I have a MySQL table called connections, with two unsigned integer fields, called a and b (which are unique), with the following values (and many more).
A, B
1, 2
1, 3
8, 9
4, 6
5, 1
3, 7
1 is connected to 2, 3, and 5. 3 is connected to 7. I have been struggling to come up with a query that will select all connections for a given number. For example, number 2 would return the following...
1
2
3
5
7
Number 5 would also return...
1
2
3
5
7
If someone can help me out I would greatly appreciate it. I have been stuck on this for the last week, and have made little to no progress. :(
Thanks in advance!
I don't think your problem cannot be solved with a single select statement. Solution requires multipass scan of your tables to arrive at the final answer
It sounds like you need a hierarchical query here, which exists in Oracle, but I don't think it is in mySql. Here is a link to an article which explains how to get hierarchical functionality in mySql. I have not tried it myself.
http://explainextended.com/2009/03/17/hierarchical-queries-in-mysql/
Related
I have a steps table. It describes the steps a person takes towards a specific goal. For example, 3 steps to one goal, 5 steps to the second. And every time I take a new step, I write a new step into this table. Question: how can I determine, for example, whether step No. X has been made? The given record can and not exist?
For example, I have 2 steps out of 3 recorded, and for example I want to select all records that do not have step number 3
Example:
id, name, step
1 Mark 1
2 Mark 2
3 Mark 3
4, Jenny, 1
5, Jenny, 2
6, Tony, 1
7, Tony, 2
8, Tony, 3
9, Joe, 1
For example, I want to show everyone who does not have step number 3
How can I do it?
For the question to decide whether step No. X has been made, the code is like:
select count() from steps where step = X
This will return the total number of records of X.
For the question to select all records that do not have step number 3:
select * from steps where step != 3
Let's say there is a table like the following:
id | number
----|----------
1 | 1
4 | 6
5 | 2
14 | 3
now I need all numbers that are not set between 1 and the highest number that is set. Here the highest number is 6, so I need all numbers that are not set between 1 and 6. Here it is: 4 and 5
But how can I achieve this with spring and hibernate jpa with a MySQL database?
The highest number is easy. Sort numbers DESC and then the first one. But then return all missing numbers that are not in the database? Is this possible?
One way: select each number that is smaller then the highest number and check if the returned object is null. So first check 5, then 4, then 3, ... But this is of course very slow on big databases.
So another idea was: get all numbers that are set and get the missing numbers on java side with the difference of two lists (one list with the numbers out of the database, the other list with the numbers from 1 to the highest number of the database). But on big databases it is also dumb to get everything. (Let's say, there are 1 million entries and only one number is missing.)
The third idea: something like select where number NULL would be perfect. But for this the database would have to be initialized with all possible numbers ever there. So that is also not possible.
Is there a possible way? Am I overseeing something?
I have a simple question that I wasn't really sure how to search for (or title!). I apologize if this has been asked a million times. For the following table, how do I generate a report that will detail the number of companies that a person has worked for and how many people have also worked for that same number? So, for example, this table should return:
people, companiesperperson
1, 1
2, 2
1, 3
for the following table called personalinfo:
id_number first last company
1 John Doe Intel
2 John Doe Microsoft
3 Phil Jenkins Amgen
4 Phil Jenkins Bayer
5 Phil Jenkins Sanofi
6 Josh Edwards Walgreens
7 Amy Dill URS
8 Amy Dill ARCADIS
Let me know if this is still confusing and if I can further clarify what I am looking to do.
Thanks!
This is a rough estimate of the query but
SELECT count as companiesperperson, COUNT(first, last) as people FROM
(SELECT COUNT(company) as count, first, last FROM personalinfo GROUP BY (first, last)) as a
GROUP BY count
To explain the query first in the subquery we are asking for the names and count of companies after splitting up all the rows by names
Then in the outer query we split up all the rows by their count and ask how many unique names can be found in each group.
There may be a few syntax errors I've left straggling but the group by feature is really what's essential to understanding how to solve this question.
I have a table that holds items and their "weight" and it looks like this:
items
-----
id weight
---------- ----------
1 1
2 5
3 2
4 9
5 8
6 4
7 1
8 2
What I'm trying to get is a group where the sum(weight) is exactly X, while honouring the order in which were inserted.
For example, if I were looking for X = 3, this should return:
id weight
---------- ----------
1 1
3 2
Even though the sum of ids 7 and 8 is 3 as well.
Or if I were looking for X = 7 should return
id weight
---------- ----------
2 5
3 2
Although the sum of the ids 1, 3 and 6 also sums 7.
I'm kind of lost in this problem and haven't been able to come up with a query that does at least something similar, but thinking this problem through, it might get extremely complex for the RDBMS to handle. Could this be done with a query? if not, what's the best way I can query the database to get the minimum amount of data to work with?
Edit: As Twelfth says, I need to return the sum, regardless of the amount of rows it returns, so if I were to ask for X = 20, I should get:
id weight
---------- ----------
1 1
3 2
4 9
5 8
This could turn out to be very difficult in sql. What you're attempting to do is solve the knapsack problem, which is non-trivial.
The knapsack problem is interesting from the perspective of computer science for many reasons:
The decision problem form of the knapsack problem (Can a value of at least V be achieved without exceeding the weight W?) is NP-complete, thus there is no possible algorithm both correct and fast (polynomial-time) on all cases, unless P=NP.
While the decision problem is NP-complete, the optimization problem is NP-hard, its resolution is at least as difficult as the decision problem, and there is no known polynomial algorithm which can tell, given a solution, whether it is optimal (which would mean that there is no solution with a larger, thus solving the decision problem NP-complete).
There is a pseudo-polynomial time algorithm using dynamic programming.
There is a fully polynomial-time approximation scheme, which uses the pseudo-polynomial time algorithm as a subroutine, described below.
Many cases that arise in practice, and "random instances" from some distributions, can nonetheless be solved exactly.
My users answer two different questions, they are added to my answers_table as one row each. I'm trying to count how many users have answered my questions which seems simple at first. Count(*) and divide by 2. YES! But: the users can add new questions as they go, so suddenly there might be three questions, all answers/3 becomes an incorrect number of users if there was two questions for a while and then three, for example!
Can anyone figure out a query for doing this? So far I've got
SELECT COUNT(DISTINCT date) as totNum FROM login_answers
But it doesn't take the time in consideration. Meaning i just get the amount of answers in total for that day, not knowing how many questions was available that day.
Here's my table:
qid is the QuestionID so i guess thats somewhere to start?
id qid answer date deviceID
1502 2 2 2012-10-19 08:42:41 7
1503 1 3 2012-10-19 08:51:53 7
1504 2 2 2012-10-19 08:51:53 7
1505 1 4 2012-10-19 09:05:23 7
1506 2 2 2012-10-19 09:05:23 7
1507 1 4 2012-10-19 09:40:59 7
My proposed solution would be to store a session Id or some auto-generated key against the answers to uniquely identify a "session" of 2, 3 or more answers from a single person.
The query would simply be
SELECT COUNT(DISTINCT session_id) totNum
FROM login_answers
With your current schema with no identifiable key, it's quite hard to answer your question correctly if at all possible.