For some reason, I am not able to use GROUP BY but I can use DISTINCT like this:
SELECT DISTINCT(name), id, email from myTable
However above query lists people with same name also because I am selecting more than one columns whereas I want to select only unique names. Is there someway to get unique names without using GROUP BY ?
Although using GROUP BY is the most direct way, you can do other things if it is prohibited. For example, you can use NOT EXISTS with a subquery, like this:
SELECT name, id, email
FROM myTable t
WHERE NOT EXISTS (SELECT * FROM myTable tt WHERE tt.name=t.name AND tt.id < t.id)
This query uses NOT EXISTS to eliminate rows with the same name and IDs higher than the one selected. Note that since this query must pick a single user per name, it may eliminate some users based on their ID, which is a rather arbitrary criterion.
I would like to query a DB2 table and get all the results of a query in addition to all of the rows returned by the select statement in a separate column.
E.g., if the table contains columns 'id' and 'user_id', assuming 100 rows, the result of the query would appear in this format: (id) | (user_id) | 100.
I do not wish to use a 'group by' clause in the query. (Just in case you are confused about what i am asking) Also, I could not find an example here: http://mysite.verizon.net/Graeme_Birchall/cookbook/DB2V97CK.PDF.
Also, if there is a more efficient way of getting both these results (values + count), I would welcome any ideas. My environment uses zend framework 1.x, which does not have an ODBC adapter for DB2. (See issue http://framework.zend.com/issues/browse/ZF-905.)
If I understand what you are asking for, then the answer should be
select t.*, g.tally
from mytable t,
(select count(*) as tally
from mytable
) as g;
If this is not what you want, then please give an actual example of desired output, supposing there are 3 to 5 records, so that we can see exactly what you want.
You would use window/analytic functions for this:
select t.*, count(*) over() as NumRows
from table t;
This will work for whatever kind of query you have.
You cannot (should not) put non-aggregates in the SELECT line of a GROUP BY query.
I would however like access the one of the non-aggregates associated with the max. In plain english, I want a table with the oldest id of each kind.
CREATE TABLE stuff (
id int,
kind int,
age int
);
This query gives me the information I'm after:
SELECT kind, MAX(age)
FROM stuff
GROUP BY kind;
But it's not in the most useful form. I really want the id associated with each row so I can use it in later queries.
I'm looking for something like this:
SELECT id, kind, MAX(age)
FROM stuff
GROUP BY kind;
That outputs this:
SELECT stuff.*
FROM
stuff,
( SELECT kind, MAX(age)
FROM stuff
GROUP BY kind) maxes
WHERE
stuff.kind = maxes.kind AND
stuff.age = maxes.age
It really seems like there should be a way to get this information without needing to join. I just need the SQL engine to remember the other columns when it's calculating the max.
You can't get the Id of the row that MAX found, because there might not be only one id with the maximum age.
You cannot (should not) put non-aggregates in the SELECT line of a GROUP BY query.
You can, and have to, define what you are grouping by for the aggregate function to return the correct result.
MySQL (and SQLite) decided in their infinite wisdom that they would go against spec, and allow queries to accept GROUP BY clauses missing columns quoted in the SELECT - it effectively makes these queries not portable.
It really seems like there should be a way to get this information without needing to join.
Without access to the analytic/ranking/windowing functions that MySQL doesn't support, the self join to a derived table/inline view is the most portable means of getting the result you desire.
I think it's tempting indeed to ask the system to solve the problem in one pass rather than having to do the job twice (find the max, and the find the corresponding id). You can do using CONCAT (as suggested in Naktibalda refered article), not sure that would be more effeciant
SELECT MAX( CONCAT( LPAD(age, 10, '0'), '-', id)
FROM STUFF1
GROUP BY kind;
Should work, you have to split the answer to get the age and the id.
(That's really ugly though)
In recent databases you can use sum() over (parition by ...) to solve this problem:
select id, kind, age as max_age from (
select id, kind, age, max(age) over (partition by kind) as mage
from table)
where age = mage
This can then be single pass
PostgesSQL's DISTINCT ON will be useful here.
SELECT DISTINCT ON (kind) kind, id, age
FROM stuff
ORDER BY kind, age DESC;
This groups by kind and returns the first row in the ordered format. As we have ordered by age in descending order, we will get the row with max age for kind.
P.S. columns in DISTINCT ON should appear first in order by
You have to have a join because the aggregate function max retrieves many rows and chooses the max.
So you need a join to choose the one that the agregate function has found.
To put it a different way how would you expect the query to behave if you replaced max with sum?
An inner join might be more efficient than your sub query though.
i tried using this query:
"SELECT * FROM guests WHERE event_id=".$id." GROUP BY member_id;"
and I'm getting this error:
ERROR: column "guests.id" must appear in the GROUP BY clause or be used in an aggregate function
can anyone explain how i can work around this?
You can't Group By without letting the Select know what to take, and how to group.
Try
SELECT guests.member_id FROM guests WHERE event_id=".$id." GROUP BY member_id;
IF you need to get more info from this table about the guests, you'll need to add it to the Group By.
Plus, it seems like your select should actually be
SELECT guests.id FROM guests WHERE event_id=".$id." GROUP BY id;
Each of the columns used in a group by query needs to be specifically called out (ie, don't do SELECT * FROM ...), as you need to use them in some sort of aggregate function (min/max/sum/avg/count/etc) or be part of the group by clause.
For example:
SELECT instrument, detector, min(date_obs), max(date_obs)
FROM observations
WHERE observatory='SOHO'
GROUP BY instrument, detector;
Is there a better way of doing a query like this:
SELECT COUNT(*)
FROM (SELECT DISTINCT DocumentId, DocumentSessionId
FROM DocumentOutputItems) AS internalQuery
I need to count the number of distinct items from this table but the distinct is over two columns.
My query works fine but I was wondering if I can get the final result using just one query (without using a sub-query)
If you are trying to improve performance, you could try creating a persisted computed column on either a hash or concatenated value of the two columns.
Once it is persisted, provided the column is deterministic and you are using "sane" database settings, it can be indexed and / or statistics can be created on it.
I believe a distinct count of the computed column would be equivalent to your query.
Edit: Altered from the less-than-reliable checksum-only query
I've discovered a way to do this (in SQL Server 2005) that works pretty well for me and I can use as many columns as I need (by adding them to the CHECKSUM() function). The REVERSE() function turns the ints into varchars to make the distinct more reliable
SELECT COUNT(DISTINCT (CHECKSUM(DocumentId,DocumentSessionId)) + CHECKSUM(REVERSE(DocumentId),REVERSE(DocumentSessionId)) )
FROM DocumentOutPutItems
What is it about your existing query that you don't like? If you are concerned that DISTINCT across two columns does not return just the unique permutations why not try it?
It certainly works as you might expect in Oracle.
SQL> select distinct deptno, job from emp
2 order by deptno, job
3 /
DEPTNO JOB
---------- ---------
10 CLERK
10 MANAGER
10 PRESIDENT
20 ANALYST
20 CLERK
20 MANAGER
30 CLERK
30 MANAGER
30 SALESMAN
9 rows selected.
SQL> select count(*) from (
2 select distinct deptno, job from emp
3 )
4 /
COUNT(*)
----------
9
SQL>
edit
I went down a blind alley with analytics but the answer was depressingly obvious...
SQL> select count(distinct concat(deptno,job)) from emp
2 /
COUNT(DISTINCTCONCAT(DEPTNO,JOB))
---------------------------------
9
SQL>
edit 2
Given the following data the concatenating solution provided above will miscount:
col1 col2
---- ----
A AA
AA A
So we to include a separator...
select col1 + '*' + col2 from t23
/
Obviously the chosen separator must be a character, or set of characters, which can never appear in either column.
To run as a single query, concatenate the columns, then get the distinct count of instances of the concatenated string.
SELECT count(DISTINCT concat(DocumentId, DocumentSessionId)) FROM DocumentOutputItems;
In MySQL you can do the same thing without the concatenation step as follows:
SELECT count(DISTINCT DocumentId, DocumentSessionId) FROM DocumentOutputItems;
This feature is mentioned in the MySQL documentation:
http://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html#function_count-distinct
How about something like:
select count(*)
from
(select count(*) cnt
from DocumentOutputItems
group by DocumentId, DocumentSessionId) t1
Probably just does the same as you are already though but it avoids the DISTINCT.
Some SQL databases can work with a tuple expression so you can just do:
SELECT COUNT(DISTINCT (DocumentId, DocumentSessionId))
FROM DocumentOutputItems;
If your database doesn't support this, it can be simulated as per #oncel-umut-turer's suggestion of CHECKSUM or other scalar function providing good uniqueness e.g.
COUNT(DISTINCT CONCAT(DocumentId, ':', DocumentSessionId)).
MySQL specifically supports COUNT(DISTINCT expr, expr, ...) which is non-SQL standard syntax. It also notes In standard SQL, you would have to do a concatenation of all expressions inside COUNT(DISTINCT ...).
A related use of tuples is performing IN queries such as:
SELECT * FROM DocumentOutputItems
WHERE (DocumentId, DocumentSessionId) in (('a', '1'), ('b', '2'));
Here's a shorter version without the subselect:
SELECT COUNT(DISTINCT DocumentId, DocumentSessionId) FROM DocumentOutputItems
It works fine in MySQL, and I think that the optimizer has an easier time understanding this one.
Edit: Apparently I misread MSSQL and MySQL - sorry about that, but maybe it helps anyway.
I have used this approach and it has worked for me.
SELECT COUNT(DISTINCT DocumentID || DocumentSessionId)
FROM DocumentOutputItems
For my case, it provides correct result.
There's nothing wrong with your query, but you could also do it this way:
WITH internalQuery (Amount)
AS
(
SELECT (0)
FROM DocumentOutputItems
GROUP BY DocumentId, DocumentSessionId
)
SELECT COUNT(*) AS NumberOfDistinctRows
FROM internalQuery
If you're working with datatypes of fixed length, you can cast to binary to do this very easily and very quickly. Assuming DocumentId and DocumentSessionId are both ints, and are therefore 4 bytes long...
SELECT COUNT(DISTINCT CAST(DocumentId as binary(4)) + CAST(DocumentSessionId as binary(4)))
FROM DocumentOutputItems
My specific problem required me to divide a SUM by the COUNT of the distinct combination of various foreign keys and a date field, grouping by another foreign key and occasionally filtering by certain values or keys. The table is very large, and using a sub-query dramatically increased the query time. And due to the complexity, statistics simply wasn't a viable option. The CHECKSUM solution was also far too slow in its conversion, particularly as a result of the various data types, and I couldn't risk its unreliability.
However, using the above solution had virtually no increase on the query time (comparing with using simply the SUM), and should be completely reliable! It should be able to help others in a similar situation so I'm posting it here.
if you had only one field to "DISTINCT", you could use:
SELECT COUNT(DISTINCT DocumentId)
FROM DocumentOutputItems
and that does return the same query plan as the original, as tested with SET SHOWPLAN_ALL ON. However you are using two fields so you could try something crazy like:
SELECT COUNT(DISTINCT convert(varchar(15),DocumentId)+'|~|'+convert(varchar(15), DocumentSessionId))
FROM DocumentOutputItems
but you'll have issues if NULLs are involved. I'd just stick with the original query.
Hope this works i am writing on prima vista
SELECT COUNT(*)
FROM DocumentOutputItems
GROUP BY DocumentId, DocumentSessionId
I wish MS SQL could also do something like COUNT(DISTINCT A, B). But it can't.
At first JayTee's answer seemed like a solution to me bu after some tests CHECKSUM() failed to create unique values. A quick example is, both CHECKSUM(31,467,519) and CHECKSUM(69,1120,823) gives the same answer which is 55.
Then I made some research and found that Microsoft does NOT recommend using CHECKSUM for change detection purposes. In some forums some suggested using
SELECT COUNT(DISTINCT CHECKSUM(value1, value2, ..., valueN) + CHECKSUM(valueN, value(N-1), ..., value1))
but this is also not conforting.
You can use HASHBYTES() function as suggested in TSQL CHECKSUM conundrum. However this also has a small chance of not returning unique results.
I would suggest using
SELECT COUNT(DISTINCT CAST(DocumentId AS VARCHAR)+'-'+CAST(DocumentSessionId AS VARCHAR)) FROM DocumentOutputItems
I found this when I Googled for my own issue, found that if you count DISTINCT objects, you get the correct number returned (I'm using MySQL)
SELECT COUNT(DISTINCT DocumentID) AS Count1,
COUNT(DISTINCT DocumentSessionId) AS Count2
FROM DocumentOutputItems
How about this,
Select DocumentId, DocumentSessionId, count(*) as c
from DocumentOutputItems
group by DocumentId, DocumentSessionId;
This will get us the count of all possible combinations of DocumentId, and DocumentSessionId
It works for me. In oracle:
SELECT SUM(DECODE(COUNT(*),1,1,1))
FROM DocumentOutputItems GROUP BY DocumentId, DocumentSessionId;
In jpql:
SELECT SUM(CASE WHEN COUNT(i)=1 THEN 1 ELSE 1 END)
FROM DocumentOutputItems i GROUP BY i.DocumentId, i.DocumentSessionId;
I had a similar question but the query I had was a sub-query with the comparison data in the main query. something like:
Select code, id, title, name
(select count(distinct col1) from mytable where code = a.code and length(title) >0)
from mytable a
group by code, id, title, name
--needs distinct over col2 as well as col1
ignoring the complexities of this, I realized I couldn't get the value of a.code into the subquery with the double sub query described in the original question
Select count(1) from (select distinct col1, col2 from mytable where code = a.code...)
--this doesn't work because the sub-query doesn't know what "a" is
So eventually I figured out I could cheat, and combine the columns:
Select count(distinct(col1 || col2)) from mytable where code = a.code...
This is what ended up working
This code uses distinct on 2 parameters and provides count of number of rows specific to those distinct values row count. It worked for me in MySQL like a charm.
select DISTINCT DocumentId as i, DocumentSessionId as s , count(*)
from DocumentOutputItems
group by i ,s;
You can just use the Count Function Twice.
In this case, it would be:
SELECT COUNT (DISTINCT DocumentId), COUNT (DISTINCT DocumentSessionId)
FROM DocumentOutputItems