I have the following code and for some reason the left outer join is not producing the correct results.
Dim StudentCourseList = From stud in students
Group Join cour in courses
on stud.id equals cour.id into joinedlist = Group
From j in joinedlist.defaultifempty
select stud
The count before the left outer join of students is 12 and courses is 4. However, after the join, the student count is 14 due to some reason. It should be 12 if not less than 12. Am I doing something wrong here?
Edit - The query is fine. The problems in with the courses list. It has repeating/duplicate items in it. The question now would be how to get the distinct results?
You have not posted your table data. So this is just an assumption.
Why count is 14?
If one student have more than one course for example having two course than it would result in two records.
Use group by and then select first like below query.
var query = from person in people
join pet in pets on person.Id equals pet.OwnerId into gj
from subpet in gj.DefaultIfEmpty()
group person by person.Id into temp1
select temp1.First();
here is working fiddle. sorry I am not a vb guy so posted answer in c#.
Related
I'm generating a table which will in turn be used to format several different statistics and graphs.
Some columns of this table, are a result of subqueries which use a nearly identical structure. My query works, but it is very inefficient even in a simplified example like the following one.
SELECT
o.order,
o.date,
c.clienttype,
o.producttype,
(SELECT date FROM orders_interactions LEFT JOIN categories WHERE order=o.order AND category=3) as completiondate,
(SELECT amount FROM orders_interactions LEFT JOIN categories WHERE order=o.order AND category=3) as amount,
DATEDIFF((select date from orders_interactions LEFT JOIN categories where order=o.order AND category=3),o.date) as elapseddays
FROM orders o
LEFT JOIN clients c ON c.idClient=o.idClient
Being this a simplified example of a much more complex query, I would like to know the recommended approaches for a query like this one, taking into account query times, and readability.
As the example shows, I had to repeat a subquery (the one with date), just to calculate a datediff, since I cannot directly reference the column 'completiondate'
Thank you
You can try a left join.
SELECT o.order,
o.date,
o.producttype,
oi.date completiondate,
oi.amount,
datediff(oi.date, o.date) completiondate
FROM orders o
LEFT JOIN orders_interactions oi
ON oi.order = o.order
AND oi.category = 3;
That doesn't necessarily perform better but there are good chances. For performance an index on order_interactions (order, category) might help in any case.
And if you consider it more readable is up to you. But at least it's less repetitive (Which doesn't necessarily translates to more performance. Just because an expression is repeated in a query doesn't necessarily mean it repeatedly calculated.)
It seems I might have found the answer.
In my opinion, it improves readability quite a bit, and in my real usage scenario, both profile and execution plans are way more efficient, and results are returned in less than 1/3 of the time.
My answer relies on using a SELECT inside the LEFT JOIN, hence, using a subquery as the JOINs 'input'.
SELECT
o.order,
o.date,
c.clienttype,
o.producttype,
tmp.date,
tmp.amount,
DATEDIFF(tmp.date,o.date) as elapseddays
FROM orders o
LEFT JOIN clients c ON c.idClient=o.idClient
LEFT JOIN (SELECT order,date,amount FROM orders_interactions oi LEFT JOIN categories ct ON ct.order=oi.order AND category=3) AS tmp ON tmp.order=o.order
The answer idea, and the explanation about how and why it works, came from this post: Mysql Reference subquery result in parent where clause
I have this sqlite database:
And I need to get the set from AssignmentMetric for a certain Athlete_id AND for Assignments which include a skill with Value X.
I've looked all over JOIN, LEFT JOIN but cannot find any examples that seems to match exactly this. I can query to get Assignment metrics for a particular Athlete ID And Assignment ID, but I am trying to narrow the Assignments to ones that are for a specific skill "of X value". How do I "reach through" the Assignment table to grab the value from the Skill table?
You could do:
SELECT ...
FROM
AssignmentMetric
INNER JOIN Assignment
ON AssignmentMetric.Assignment_id =
Assignment.Assignment_id
INNER JOIN
Skill
ON Skill.Skill_id = Assignment.Skill_id
WHERE ...
In the WHERE you can refer to the colums of the tables using the tablename.columnname notation.
I have been searching and searching and have to resolved to post! I'm still pretty new to R.
I have 2 data frames. The large one is HEAT and the small one is EE.
I have manage to do a left join to get EE matched up with HEAT.
df(HEAT)
Date Time. EVENT. Person. PersonID
DTgroup1. X. Code. Code
DTgroup2. X Code. Code
DTgroup3. Y. Code. Code
....
Then there is:
df(EE)
Person ID. Type. var 3. var 4 var 5
here is the merge that I used:
merge <- left_join(HEAT, EE)
I have managed to merge the two data frames but I loose all the data in df(EE) except for the PersonID that it share with df(HEAT).
Does anyone have any advice about what I am doing wrong?
Thanks a bunch!
A left join will keep all rows on the left side, in your case HEAT, and include data where there is a match on the right hand side.
An inner join, would only return records where there is a valid join on both sides, in your case, one record would be returned.
See What is the difference between “INNER JOIN” and “OUTER JOIN”? for more info.
Obviously, you want a
merge <- full_join(HEAT, EE)
Here is a nice Cheat sheet page http://stat545.com/bit001_dplyr-cheatsheet.html
And here a super nice graphics http://r4ds.had.co.nz/relational-data.html
I have 3 tables in a SQLite database for an Android app. This picture below shows the relevant tables that I'm working with.
Tables
I'm trying to get two fields, value and name, from measurement_lines and competences respectively, tied to a specific person_id in measurements. I'm trying to make a query that returns these fields but I'm having little luck. The best I've got so far is the following query:
SELECT name, value
FROM measurements, measurement_lines, competences
WHERE measurements.id = measurement_lines.measurements_id
AND measurement_lines.competences_id = competences.id
AND measurements.persons_id = 1
This, however, has one issue. This query won't return any records when a person has no entries in measurements (and subsequently, nothing in measurement_lines). What I want is to always get a list of competence names, even if the value column is empty. I'm guessing I need a Left Outer Join for this but I can't seem to make it work. The following query just returns no records:
SELECT name, value
FROM measurements AS m, competences AS c
LEFT OUTER JOIN measurement_lines AS ml ON c._id = ml.competence_id
WHERE ml.measurement_id = m._id AND m.persons_id = 1
For inner joins, you can be sloppy with the distinction between join conditions and selection predicates, but when outer joins are involved that makes a difference. Any criterion appearing in the WHERE clause filters your result rows after all joins are performed (logically, at least), which can remove result rows associated with outer tables.
In addition, if you're ever uncertain about join order, you can use parentheses to make your intent clear. At least in many DBMSs. It lokos like SQLite doesn't support them.
It looks like you may want this: (edited to avoid use of parentheses)
SELECT c.name, pm.value
FROM competences c
LEFT OUTER JOIN (
SELECT ml.competences_id AS cid,
ml.value AS value
FROM measurement_lines ml
INNER JOIN measurements m
ON m.id = ml.measurements_id
WHERE m.person_id = 1
) pm
ON pm.cid = c.id
I have two tables, one contains a list of items which is called watch_list with some important attributes and the other is just a list of prices which is called price_history. What I would like to do is group together 10 of the lowest prices into a single column with a group_concat operation and then create a row with item attributes from watch_list along with the 10 lowest prices for each item in watch_list. First I tried joins but then I realized that the operations where happening in the wrong order so there was no way I could get the desired result with a join operation. Then I tried the obvious thing and just queried the price_history for every row in the watch_list and just glued everything together in the host environment which worked but seemed very inefficient. Now I have the following query which looks like it should work but it's not giving me the results that I want. I would like to know what is wrong with the following statement:
select w.asin,w.title,
(select group_concat(lowest_used_price) from price_history as p
where p.asin=w.asin limit 10)
as lowest_used
from watch_list as w
Basically I want the limit operation to happen before group_concat does anything but I can't think of a sql statement that will do that.
Figured it out, as somebody once said "All problems in computer science can be solved by another level of indirection." and in this case an extra select subquery did the trick:
select w.asin,w.title,
(select group_concat(lowest_used_price)
from (select lowest_used_price from price_history as p
where p.asin=w.asin limit 10)) as lowest_used
from watch_list as w