I am using the following sql in mySQL.
I don't understand why I get only one row as output.
select category, count(*) as counts
from table_1
where date_add between '2016-07-01' and '2016-07-31'
group by category
How many categories do you have in that date range would be the first thing you'd have to look for since you're grouping by categories. If you have 'n' number of categories within the specified date period, ideally you result set should have 'n' rows.
There is nothing wrong with your query per se. I tried the query with two categories and I do get the expected result set.
If you want all the categories in the result set irrespective of whether they are in the result set or not, then you can something of this sort :
select category, count(*) as counts
from tab
where date_add between '2016-07-01' and '2016-07-31'
group by category
union
select category, 0 as counts
from tab
where date_add not between '2016-07-01' and '2016-07-31'
group by category
Mind you this is a simple way of doing this but certainly not the best
This will show all the categories irrespective of the date peroids. You can see this here -> http://sqlfiddle.com/#!9/a3a9b/2
See this here -> http://sqlfiddle.com/#!9/bdb979/1
If you modify the data set in the fiddle to have three categories, then again, the result set returns the expected 3 rows.
See this here -> http://sqlfiddle.com/#!9/ec7a8/1
Again, its important to note that the categories have to be present within that date period for them to show up in the result set.
Hope this helps!!!
Just do some changes and try :
select category, count(*) as counts
from table_1
group by category
Use above code and see if it return more than one row , it means that you query is correct but only one row is matching below criteria ie:
where date_add between '2016-07-01' and '2016-07-31'
group by category
So Query returning one row, when your database table will have more data match this criteria htne it will return more row. thanks.
You are clearly only getting one row because only one category matches the where condition.
If you want all categories with the count of adds in that range, then use conditional aggregation:
select category,
sum(date_add between '2016-07-01' and '2016-07-31') as counts
from table_1
group by category;
Related
Why do I not get the same results when running the two queries? If I run the second one I get the course with the smallest amount of credits and when I run the first one I get the courses ordered by courseid
select min(credits), title, courseid
from course
group by title, courseid
select min(credits)
from course
An aggregation query is any query that has a group by or an aggregation function in the select.
An aggregation query returns one row per group, where a "group" is defined as the unique combination of values of the keys in the group by clause. If there is no group by clause, then all rows are taken to be a single group and one row is returned.
So, your first query returns one row for each combination of title and courseid in the course table. That row contains the minimum value of credits for that combination. If the course table has only one row per courseid, then the results are very similar to the contents of the table.
The second query returns one row overall, with the minimum number of credits of all rows.
If you want to get one row from with the minimum number of credits, then you don't want an aggregation query. Instead, you can use:
select c.*
from course c
order by c.credits
limit 1;
When you use a group by, you are using a sort of "filter", in the first query you group by title, then all the same titles are grouped by courseid, in the second you only select the minimum value of credits without filtering.
Take a look at a group by doc maybe with some graphical examples like this:
https://www.geeksforgeeks.org/sql-group-by/
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 have a MySQL table :
when i count the entries of month January using the query :
SELECT COUNT(*) AS entries FROM daily_call_reports WHERE Month(datetime_in)='01' AND emp_id='E0001'
I got the result 5.
But i want to count same date rows as one.. in the table there are two row of same date 2016-01-21. Now how to count these two row as one..
try this
select count(*)
from
(SELECT distinct id,client_id,emp_id,...., CAST(re.datetime_in AS DATE) AS DATE_PURCHASED
FROM daily_call_reports re
WHERE Month(datetime_in)='01' AND emp_id='E0001')
In a table, a column may contain many duplicate values; and sometimes
you only want to list the different (distinct) values.
The DISTINCT keyword can be used to return only distinct (different)
values.
You have to use DISTINCT for counting the different rows.
SELECT COUNT(DISTINCT DATE_FORMAT(`datetime_in`, '%Y-%m-%d')) as entries FROM daily_call_reports WHERE month(datetime_in)='01' AND emp_id='E0001'
Try this one and let me know.
I am working on a query that needs to output 'total engagements' by users in columns like 1 -eng column will display users who have one engagements, second column 2-eng which will display users who have done 2 engagements. Likewise 3eng, and so on. Note that the display should be like this. I have a engagement table which has userID. So I get distinct users like this
select count(distinct userID) from engagements
and I get engagements as
select count(*) from engagements
Engagements here refers to users who have either liked,replied,or shared the content
Please help. Thanks! I have used CASE and IF but unable to display in the below form
1eng 2eng 3eng
100 200 100
Consider returning the results in rows and pivoting them afterwards in your application.
To return the desired results in rows, you could use the following query:
SELECT
engagementCount,
COUNT(*) AS userCount
FROM (
SELECT
userID,
COUNT(*) AS engagementCount
FROM engagements
GROUP BY userID
) AS s
GROUP BY engagementCount
;
Basically, you first group the engagements rows by userID and get the row counts per userID. Afterwards, you use the counts as the grouping criterion and count how many users were found with that count.
If you insist on returning the columnar view in SQL, you'll need to resort to dynamic SQL because of the indefinite number of columns in the final result set. You'd probably need to store the results of the inner SELECT temporarily, scan it to build the list of count expressions for every engagementCount value and ultimately construct a query of this kind:
SELECT
COUNT(engagementCount = 1 OR NULL) AS `1eng`,
COUNT(engagementCount = 2 OR NULL) AS `2eng`,
COUNT(engagementCount = 3 OR NULL) AS `3eng`,
...
FROM temporary_storage
;
Or SUM(engagementCount = value) instead COUNT(engagementCount = value OR NULL). (For me, the latter expresses the intention more explicitly, hence why I've suggested it first, but, in case you happen to prefer the SUM technique, there should be no discernible difference in performance between the two. The OR NULL trick is explained here.)
I'm writing a query where I group a selection of rows to find the MIN value for one of the columns.
I'd also like to return the other column values associated with the MIN row returned.
e.g
ID QTY PRODUCT TYPE
--------------------
1 2 Orange Fruit
2 4 Banana Fruit
3 3 Apple Fruit
If I GROUP this table by the column 'TYPE' and select the MIN qty, it won't return the corresponding product for the MIN row which in the case above is 'Apple'.
Adding an ORDER BY clause before grouping seems to solve the problem. However, before I go ahead and include this query in my application I'd just like to know whether this method will always return the correct value. Is this the correct approach? I've seen some examples where subqueries are used, however I have also read that this inefficient.
Thanks in advance.
Adding an ORDER BY clause before grouping seems to solve the problem. However, before I go ahead and include this query in my application I'd just like to know whether this method will always return the correct value. Is this the correct approach? I've seen some examples where subqueries are used, however I have also read that this inefficient.
No, this is not the correct approach.
I believe you are talking about a query like this:
SELECT product.*, MIN(qty)
FROM product
GROUP BY
type
ORDER BY
qty
What you are doing here is using MySQL's extension that allows you to select unaggregated/ungrouped columns in a GROUP BY query.
This is mostly used in the queries containing both a JOIN and a GROUP BY on a PRIMARY KEY, like this:
SELECT order.id, order.customer, SUM(price)
FROM order
JOIN orderline
ON orderline.order_id = order.id
GROUP BY
order.id
Here, order.customer is neither grouped nor aggregated, but since you are grouping on order.id, it is guaranteed to have the same value within each group.
In your case, all values of qty have different values within the group.
It is not guaranteed from which record within the group the engine will take the value.
You should do this:
SELECT p.*
FROM (
SELECT DISTINCT type
FROM product p
) pd
JOIN p
ON p.id =
(
SELECT pi.id
FROM product pi
WHERE pi.type = pd.type
ORDER BY
type, qty, id
LIMIT 1
)
If you create an index on product (type, qty, id), this query will work fast.
It's difficult to follow you properly without an example of the query you try.
From your comments I guess you query something like,
SELECT ID, COUNT(*) AS QTY, PRODUCT_TYPE
FROM PRODUCTS
GROUP BY PRODUCT_TYPE
ORDER BY COUNT(*) DESC;
My advice, you group by concept (in this case PRODUCT_TYPE) and you order by the times it appears count(*). The query above would do what you want.
The sub-queries are mostly for sorting or dismissing rows that are not interested.
The MIN you look is not exactly a MIN, it is an occurrence and you want to see first the one who gives less occurrences (meaning appears less times, I guess).
Cheers,