I'm having translating the following logic into a MySQL CASE statement, I'm sure there is an elegant way to do it but I think I have been working on for way too long and confused my self!
The business rules:
Programmes have an programme_start and programme_end
Individuals have an attend_start and attend_end
Individuals attend multiple programmes during one period, they can only have one of the following example attendance spans:
What I need to do is categorise each span into a category type i.e. Type 1 ... 9. but I'm having trouble building up a CASE statement that can mutually exclusively cover each span.
Has anyone done something like this before? If not could someone help with the logic behind it?
Thanks in advance!
The cases are quite simple..
In the case of a single time, there are only 3 ways a time can be classified - before, on and after. There are two times involved, so 3 x 3 = 9 results.
There is something to be said about consistency though, because for _start, it goes:
on - after - before
Whereas for the _end time, it goes
on - before - after
Combine them using (assuming no consideration for NULLs):
case when programme_start = attend_start then 0
when programme_start < attend_start then 3
else 6 end
+
case when programme_end = attend_end then 1
when programme_end > attend_end then 2
else 3 end
AS Category
Related
I have an imported table of several thousand customers, the development I am working on runs on the basis of anonymity for purchase checkouts (customers do not need to log in to check out), but if enough of their details match the database record then do a soft match and email the (probably new) email address and eventually associate the anonymous checkout with the account record on file.
This is rolling out this way due to the age of the records, many people have the same postal address or names but not the same email address, likewise some people will have moved house and some people will have changed name (marriage etc).
What I think I am looking for is a MySQL CASE system, however the CASE questions on Stack Overflow I've found don't appear to cover what I'm trying to get from this query.
The query should work something like this:
$input[0] = postcode (zip code)
$input[1] = postal address
$input[2] = phone number
$input[3] = surname
$input[4] = forename
SELECT account_id FROM account WHERE <4 or more of the variables listed match the same row>
The only way I KNOW I can do this is with a massive bunch of OR statements but that's excessive and I'm sure there's a cleaner more concise method.
I also apologise in advance if this is relatively easy but I don't [think I] know the keyword to research constructing this. As I say, CASE is my best guess.
I'm having trouble working out how to manipulate CASE to fit what I'm trying to do. I do not need to return the values only the account_id from the valid row (only) that matches 4 or 5 of the given inputs.
I imagine that I could construct a layout that does this:
SELECT account_id CASE <if postcode_column=postcode_var> X=X+1
CASE <if surname_column=surname_var> X=X+1
...
...
WHERE X > 3
Is CASE the right idea?
If not, What is the process I need to use to achieve the desired results?
What is [another] MySQL keyword / syntax I need to research, if not CASE.
Here is your pseudo query:
SELECT account_id
FROM account
WHERE (postcode = 'pc')+
(postal_address = 'pa')+
(phone_number = '12345678901')+
(surname = 'sn')+
(forename= 'fn') > 3
TL;DR? See Edit 2
I've got a little Rails application that has a few different sort of games people can play: it's based around sports, so they can pick the winners of each game every week (model PickEm, attribute correct boolean with nil for unfinished games), and predict the outcome of a specific team's game (model Guess, attribute score with integer, nil for unfinished games). Every User has_many PickEms and Guesses. And I'm trying to display standings (correct/total - total being all non-nil, score/total possible).
What I'm finding is that I can gather the users and their associated records, but in trying to display standings I'm discovering that every single User is triggering another query - slow and not sustainable as the user base increases. That's because #user.pick_em_score is pick_ems.where(correct: true).size and #user.guess_Score is guesses.where.not(score: nil).sum(:score). So I call user.pick_em_score and it runs that query. I feel like there should be a way to get every User, as well as these specific counts, at once, rather than buffering a whole bunch of needless extra stuff.
What I need:
User record
User.pick_em_score (calculated by counting correct records)
User.pick_ems count where NOT NULL
User.guesses_score (calculated by guesses.sum(:score))
User.guesses count where NOT NULL
Most of the stuff I find on Rails's ActiveRecord helpers, especially related to calculations, is for retrieving only the calculation. It looks like I'll probably need to delve directly into select() etc. But I can't get it working. Can someone point me in the right direction?
Edit
For clarification: I'm aware that I can write this information to the User model, but this is overly restrictive: next season, I'll need to add a new column to the User for that year's results, etc. In addition, this is a third degree of callback updating related models – the Match model already updates related PickEms and Guesses on save. I'm looking for the simplest ActiveRecord query or queries to be able to work with this information, as indicated by the title. Ideally one query that returns the above information, but if it needs to a few, that's OK.
I used to work directly in MySQL with PHP, but those skills have rusted (in raw MySQL, I imagine, I'd have several sub-select statements to help pull these counts) and I'd also like to be able to use Rails's ActiveRecord helpers and such, and avoid constructing raw SQL as much as possible.
Second Edit:
I seem to have it down to one call that starts to work, but I'm writing a lot of SQL. It's also brittle, IMO, and trying to run with it has failed. It also looks like I'm just pushing the million singular SELECT queries from Rails right into SQL, but that may still be a step up.
User.unscoped.select('users.*',
'(SELECT COUNT(*) FROM pick_ems WHERE pick_ems.user_id = users.id AND pick_ems.correct) AS correct_pick_ems',
'(SELECT COUNT(*) FROM pick_ems WHERE pick_ems.user_id = users.id AND pick_ems.correct IS NOT NULL) AS total_pick_ems',
'(SELECT SUM(guesses.score) FROM guesses WHERE guesses.user_id = users.id AND guesses.score IS NOT NULL) AS guesses_score',
'(SELECT COUNT(*) FROM guesses WHERE guesses.user_id = users.id AND guesses.score IS NOT NULL) AS guesses_count' )
The issue seems to be: is there a way to use Rails, and not raw SQL, to link up users.id that we see there with these subqueries? Or just … a better way to construct this, in general?
In addition, I'm running another set of SELECTs for the WHERE, which would hinge on total_pick_ems and guesses_count being > 0 but since I can't use those aliased columns, I have to call the SELECT one more time.
Welcome to AR. Its really only good for simple CRUD like queries. Once you actually want to query your data in anger it just doesn't have the capababilities to do the queries you want without resorting to wholesale SQL strings and often abandoning the ability to chain as a result.
Its precisely why I moved to Sequel as it does have the features to compose queries using a much fuller SQL feature set, including join conditions, window functions, recursive common table expressions, and advanced eager loading. The author is incredibly responsive and documentation is excellent compared to AR and Arel.
I don't expect you will like this answer but a time will come when you will start to look outside the opinionated components that come with rails which I have to say are hardly best of breed. Sequel also sped my application up many times over what I was able to get with AR as well, it not just developer happiness, it means less servers to run. Yes it will be a learning curve but IMO its better to learn tools that have your back covered.
Joins might work. Smthing like below
User.unscoped.joins(:guesses).joins(:pick_ems).
where("guesses.score IS NOT NULL").
select("users.*,
sum(guesses.score) as guesses_score,
count(guesses.id) as guesses_count,
count(case when pick_ems.correct = True then 1 else null end)
as correct_pick_ems,
count(case when pick_ems.correct != null then 1 else null end)
as total_pick_ems,
").
group("users.id")
If you need this information for a limited number of users at a time then above query or eager loading (User.includes(:guesses, :pick_ems)) with class methods like
def correct_pick_ems
pick_ems.count(&:correct)
end
would work.
However If you need this information for all the users most of the time, cached counters within the users table would be more optimal.
What you need is some sort of custom (smart) counter_cache to count only at certain conditions (e.g correct is true)
You can achive this using conditional after_save & after_destroy triggers to build your own custom counter_cache that looks like this:
class PickEm
belongs_to :user
after_save :increment_finished_counter_cache, if: Proc.new { |pick_em| pick_em.correct }
after_destroy :decrement_finished_counter_cache, if: Proc.new { |pick_em| pick_em.correct }
private
def increment_finished_counter_cache
self.user.update_column(:finished_games_counter, self.user.finished_games_counter + 1) #update_column should not trigger any validations or callbacks
end
def decrement_finished_counter_cache
self.user.update_column(:finished_games_counter, self.user.finished_games_counter - 1) #update_column should not trigger any validations or callbacks
end
end
Notes:
Code not tested (only to show the idea)
Some guys said it's better to avoid naming custom counters as rails name them (foo_counter_cache)
You should benchmark it, but my hunch is that adding all of that data into a single SELECT isn't going to be much faster than breaking it up into separate SELECTs (I've actually had cases where the latter was faster). By breaking it up, you can also stick to more ActiveRecord and less raw SQL, e.g.:
user_ids_to_pick_em_score = User.joins(:pick_ems).where(pick_ems: {correct: true}).group(:user_id).count
user_ids_to_pick_ems_count = User.joins(:pick_ems).where.not(pick_ems: {correct: nil}).group(:user_id).count
user_ids_to_guesses_score = Hash[User.select("users.id, SUM(guesses.score) AS total_score").joins(:guesses).group(:user_id).map{|u| [u.id, u.total_score]}]
user_ids_to_guesses_count = User.joins(:guesses).where.not(guesses: {score: nil}).group(:user_id).count
Edit: To display them, you could do like so:
<%- User.select(:id, :name).find_each do |u| -%>
Name: <%= u.name %>
Picks Correct: <%= user_ids_to_pick_em_score[u.id] %>/<%= user_ids_to_pick_ems_count[u.id] %>
Total Score: <%= user_ids_to_guesses_score[u.id] %>/<%= user_ids_to_guesses_count[u.id] %>
<%- end -%>
I'm writing a video game in javascript on a server that saves info in a mysql database and I am trying to make my first effect attached to simple healing potion item. To implement the effect I call a spells table using spell_id and it gets a field called effect containing the code to execute on my server. I use the eval() function to execute the code in the string. In order to optimize the game I want to run as few queries as possible. For this instance (and I think the answer will help me evaluate other similar effects) I want to update the 'player' table which contains a stat column like 'health' then I want it to add n which will be a decreasing number 15 then 250 ms later add 14 then 13 until that n=1 the net effect is a large jump in health then smaller and smaller accomplishing this is relatively easy if the player's health reaches his maximum allowed limit the effect will stop immediately...
but I'd like to do a single update statement for each increase rather than a select and an update every 250ms to check if health > max_health and make sure the player's health doesn't go above his max health. So to digress a bit I'd like a single update that given this data
player_id health max_health
========= ====== ==========
1 90 100
will add 15 to health unless (max_health-health) < 15... in this case it should only add 10.
An easier solution might be
if I could just return health and max health after each update I update it I don't mind doing a final
pseudo code
if health > max_health
update health set health = max health
So if anyone could explain how to return fields after an update that would help.
Or if anyone could show how to use logic within the update that would also help.
Also, If I didn't give enough information I'm sorry I'd be glad to provide more I just didn't want to make the question hard to understand.
update health
set health = least(max_health, health +<potion effect>)
where player_id = ...
EDIT
For your other question : normally, i think that update returns the number of affected rows. So if you try to update health when health is already = max_health, it should return 0.
I'd know how to do this in php, for example, but just said you where using javascript... so ?
http://dev.mysql.com/doc/refman/5.6/en/update.html
UPDATE returns the number of rows that were actually changed. The
mysql_info() C API function returns the number of rows that were
matched and updated and the number of warnings that occurred during
the UPDATE.
Use the ANSI standard CASE function, or the mysql only least function as in the other answer
UPDATE player
SET health = CASE WHEN health + [potion] > max_health
THEN max_health
ELSE health + [potion]
END CASE
WHERE player_id = [player_id]
I am building a query covering several WHERE AND conditions. Here is an example:
...
WHERE stage1 = 3
AND stage2 = 3
AND stage3 = 3
AND stage4 = 3
AND stage5 = 3
AND submitted = 0
Is there a shorter way of writing this, perhaps like WHERE TRUE(stage1=3 & stage2=3 ...) ?
Edit - just for info, here's the table:
The shorter way to write that would be:
WHERE (stage1, stage2, stage3, stage4, stage5, submitted)
= (3, 3, 3, 3, 3, 0)
It won't be any faster though, either way you write it.
No, there is not. And the notation you suggest is not really shorter. :)
Why would you need it to be shorter, and what table structure do you have where all these stage fields should be 3?
[edit] Added a solution for querying stages from a hypothetical detail table.
select
*
from
FormsTable f
where
f.submitted = 0 and
not exists
(select 'x'
from
PageDetailTable d
where
d.FormId = f.FormId and
d.State <> 3)
This is hardly shorter at the moment, but when you get more complex form that consist of more (or less) pages, you can support this more easily. When you create a form, you create a record for each page it consists of. In the page record you can store not only the state, but also other data you may need. When you get a different type of form with more or less pages, you can just create those pages. This query just checks if the form is not submitted and if it doesn't have any pages that are not finished yet. It doesn't matter how many pages there are.
We want to set up a directory of all the organizations working with us. They are incredibly diverse (government, embassy, private companies, and organizations depending on them ). So, I've resolved to create 2 tables. Table 1 will treat all the organizations equally, i.e. it'll collect all the basic information (name, address, phone number, etc.). Table 2 will establish the hierarchy among all the organizations. For instance, Program for illiterate adults depends on the National Institute for Social Security which depends on the Labor Ministry.
In the Hierarchy table, each column represents a level. So, for the example above, (i)Labor Ministry - Level1(column1), (ii)National Institute for Social Security - Level2(column2), (iii)Program for illiterate adults - Level3(column3).
To attach an organization to an hierarchy, the user needs to go level by level(i.e. column by column). So, there will be at least 3 situations:
If an adequate hierarchy exists for an organization(for instance, level1: US Embassy), that organization can be added (For instance, level2: USAID).--> US Embassy/USAID, and so on.
How about if one or more levels are missing? - then they need to be added
How about if the hierarchy need to be modified? -- not every thing need to be modified.
I do not have any choice but working by level (i.e. column by column). I does not make sense to have all the levels in one form as the user need to navigate hierarchies to find the right one to attach an organization.
Let's say, I have those queries in my repository (just that you get the idea).
Query1
var orgHierarchy = (from orgH in db.Hierarchy
select orgH.Level1).FirstOrDefault;
Query2
var orgHierarchy = (from orgH in db.Hierarchy
select orgH.Level2).FirstOrDefault;
Query3, Query4, etc.
The above queries are the same except for the property queried (level1, level2, level3, etc.)
Question: Is there a general way of writing the above queries in one? So that the user can track an hierarchy level by level to attach an organization.
In other words, not knowing in advance which column to query, I still need to be able to do so depending on some conditions. For instance, an organization X depends on Y. Knowing that Y is somewhere on the 3rd level, I'll go to the 4th level, linking X to Y.
I need to select (not manually) a column with only one query that takes parameters.
=======================
EDIT
As I just said to #Mark Byers, all I want is just to be able to query a column not knowing in advance which one. Check this out:
How about this
Public Hierarchy GetHierarchy(string name)
{
var myHierarchy = from hierarc in db.Hierarchy
where (hierarc.Level1 == name)
select hierarc;
retuen myHierarchy;
}
Above, the query depends on name which is a variable. It mighbe Planning Ministry, Embassy, Local Phone, etc.
Can I write the same query, but this time instead of looking to much a value in the DB, I impose my query to select a particular column.
var myVar = from orgH in db.Hierarchy
where (orgH.Level1 == "Government")
select orgH.where(level == myVariable);
return myVar;
I don't pretend that select orgH.where(level == myVariable) is even close to be valid. But that is what I want: to be able to select a column depending on a variable (i.e. the value is not known in advance like with name).
Thanks for helping
How about using DynamicQueryable?
http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx
Your database is not normalized so you should start by changing the heirarchy table to, for example:
OrganizationId Parent
1 NULL
2 1
3 1
4 3
To query this you might need to use recursive queries. This is difficult (but not impossible) using LINQ, so you might instead prefer to create a parameterized stored procedure using a recursive CTE and put the query there.