The results should get: Apple, Tire, and Fork only since they are the first results of the second column (category)
Details
Category
Apple
Fruits
Banana
Fruits
Tire
Car
Engine
Car
Fork
Silverware
Spoon
Silverware
Knife
Silverware
Usually we can use windowing functions like ROW_NUMBER() to simplify these types of queries, however your requested record set does not have a natural sort order that could be used that would result in the output you are expecting.
The following is a simple solution that uses ROW_NUMBER(), however it will not result as you have requested:
SELECT Category, Details
FROM
(
SELECT Category, Details, row_number() over (partition by category order by details) as rn
FROM SpecificResults
) as numberedRecords
WHERE rn = 1;
Results:
Category
Details
Car
Engine
Fruits
Apple
Silverware
Fork
You requested an output of: Apple, Tire, and Fork
The next query might produce the expected output, because we do not specify the sort, however due to this the output is non-deterministic, that is we cannot gaurantee it, due to database internals over time or even after instantaneously repeated queries the result might be different.
There are many discussions on non-deterministic queries in SQL, have a read through this thread on SO: The order of a SQL Select statement without Order By clause
SELECT Category, details.Details
FROM SpecificResults byCategory
CROSS APPLY (
SELECT TOP 1 Details
FROM SpecificResults lookup
WHERE lookup.Category = byCategory.Category
--ORDER BY Details
) as details
GROUP BY Category, details.Details;
Results in:
Category
Details
Car
Tire
Fruits
Apple
Silverware
Fork
I have setup a SQL Fiddle for you to explore this further: http://sqlfiddle.com/#!18/68530/12
Real World Solution
In the real world, your dataset will have a primary key, and in many cases that key value might be incrementally tallied, if not there may be other columns that could be used to determine the sort order that will match your expected results.
Assuming that your dataset has an integer column called Id and that column is an Identity column, then a simple change to the original query using ROW_NUMBER() will achieve the desired result:
SELECT Category, Details
FROM
(
SELECT Category, Details, row_number() over (partition by category order by Id) as rn
FROM OrderedResults
) as numberedRecords
WHERE rn = 1;
I have updated the SQL Fiddle with this variation: http://sqlfiddle.com/#!18/3f7bd/2
If there is a Created date or some other Timestamp or DateTime based column in your recordset then you you could consider those as candidates for your ORDER BY clause.
SQL table represent unordered sets. There is no "first" value unless a column specifies the value. If you have such a column, then you can use row_number():
select t.*
from (select t.*,
row_number() over (partition by category order by <ordering col>) as seqnum
from t
) t
where seqnum = 1;
If you don't have such a column, then you simply cannot ask such a question in a relational database. The data doesn't support the question.
If I understand it correctly, try this -
select category, details from ( select *, row number() over (partition by category order by details) as rn from tablename) where rn = 1
Let's say I have an Oracle table with measurements in different categories:
CREATE TABLE measurements (
category CHAR(8),
value NUMBER,
error NUMBER,
created DATE
)
Now I want to find the "best" row in each category, where "best" is defined like this:
It has the lowest errror.
If there are multiple measurements with the same error, the one that was created most recently is the considered to be the best.
This is a variation of the greatest N per group problem, but including two columns instead of one. How can I express this in SQL?
Use ROW_NUMBER:
WITH cte AS (
SELECT m.*, ROW_NUMBER() OVER (PARTITION BY category ORDER BY error, created DESC) rn
FROM measurements m
)
SELECT category, value, error, created
FROM cte
WHERE rn = 1;
For a brief explanation, the PARTITION BY clause instructs the DB to generate a separate row number for each group of records in the same category. The ORDER BY clause places those records with the smallest error first. Should two or more records in the same category be tied with the lowest error, then the next sorting level would place the record with the most recent creation date first.
I am in need of some help in showing all rows that have different dates based on a unique number in a table, below is a dummy table I have created which is similar to the one I need to analyse.
What I need to show is all rows where the expected date has changed but the VisitNumber is the same, I have highlighted four rows as an example;
The First Entry Was Null
The Second Entry Was 25/04/16
Then the final entry was 24/06/16
Sometimes the there could be an entry where the Visit Number is the same as a row that has changed but the Expected Dates are the same, in this case, I need to ignore them rows.
Additionally, there could be multiple entries with the same Unique ID but the Visit Number is different.
This should do what you want:
SELECT UniqueID, VisitNumber, Min(ExpectedDate), Max(ExpectedDate)
FROM Table
GROUP BY uniqueID, VisitNumber
HAVING Min(ExpectedDate) <> Max(ExpectedDate)
I think you can do this using window functions:
select t.*
from (select t.*,
min(ExpectedDate) over (partition by UniqueId, VisitNumber) as mined,
max(ExpectedDate) over (partition by UniqueId, VisitNumber) as maxed
from t
) t
where mined <> maxed and ExpectedDate is not null;
Say that I have some SELECT statement:
SELECT id, name FROM people
ORDER BY name ASC;
I have a few million rows in the people table and the ORDER BY clause can be much more complex than what I have shown here (possibly operating on a dozen columns).
I retrieve only a small subset of the rows (say rows 1..11) in order to display them in the UI. Now, I would like to solve following problems:
Find the number of a row with a given id.
Display the 5 items before and the 5 items after a row with a given id.
Problem 2 is easy to solve once I have solved problem 1, as I can then use something like this if I know that the item I was looking for has row number 1000 in the sorted result set (this is the Firebird SQL dialect):
SELECT id, name FROM people
ORDER BY name ASC
ROWS 995 TO 1005;
I also know that I can find the rank of a row by counting all of the rows which come before the one I am looking for, but this can lead to very long WHERE clauses with tons of OR and AND in the condition. And I have to do this repeatedly. With my test data, this takes hundreds of milliseconds, even when using properly indexed columns, which is way too slow.
Is there some means of achieving this by using some SQL:2003 features (such as row_number supported in Firebird 3.0)? I am by no way an SQL guru and I need some pointers here. Could I create a cached view where the result would include a rank/dense rank/row index?
Firebird appears to support window functions (called analytic functions in Oracle). So you can do the following:
To find the "row" number of a a row with a given id:
select id, row_number() over (partition by NULL order by name, id)
from t
where id = <id>
This assumes the id's are unique.
To solve the second problem:
select t.*
from (select id, row_number() over (partition by NULL order by name, id) as rownum
from t
) t join
(select id, row_number() over (partition by NULL order by name, id) as rownum
from t
where id = <id>
) tid
on t.rownum between tid.rownum - 5 and tid.rownum + 5
I might suggest something else, though, if you can modify the table structure. Most databases offer the ability to add an auto-increment column when a row is inserted. If your records are never deleted, this can server as your counter, simplifying your queries.