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  • What are some useful SQL statements that should be known by all developers who may touch the Back en

    - by Jian Lin
    What are some useful SQL statements that should be known by all developers who may touch the Back end side of the project? (Update: just like in algorithm, we know there are sorting problems, shuffling problems, and we know some solutions to them. This question is aiming at the same thing). For example, ones I can think of are: Get a list of Employees and their boss. Or one with the employee's salary greater than the boss. (Self-join) Get a list of the most popular Classes registered by students, from the greatest number to the smallest. (Count, group by, order by) Get a list of Classes that are not registered by any students. (Outer join and check whether the match is NULL, or by Get from Classes table, all ClassIDs which are NOT IN (a subquery to get all ClassIDs from the Registrations table)) Are there some SQL statements that should be under the sleeve of all developers that might touch back end data?

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  • In HAML on Ruby on Rails, how to use the :sass filter?

    - by Jian Lin
    If using HAML on Ruby on Rails, then :sass #someDiv border: 3px dashed orange won't have any <style> tag around them. and then :css :sass #someDiv border: 3px dashed orange won't kick on the :sass filter, but :css :sass #someDiv border: 3px dashed orange will kick on the :sass filter, but it is outside of the <style> tag. So how can the :sass filter be used?

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  • Can a binary tree or tree be always represented in a Database table as 1 table and self-referencing?

    - by Jian Lin
    I didn't feel this rule before, but it seems that a binary tree or any tree (each node can have many children but children cannot point back to any parent), then this data structure can be represented as 1 table in a database, with each row having an ID for itself and a parentID that points back to the parent node. That is in fact the classical Employee - Manager diagram: one boss can have many people under him... and each person under him can have n people under him, etc. This is a tree structure and is represented in database books as a common example as a single table Employee.

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  • Using Ruby on Rails, can a polymorphic database be created in a few steps (with polymorphic associat

    - by Jian Lin
    I thought it could be created in a few steps but it can't yet: rails poly cd poly ruby script/generate scaffold animal name:string obj_type:string obj_id:integer rake:migrate ruby script/generate scaffold human name:string passportNumber:string rake:migrate ruby script/generate scaffold dog name:string registrationNumber:string rake:migrate and now change app/models/animal.rb to: class Animal < ActiveRecord::Base belongs_to :obj, :polymorphic => true end and run ruby script/server and go to http://localhost:3000 I thought on the server then if I create an Michael, Human, J123456 and then Woofie, Dog, L23456 then the database will have entries in the Dogs table and "Humen" or "Humans" table as well as in the Animals table? But only the Animals table has records, Dogs and "Humen" do not for some reason. Is there some steps missing?

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  • In MVC framworks (such as Ruby on Rails), does usually Model spell as singular and controller and vi

    - by Jian Lin
    I usually see Ruby on Rails books using script/generate model Story name:string link:string which is a singular Story, while when it is controller script/generate controller Stories index then the Story now is Stories, which is plural. Is this a standard on Ruby on Rails? Is it true in other MVC frameworks too, like CakePHP, Symfony, Django, or TurboGears? I see that in the book Rails Space, the controller is also called User, which is the same as the model name, and it is the only exception I see. Update: also, when scaffold is done on Ruby on Rails, then automatically, the model is singular and the controller and view are both plural.

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  • What are some useful SQL statements / usage patterns that should be known by all developers who may

    - by Jian Lin
    What are some useful SQL statements that should be known by all developers who may touch the Back end side of the project? (Update: just like in algorithm, we know there are sorting problems, shuffling problems, and we know some solutions to them. This question is aiming at the same thing). For example, ones I can think of are: Get a list of Employees and their boss. Or one with the employee's salary greater than the boss. (Self-join) Get a list of the most popular Classes registered by students, from the greatest number to the smallest. (Count, group by, order by) Get a list of Classes that are not registered by any students. (Outer join and check whether the match is NULL, or by Get from Classes table, all ClassIDs which are NOT IN (a subquery to get all ClassIDs from the Registrations table)) Are there some SQL statements that should be under the sleeve of all developers that might touch back end data?

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  • Do we need Record Level Locking when we already have Transaction for online ordering? (of concert ti

    - by Jian Lin
    For online ordering of concert seat or airline ticket, do we need Record Level Locking or is Transaction good enough? For concert ticket (say, seat Number 20B), or airline ticket (even with overbooking, the limit is 210, for example), I think the website cannot lock any record or begin transaction when showing the ticket purchase screen. But after the user clicks "Confirm Purchase", then the server should Begin a Transaction, Purchase Seat Number 20B, and try to Commit. If another user already bought Seat 20B in a previous transaction, then it is the "Commit" part that the current transaction will fail? So... we don't need Record Level Locking? Do Transactions always go serialized (one after another), so that's why we can know for sure there is no "race condition"? In what situation is Record Level Locking needed then?

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  • In SQL, what does Group By mean without Count(*), or Sum(), Max(), avg(), ..., and what are some use

    - by Jian Lin
    In SQL, if we use Group By without Count(*) or Sum(), etc, then the result is as follows: mysql> select * from sentGifts; +--------+------------+--------+------+---------------------+--------+ | sentID | whenSent | fromID | toID | trytryWhen | giftID | +--------+------------+--------+------+---------------------+--------+ | 1 | 2010-04-24 | 123 | 456 | 2010-04-24 01:52:20 | 100 | | 2 | 2010-04-24 | 123 | 4568 | 2010-04-24 01:56:04 | 100 | | 3 | 2010-04-24 | 123 | NULL | NULL | 1 | | 4 | 2010-04-24 | NULL | 111 | 2010-04-24 03:10:42 | 2 | | 5 | 2010-03-03 | 11 | 22 | 2010-03-03 00:00:00 | 6 | | 6 | 2010-04-24 | 11 | 222 | 2010-04-24 03:54:49 | 6 | | 7 | 2010-04-24 | 1 | 2 | 2010-04-24 03:58:45 | 6 | +--------+------------+--------+------+---------------------+--------+ 7 rows in set (0.00 sec) mysql> select *, count(*) from sentGifts group by whenSent; +--------+------------+--------+------+---------------------+--------+----------+ | sentID | whenSent | fromID | toID | trytryWhen | giftID | count(*) | +--------+------------+--------+------+---------------------+--------+----------+ | 5 | 2010-03-03 | 11 | 22 | 2010-03-03 00:00:00 | 6 | 1 | | 1 | 2010-04-24 | 123 | 456 | 2010-04-24 01:52:20 | 100 | 6 | +--------+------------+--------+------+---------------------+--------+----------+ 2 rows in set (0.00 sec) mysql> select * from sentGifts group by whenSent; +--------+------------+--------+------+---------------------+--------+ | sentID | whenSent | fromID | toID | trytryWhen | giftID | +--------+------------+--------+------+---------------------+--------+ | 5 | 2010-03-03 | 11 | 22 | 2010-03-03 00:00:00 | 6 | | 1 | 2010-04-24 | 123 | 456 | 2010-04-24 01:52:20 | 100 | +--------+------------+--------+------+---------------------+--------+ 2 rows in set (0.00 sec) Only 1 row is returned per "group". What does it mean when there is no "Count(*)", etc when using "Group By", and what are it uses? thanks.

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  • Is it good to commit files often if using Mercurial or Git?

    - by Jian Lin
    It seems that it is suggested we can commit often to keep track of intermediate changes of code we wrote… such as on hginit.com, when using Mercurial or Git. However, let's say if we work on a project, and we commit files often. Now for one reason or another, the manager wants part of the feature to go out, so we need to do a push, but I heard that on Mercurial or Git, there is no way to push individual files or a folder… either everything committed gets pushed or nothing get pushed. So we either have to revert all those files we don't want to push, or we just never should commit until before we push -- right after commit, we push?

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  • Why TortoiseHg not show the "merge conflict"?

    - by Jian Lin
    Short version of the question: Since I already have TortoiseHg, I right clicked on that file trying to see the merge conflict visually, but there is no way to see it? Details: To make a simple case of merge conflict, I hg init a repo on Win 7, and then clone it to another folder. Now, in one working directory, i added the line "the code is 123", committed. And in the other folder, i did an "hg pull" and "hg update" Now, I go back to the first folder, and change "123" to "123abc", and then do an "hg commit" And then I go to the other folder and edit "123" to "123xyz" over there, and do an "hg commit", and when "hg push", it says it can't. So I try to use any visual tool to see how the conflict is like, but ... TortoiseHg doesn't seem to have any option to do that?

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  • Using Mercurial (hg), can you just "hg backout" all the commits you did for the files you don't want

    - by Jian Lin
    Using Mercurial (hg), can you just "hg backout" all the commits you did for the files you don't want to push, and then do a push? Because Mercurial (or Git) won't let us push a single file or a single folder to another repository, so I am thinking: 1) How about, we just look at the commit we did, and hg backout the ones we don't want to push. 2) hg out -v to see the list of files that will be pushed 3) now do the push by hg push Is this a good way? This is because I got the following advice: 1) Don't commit that file if you don't want it to be pushed (but sometimes even just for experimentation, I do want to keep the intermediate revisions) (-- maybe I can hg commit and hg backout right away to prevent it from being pushed.) 2) Some people told me just to hg clone tmp from that repository i want to push to, and then copy the local file over to this tmp working directory, hg commit to this tmp repository, and then do a push. But I found that the hg clone tmp will take up 400MB of new data and files, and make the hard drive work very hard, just to push 1 file? So I would rather not use this method.

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  • nicely display file rename history in git log

    - by Jian
    The git command git log --format='%H' --follow -- foo.txt will give you the series of commits that touch foo.txt, following it across renames. I'm wondering if there's a git log command that will also print the corresponding historical file name beside each commit. It would be something like this, where we can interpret '%F' to be the (actually non-existent) placeholder for filename. git log --format='%H %F' --follow -- foo.txt I know this could be accomplished with git log --format='%H' --follow --numstat -- foo.txt but the output is not ideal since it requires some non-trivial parsing; each commit is strewn across multiple lines, and you'll still need to parse the file rename syntax ("bar.txt => foo.txt") to find what you're looking for.

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  • In SQL, we can use "Union" to merge two tables. What are different ways to do "Intersection"?

    - by Jian Lin
    In SQL, there is an operator to "Union" two tables. In an interview, I was told that, say one table has just 1 field with 1, 2, 7, 8 in it, and another table also has just 1 field with 2, and 7 in it, how do I get the intersection. I was stunned at first, because I never saw it that way. Later on, I found that it is actually a "Join" (inner join), which is just select * from t1, t2 where t1.number = t2.number (although the name "join" feels more like "union" rather than "intersect") another solution seems to be select * from t1 INTERSECT select * from t2 but it is not supported in MySQL. Are there different ways to get the intersection besides these two methods?

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  • Why does FastCGI not work well with Ruby on Rails?

    - by Jian Lin
    It is said that FastCGI doesn't work well with Ruby on Rails deployment. Why is that? In previous experience, something either works quite well or it might be fundamentally wrong. So if FastCGI is a viable solution, why is it not reliable with RoR? Does FastCGI work well with most any language / frameworks?

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  • In SQL / MySQL, what is the difference between "On" and "Where" in a join statement?

    - by Jian Lin
    The following statements give the same result (one is using "on", and the other using "where"): mysql> select * from gifts INNER JOIN sentGifts on gifts.giftID = sentGifts.giftID; mysql> select * from gifts INNER JOIN sentGifts where gifts.giftID = sentGifts.giftID; I can only see in a case of a Left Outer Join finding the "unmatched" cases: (to find out the gifts that were never sent by anybody) mysql> select name from gifts LEFT OUTER JOIN sentgifts on gifts.giftID = sentgifts.giftID where sentgifts.giftID IS NULL; In this case, it is first using "on", and then "where". Does the "on" first do the matching, and then "where" does the "secondary" filtering? Or is there a more general rule of using "on" versus "where"? Thanks.

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  • How to change a primary key in SQL to auto_increment?

    - by Jian Lin
    I have a table in MySQL that has a primary key: mysql> desc gifts; +---------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------------+-------------+------+-----+---------+-------+ | giftID | int(11) | NO | PRI | NULL | | | name | varchar(80) | YES | | NULL | | | filename | varchar(80) | YES | | NULL | | | effectiveTime | datetime | YES | | NULL | | +---------------+-------------+------+-----+---------+-------+ but I wanted to make it auto_increment. The following statement failed. How can it be modified so that it can work? thanks mysql> alter table gifts modify giftID int primary key auto_increment; ERROR 1068 (42000): Multiple primary key defined

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  • It is said that Mercurial's "hg clone" is very cheap... but it is 400MB on my hard drive? (on Mac OS

    - by Jian Lin
    I have a project I cloned over the network to the Mac hard drive (OS X Snow Leopard). The project is about 1GB in the hard drive du -s 2073848 . so when I hg clone proj proj2 then when I MacBook-Pro ~/development $ du -s proj 2073848 proj MacBook-Pro ~/development $ du -s proj2 894840 proj2 MacBook-Pro ~/development $ du -s 2397928 . so the clone seems not so cheap... probably around 400MB... is that so? also, the whole folder grew by about 200MB, which is not the total of proj and proj2 by the way... are there some links and some are not links, that's why the overlapping is not counted twice?

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