I have a mysql query
select count(*) as TotalCount,
pd.Product_Modified_Date,
psc.Product_Subcategory_Name,
pd.Product_Image_URL
from product_subcategory psc
inner join product_details pd on psc.Product_Subcategory_ID = pd.Product_Subcategory_Reference_ID
where pd.Product_Status = 0 and
psc.Product_Subcategory_Status = 0
group by psc.Product_Subcategory_Name
order by pd.Product_Modified_Date desc
In my product_details table have new image urls. But i could not get it by the above query.
How can i do it?
You are grouping by one column, Product_Subcategory_Name, but you have other columns Product_Image_URL and Product_Modified_Date in your select-list.
If you have cases where the group has multiple rows (which you do, since the count is 14 or more in each group), MySQL can only present one value for the Product_Image_URL. So it picks some row in the group, and uses the value in that row. The URL value for all other rows in the group is ignored.
To fix this, you must group by all columns in your select-list that are not part of an aggregate function. Any column you don't want to use to form a new group must go into an aggregate function.
Roland Bouman wrote an excellent blog detailing how to use GROUP BY properly: http://rpbouman.blogspot.com/2007/05/debunking-group-by-myths.html
Combining GROUP BY and ORDER BY is problematic and your problem is most likely covered in another question on Stack Exchange : MySQL wrong results with GROUP BY and ORDER BY
Related
My database is called: (training_session)
I try to print out some information from my data, but I do not want to have any duplicates. I do get it somehow, may someone tell me what I do wrong?
SELECT DISTINCT athlete_id AND duration FROM training_session
SELECT DISTINCT athlete_id, duration FROM training_session
It works perfectly if i use only one column, but when I add another. it does not work.
I think you misunderstood the use of DISTINCT.
There is big difference between using DISTINCT and GROUP BY.
Both have some sort of goal, but they have different purpose.
You use DISTINCT if you want to show a series of columns and never repeat. That means you dont care about calculations or group function aggregates. DISTINCT will show different RESULTS if you keep adding more columns in your SELECT (if the table has many columns)
You use GROUP BY if you want to show "distinctively" on a certain selected columns and you use group function to calculate the data related to it. Therefore you use GROUP BY if you want to use group functions.
Please check group functions you can use in this link.
https://dev.mysql.com/doc/refman/8.0/en/group-by-functions.html
EDIT 1:
It seems like you are trying to get the "latest" of a certain athlete, I'll assume the current scenario if there is no ID.
Here is my alternate solution:
SELECT a.athlete_id ,
( SELECT b.duration
FROM training_session as b
WHERE b.athlete_id = a.athlete_id -- connect
ORDER BY [latest column to sort] DESC
LIMIT 1
) last_duration
FROM training_session as a
GROUP BY a.athlete_id
ORDER BY a.athlete_id
This syntax is called IN-SELECT subquery. With the help of LIMIT 1, it shows the topmost record. In-select subquery must have 1 record to return or else it shows error.
MySQL's DISTINCT clause is used to filter out duplicate recordsets.
If your query was SELECT DISTINCT athlete_id FROM training_session then your output would be:
athlete_id
----------
1
2
3
4
5
6
As soon as you add another column to your query (in your example, the column called duration) then each record resulting from your query are unique, hence the results you're getting. In other words the query is working correctly.
I'm trying to get the highest version within a group. My query:
SELECT
rubric_id,
max(version) as version,
group_id
FROM
rubrics
WHERE
client_id = 1
GROUP BY
group_id
The Data:
The Results:
The rubric of ID 2 does not have a version of 2, why is this being mismatched? What do I need to do to correct this?
Edit, not a duplicate:
This is not a duplicate of SQL Select only rows with Max Value on a Column , which is a post I have read and referenced before writing this. My question is not how to find the max, my question is why is the version not matched to the correct ID
MySQL is confusing you by letting you get away with having a column in your select that isn't in your group by. To resolve the issue, make sure you don't select any field that isn't in the group by.
Instead of trying to get everything in one statement, you will need to use a subquery to find the max_version_id and then join to it.
SELECT T.*
FROM rubrics T
JOIN
(
SELECT
group_id,
max(version) as max_version
FROM
rubrics
GROUP BY
group_id
) dedupe
on T.group_id = dedupe.group_id
and T.version_id = dedupe.max_version_id
WHERE
T.client_id = 1
Edit: So MySQL allows it, but I don't think it's a good practise to use it.
You are trying to query non-aggregated data from an aggregated query. You should not do that.
A GROUP BY takes the field it should make group of rows with (in your case, what you say with your GROUP BY is: give me a result per different group_id) and gives a result (the aggregated data) based on the grouping.
Here, you try to access non aggregated data (rubric_id in your case). For some reason, the query does not crash and picks a "random" id in your aggregated data.
Is it possible to have count in the select clause with a group by which is suppressed in the count? I need the count to ignore the group by clause
I got this query which is counting the total entries. The query is generic generated and therefore I can't make any comprehensive changes like subqueries etc.
In some specific cases a group by is needed to retrieve the correct rows and because of this the group by can't be removed
SELECT count(dv.id) num
FROM `data_voucher` dv
LEFT JOIN `data_voucher_enclosure` de ON de.data_voucher_id=dv.id
WHERE IF(de.id IS NULL,0,1)=0
GROUP BY dv.id
Is it possible to have count in the select clause with a group by which is suppressed in the count? I need the count to ignore the group by clause
well, the answer to your question is simply you can't have an aggregate that works on all the results, while having a group by statement. That's the whole purpose of the group by to create groups that change the behaviour of aggregates:
The GROUP BY clause causes aggregations to occur in groups (naturally) for the columns you name.
cf this blog post which is only the first result I found on google on this topic.
You'd need to redesign your query, the easiest way being to create a subquery, or a hell of a jointure. But without the schema and a little context on what you want this query to do, I can't give you an alternative that works.
I just can tell you that you're trying to use a hammer to tighten a screw...
Have found an alternative where COUNT DISTINCT is used
SELECT count(distinct dv.id) num
FROM `data_voucher` dv
LEFT JOIN `data_voucher_enclosure` de ON de.data_voucher_id=dv.id
WHERE IF(de.id IS NULL,0,1)=0
Please consider the following query:
SELECT artist.id, COUNT(DISTINCT artist$styles.v_id)
FROM artist
LEFT JOIN artist$styles ON artist$styles.p_id = artist.id
This is the result I get:
id count
1 4
The questions are:
How come it's only selecting one row from the artist table, when there are 4 rows in it and there are no WHERE, HAVING, LIMIT or GROUP BY clauses applied to the query?
There are only three records in artist$styles having p_id of value 1, why is it counting 4?
Why if I add a GROUP BY clause to it I get the correct results?
SELECT artist.id, COUNT(DISTINCT artist$styles.v_id)
FROM artist
LEFT JOIN artist$styles ON artist$styles.p_id = artist.id
GROUP BY artist.id
----
id count
1 3
2 1
3 3
4 1
This all just doesn't make sense to me. Could this be a bug of MySQL? I'm running Community 5.5.25a
As stated in the manual page on aggregate functions (of which COUNT() is one):
If you use a group function in a statement containing no GROUP BY clause, it is equivalent to grouping on all rows.
As stated in the manual page on GROUP BY with hidden columns:
The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate.
In other words, the server has chosen one (indeterminate) value for column p_id, which happens in this case to be the value 1, whilst it has properly aggregated and counted the result for the COUNT() function.
Because you are then grouping on the correct columns, rather than on all rows.
It's not a bug; this behaviour is documented and by design.
It is a possible bug in Mysql. All non aggeregate columns should be included in Group by clause. MySQL does not force this and the result is unpredictable and hard to debug. As a rule always include all non-aggregate columns in the Group by clause. This is how all RDBMSs work
Count Function return single row result if you are not using group by clause and that's why its returning one row.
2.In your output
id count
1 4
4 is total no of results in that table not result for id 1.and it display in front of 1 because only one row produce.
3.when you use group by then a group of that column value is created that's why you get that output.
And finally its not a bug.Mysql provide a proper documentation for that you can read on mysql site.
Well how mysql works when using more than a column on group by like:
select
a.nome,
b.tb2_id,
count(c.tb2_id) as saida
from tb1 a
left join tb2 b on a.tb1_id = b.tb1_id
left join tb3 c on b.tb2_id = c.tb2_id
group by a.tb1_id, b.tb2_id
order by a.tb1_id desc
how mysql knows which column it will use to group the result set?
i thought that it would do it in order but i changed the group by to 'b.tb2_id,a.tb1_id' but it doesn't make any change, same result.
group by a.tb1_id, b.tb2_id means group by the pair of a.tb1_id and b.tb2_id, both a.tb1_id and b.tb2_id need to be same to be treated as a group.
Only the order by clause affects the order of rows.
The group by clause affects data aggregation. mysql is special in that, unlike most other databases, it allows the data to be grouped by columns not selected, and further allows non-grouped by columns to be non-aggregated. In this case of this last option being exercised (as in your query - a.nome is not being grouped by), mysql returns the first row encountered for each group. All other databases I know would throw an SQL syntax exception if you tried to execute this query.