First of, I'm pretty new to this site and coding in general so please explain in simple terms as I'm still learning! Thanks
Ok, so I've got a database of results. These are 1-6 ratings. I've already created the ability to retrieve certain results (user, group, all).
But now I'm wanting to alongside retrieving the group and all results to display at the top of the results a mean for each question.
So to start I'm wanting something like this I believe.
SELECT sum(r1), sum(r2), sum(r3) so on,
FROM table
This is where I get confused.
I think I'd need a variable to contain these and then another that counts the amount of entries to divide the total of r1 hence the mean.
Any ideas?..
To calculate a mean, use the AVG function, e.g.
SELECT AVG(r1), AVG(r2)
FROM table
See the MySQL docs.
Related
As i am a junior PHP Developer growing day by day stuck in a performance problem described here:
I am making a search engine in PHP ,my database has one table with 41 column and million's of rows obviously it is a very large dataset. In index.php i have a form for searching data.When user enters search keyword and hit submit the action is on search.php with results.The query is like this.
SELECT * FROM TABLE WHERE product_description LIKE '%mobile%' ORDER BY id ASC LIMIT 10
This is the first query.After result shows i have to run 4 other query like this:
SELECT DISTINCT(weight_u) as weight from TABLE WHERE product_description LIKE '%mobile%'
SELECT DISTINCT(country_unit) as country_unit from TABLE WHERE product_description LIKE '%mobile%'
SELECT DISTINCT(country) as country from TABLE WHERE product_description LIKE '%mobile%'
SELECT DISTINCT(hs_code) as hscode from TABLE WHERE product_description LIKE '%mobile%'
These queries are for FILTERS ,the problem is this when i submit search button ,all queries are running simultaneously at the cost of Performance issue,its very slow.
Is there any other method to fetch weight,country,country_unit,hs_code speeder or how can achieve it.
The same functionality is implemented here,Where the filter bar comes after table is filled with data,How i can achieve it .Please help
Full Functionality implemented here.
I have tried to explain my full problem ,if there is any mistake please let me know i will improve the question,i am also new to stackoverflow.
Firstly - are you sure this code is working as you expect it? The first query retrieves 10 records matching your search term. Those records might have duplicate weight_u, country_unit, country or hs_code values, so when you then execute the next 4 queries for your filter, it's entirely possible that you will get values back which are not in the first query, so the filter might not make sense.
if that's true, I would create the filter values in your client code (PHP)- finding the unique values in 10 records is going to be quick and easy, and reduces the number of database round trips.
Finally, the biggest improvement you can make is to use MySQL's fulltext searching features. The reason your app is slow is because your search terms cannot use an index - you're wild-carding the start as well as the end. It's like searching the phonebook for people whose name contains "ishra" - you have to look at every record to check for a match. Fulltext search indexes are designed for this - they also help with fuzzy matching.
I'll give you some tips that will show useful in many situations when querying a large dataset, or mostly any dataset.
If you can list the fields you want instead of querying for '*' is a better practice. The weight of this increases as you have more columns and more rows.
Always try to use the PK's to look for the data. The more specific the filter, the less it will cost.
An index in this kind of situation would come pretty handy, as it will make the search more agile.
LIKE queries are generally pretty slow and resource heavy, and more in your situation. So again, the more specific you are, the better it will get.
Also add, that if you just want to retrieve data from this tables again and again, maybe a VIEW would fit nicely.
Those are just some tips that came to my mind to ease your problem.
Hope it helps.
I need to display the number of comments a user has post. I can think about two different ways of doing it, and I would like to know which one is better.
METHOD ONE: Each time I need to display the number of comments, query the comments table to select all comments with user_id x, and count the number of results.
METHOD TWO: Add a new column to the user table to store the number of comments a particular user has post. This value will be updated each time the user enters a new comment. This way every time I need to show the number of comments, I just need to query this value in the datbase.
I think the second method is more efficient, but I would like to know other opinions.
Any comment will be appreciated.
Thanks in advance,
Sonia
Well it depends. I suppose you use SQL. Counting is pretty fast of you have correct indexes (eg. SELECT COUNT(1) FROM articles WHERE user_id = ?). If this would be bottleneck than I would consider caching of these results.
At scale, option #2 is the only one that is viable. Counts may eventually be skewed some and you may need to rebuild the stats but this is a relatively low cost compared to trying to count the number of rows matching a secondary index.
For a new version of a website, I am making a "Top User" section on the homepage. This allows people to vote for the user on a bunch of different categories. All this information is stored in a MySQL database in two main tables. One table has the information(id*autogenerated*, Username, Date added, etc.) and the other has the rates (id*autogenerated*, linkID, rowAvg, avg, avg2, etc.). The linkID from the rate table corresponds to the id from the information table. How the query works is it queries through the rate_table, orders it by highest averages then selects the first 10 results. Then using the linkID of that row, I select the actual information for that user from the info_table. This is fine and dandy as long as each user only has 1 rate. What happens when a user has 2 or more rates, and both of the rates are positive, is the query selects both of those and pulls the information for each of the 2 rows which is then displayed as two different users even though it is the same person. How can I make the rate_table query know when there are multiple rates for the same linkID and average them together instead of showing them as two different people. I was thinking of inserting each linkID into an array, then for each subsequently selected row, check if that linkID is already in the array. If not, insert it, if it is, then average them together and store it in the array and use the array to populate the table on the homepage. I feel like I am overthinking this. Sorry if this is a little confusing. If you need more information, let me know.
P.S. I'm still learning my way around MySQL queries so I apologize if I am going about this the completely wrong way and you spent the last few minutes trying to understand what I was saying :P
P.P.S. I know I shouldn't be using MySQL_Query anymore, and should be doing prepared statements. I want to master MySQL queries because that is what FMDB databases for iOS use then I will move onto learning prepared statements.
Take a look at some of the MySQL Aggregate Functions.
AVG() may be what you're looking for. The average of one result is just that result, so that should still work. Use the GROUP BY clause on the column that should be unique in order to run the aggregate calculation on the grouped rows.
I have recently written a survey application that has done it's job and all the data is gathered. Now i have to analyze the data and i'm having some time issues.
I have to find out how many people selected what option and display it all.
I'm using this query, which does do it's job:
SELECT COUNT(*)
FROM survey
WHERE users = ? AND table = ? AND col = ? AND row = ? AND selected = ?
GROUP BY users,table,col,row,selected
As evident by the "?" i'm using MySQLi (in php) to fetch the data when needed, but i fear this is causing it to be so slow.
The table consists of all the elements above (+ an unique ID) and all of them are integers.
To explain some of the fields:
Each survey was divided into 3 or 4 tables (sized from 2x3 to 5x5) with a 1 to 10 happiness grade to select form. (questions are on the right and top of the table, then you answer where the questions intersect)
users - age groups
table, row, col - explained above
selected - dooooh explained above
Now with the surveys complete and around 1 million entries in the table the query is getting very slow. Sometimes it takes like 3 minutes, sometimes (i guess) the time limit expires and you get no data at all. I also don't have access to the full database, just my empty "testing" one since the costumer is kinda paranoid :S (and his server seems to be a bit slow)
Now (after the initial essay) my questions are: I left indexing out intentionally because with a lot of data being written during the survey, it would be a bad idea. But since no new data is coming in at this point, would it make sense to index all the fields of a table? How much sense does it make to index integers that never go above 10? (as you can guess i haven't got a clue about indexes). Do i need the primary unique ID in this table? I
I read somewhere that indexing may help groups but only if you group by the first columns in a table (and since my ID is first and from my point of view useless can i remove it and gain anything by it?)
Is there another way to write my query that would basically do the same thing but in a shorter period of time?
Thanks for all your suggestions in advance!
Add an index on entries that you "GROUP BY" or do "WHERE". So that's ONE index incorporating users,table,col,row and selected in your case.
Some quick rules:
combine fields to have the WHERE first, and the GROUP BY elements last.
If you have other queries that only use part of it (e.g. users,table,col and selected) then leave the missing value (row, in this example) last.
Don't use too many indexes/indeces, as each will slow the table to updates marginally - so on really large system you need to balance queries with indexes.
Edit: do you need the GROUP BY user,col,row as these are used in the WHERE. If the WHERE has already filtered them out, you only need group by "selected".
I'm running a sql query to get basic details from a number of tables. Sorted by the last update date field. Its terribly tricky and I'm thinking if there is an alternate to using the UNION clause instead...I'm working in PHP MYSQL.
Actually I have a few tables containing news, articles, photos, events etc and need to collect all of them in one query to show a simple - whats newly added on the website kind of thing.
Maybe do it in PHP rather than MySQL - if you want the latest n items, then fetch the latest n of each of your news items, articles, photos and events, and sort in PHP (you'll need the last n of each obviously, and you'll then trim the dataset in PHP). This is probably easier than combining those with UNION given they're likely to have lots of data items which are different.
I'm not aware of an alternative to UNION that does what you want, and hopefully those fetches won't be too expensive. It would definitely be wise to profile this though.
If you use Join in your query you can select datas from differents tables who are related with foreign keys.
You can look of this from another angle: do you need absolutely updated information? (the moment someone enters new information it should appear)
If not, you can have a table holding the results of the query in the format you need (serving as cache), and update this table every 5 minutes or so. Then your query problem becomes trivial, as you can have the updates run as several updates in the background.