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  • In SQL / MySQL, can a Left Outer Join be used to find out the duplicates when there is no Primary ID

    - by Jian Lin
    I would like to try using Outer Join to find out duplicates in a table: If a table has Primary Index ID, then the following outer join can find out the duplicate names: mysql> select * from gifts; +--------+------------+-----------------+---------------------+ | giftID | name | filename | effectiveTime | +--------+------------+-----------------+---------------------+ | 2 | teddy bear | bear.jpg | 2010-04-24 04:36:03 | | 3 | coffee | coffee123.jpg | 2010-04-24 05:10:43 | | 6 | beer | beer_glass.png | 2010-04-24 05:18:12 | | 10 | heart | heart_shape.jpg | 2010-04-24 05:11:29 | | 11 | ice tea | icetea.jpg | 2010-04-24 05:19:53 | | 12 | cash | cash.png | 2010-04-24 05:27:44 | | 13 | chocolate | choco.jpg | 2010-04-25 04:04:31 | | 14 | coffee | latte.jpg | 2010-04-27 05:49:52 | | 15 | coffee | espresso.jpg | 2010-04-27 06:03:03 | +--------+------------+-----------------+---------------------+ 9 rows in set (0.00 sec) mysql> select * from gifts g1 LEFT JOIN (select * from gifts group by name) g2 on g1.giftID = g2.giftID where g2.giftID IS NULL; +--------+--------+--------------+---------------------+--------+------+----------+---------------+ | giftID | name | filename | effectiveTime | giftID | name | filename | effectiveTime | +--------+--------+--------------+---------------------+--------+------+----------+---------------+ | 14 | coffee | latte.jpg | 2010-04-27 05:49:52 | NULL | NULL | NULL | NULL | | 15 | coffee | espresso.jpg | 2010-04-27 06:03:03 | NULL | NULL | NULL | NULL | +--------+--------+--------------+---------------------+--------+------+----------+---------------+ 2 rows in set (0.00 sec) But what if the table doesn't have a Primary Index ID, then can an outer join still be used to find out duplicates?

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  • If Inner Join can be thought of as Cross Join but filtering out the records satisfying the condition

    - by Jian Lin
    If an Inner Join can be thought of as a cross join and then getting the records that satisfy the condition, then a LEFT OUTER JOIN can be thought of as that, plus ONE record on the left table that doesn't satisfy the condition. In other words, it is not a cross join that "goes easy" on the left records (even when the condition is not satisfied), because then the left record can appear many times (as many as how many records there are in the right table). So the LEFT OUTER JOIN is the Cross JOIN with the records satisfying the condition, plus ONE record from the LEFT TABLE that doesn't satisfy the condition.

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  • In Ruby on Rails, why will story.votes return an empty Array object, but story.votes.create will act

    - by Jian Lin
    In Ruby on Rails, say a Story object can "has_many" Vote objects (a story is voted "hot" by many users). So when we do a s = Story.find(:first) s is a Story object, and say s.votes returns [] and s.votes.class returns Array So clearly, s.votes is an empty Array object. At this time, when s.votes.create is called, it actually invokes a method of the Vote class? How come an Array class object can invoke a Vote class method?

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  • Does jQuery have a plugin to display a "message bar" like the Twitter "wrong password" bar at the to

    - by Jian Lin
    Twitter will pop down a message bar at the top of the screen say "Wrong password" and after 10 seconds, it will slide up and disappear. Chrome also shows "Do you want to save the password" message box using such a way. Does jQuery have a plug in to do that already? Does it also work in IE 6? Because usually, the display of relative to the viewport (using position: fixed) will not work on IE 6. thanks.

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  • In Ruby, how to implement 20 - point and point - 20 using coerce() ?

    - by Jian Lin
    In Ruby, the operation of point - 20 20 - point are to be implemented. But the following code: class Point attr_accessor :x, :y def initialize(x,y) @x, @y = x, y end def -(q) if (q.is_a? Fixnum) return Point.new(@x - q, @y - q) end Point.new(@x - q.x, @y - q.y) end def -@ Point.new(-@x, -@y) end def *(c) Point.new(@x * c, @y * c) end def coerce(something) [self, something] end end p = Point.new(100,100) q = Point.new(80,80) p (-p) p p - q p q - p p p * 3 p 5 * p p p - 30 p 30 - p Output: #<Point:0x2424e54 @x=-100, @y=-100> #<Point:0x2424dc8 @x=20, @y=20> #<Point:0x2424d3c @x=-20, @y=-20> #<Point:0x2424cc4 @x=300, @y=300> #<Point:0x2424c38 @x=500, @y=500> #<Point:0x2424bc0 @x=70, @y=70> #<Point:0x2424b20 @x=70, @y=70> 30 - p will actually be taken as p - 30 by the coerce function. Can it be made to work? I am actually surprise that the - method won't coerce the argument this way: class Fixnum def -(something) if (/* something is unknown class */) a, b = something.coerce(self) return -(a - b) # because we are doing a - b but we wanted b - a, so it is negated end end end that is, the function returns a negated version of a - b instead of just returning a - b.

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  • Can a binary tree or tree be always represented in a Database 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 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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  • Can MySQL / SQL's short hand of "Using" be used without saying "Inner Join" ?

    - by Jian Lin
    The following 2 statements are to join using gifts.giftID = sentgifts.giftID: mysql> select * from gifts, sentgifts using (giftID); ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'using (giftID)' at line 1 and the second one: mysql> select * from gifts INNER JOIN sentgifts using (giftID); +--------+------------+----------------+---------------------+--------+------------+--------+------+---------------------+ | giftID | name | filename | effectiveTime | sentID | whenSent | fromID | toID | trytryWhen | +--------+------------+----------------+---------------------+--------+------------+--------+------+---------------------+ | 2 | teddy bear | bear.jpg | 2010-04-24 04:36:03 | 4 | 2010-04-24 | NULL | 111 | 2010-04-24 03:10:42 | | 6 | beer | beer_glass.png | 2010-04-24 05:18:12 | 5 | 2010-03-03 | 11 | 22 | 2010-03-03 00:00:00 | | 6 | beer | beer_glass.png | 2010-04-24 05:18:12 | 6 | 2010-04-24 | 11 | 222 | 2010-04-24 03:54:49 | | 6 | beer | beer_glass.png | 2010-04-24 05:18:12 | 7 | 2010-04-24 | 1 | 2 | 2010-04-24 03:58:45 | +--------+------------+----------------+---------------------+--------+------------+--------+------+---------------------+ 4 rows in set (0.00 sec) Can the first statement also use the "using" shorthand? It seems that when it is used then the word "Inner Join" must be specified... but the first statement is actually an inner join?

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  • Is using Natural Join or Implicit column names not a good practice when writing SQL in a programming

    - by Jian Lin
    When we use Natural Join, we are joining the tables when both table have the same column names. But what if we write it in PHP and then the DBA add some more fields to both tables, then the Natural Join can break? The same goes for Insert, if we do a insert into gifts values (NULL, "chocolate", "choco.jpg", now()); then it will break the code as well as contaminating the table when the DBA adds some fields to the table (example as column 2 or 3). So it is always best to spell out the column names when the SQL statements are written inside a programming language and stored in a file in a big project.

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  • In Ruby, can the coerce() method know what operator it is that requires the help to coerce?

    - by Jian Lin
    In Ruby, it seems that a lot of coerce() help can be done by def coerce(something) [self, something] end that's is, when 3 + rational is needed, Fixnum 3 doesn't know how to handle adding a Rational, so it asks Rational#coerce for help by calling rational.coerce(3), and this coerce instance method will tell the caller: # I know how to handle rational + something, so I will return you the following: [self, something] # so that now you can invoke + on me, and I will deal with Fixnum to get an answer So what if most operators can use this method, but not when it is (a - b) != (b - a) situation? Can coerce() know which operator it is, and just handle those special cases, while just using the simple [self, something] to handle all the other cases where (a op b) == (b op a) ? (op is the operator).

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  • Can Ruby on Rails handle AJAX Response by Static Javascript code? by using RJS?

    - by Jian Lin
    So it looks like on RoR, when Ajax (using form_remote_tag) returns a success code, Javascript is also returned to handle the visual effects. (this is the RJS mechanism) using Fiddler, I do see the following response: try { Element.update("vote_score", "Score 58"); $("vote_score").visualEffect("highlight"); } catch (e) { alert('RJS error:\n\n' + e.toString()); alert('Element.update(\"vote_score\", \"Score 58\");\n$(\"vote_score\").visualEffect(\"highlight\");'); throw e } Will there be situation where the code is quite big (like 1 or 2k) that RJS can use some sort of static Javascript already loaded to handle the effect? Is there any other ways to use static Javascript already loaded with the page? Must it be just raw Javascript and use the Prototype's Ajax success response to call the static Javascript code, or can it be some Rails mechanism related to RJS?

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  • How to implement a link that can get bonus for 5 times by different people on a Social Network?

    - by Jian Lin
    How can we generate a link that can be clicked on by 5 different people on a Social Network game, so that each of the five people can get a bonus? The link will be published as a "newsfeed" on the Social Network such as Facebook. The link shouldn't be easily "generated" by any people as a cheating method. Also, what database table(s) should be added to handle: 1) the bonus can be claimed up to 5 times 2) must be claimed by different people ?

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  • Can GIT, Mercurial, SVN, or other version control tools work well when project tree has binary files

    - by Jian Lin
    Sometimes our project tree can have binary files, such as jpg, png, doc, xls, or pdf. Can GIT, Mercurial, SVN, or other tools do a good job when only part of a binary file is changed? For example, if the spec is written in .doc and it is part of the repository, then if it is 4MB, and edited 100 times but just for 1 or 2 lines, and checked in 100 times during the year, then it is 400MB. If it is 100 different .doc and .xls files, then it is 40GB... not a size that is easy to manage. I have tried GIT and Mercurial and see that they both seem to add a big size of data even when 1 line is changed in a .doc or .pdf. Is there other way inside of GIT or Mercurial or SVN that can do the job?

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  • Why in Ruby, a || 1 will throw an error when `a` is undefined, but a = a || 1 will not?

    - by Jian Lin
    When a is undefined, then a || 1 will throw an error, but a = a || 1 will not. Isn't that a little bit inconsistent? irb(main):001:0> a NameError: undefined local variable or method `a' for main:Object from (irb):1 from c:/ruby/bin/irb:12:in `<main>' irb(main):002:0> a || 1 NameError: undefined local variable or method `a' for main:Object from (irb):2 from c:/ruby/bin/irb:12:in `<main>' irb(main):003:0> a = a || 1 => 1

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  • Does Ruby on Rails "has_many" array provide data on a "need to know" basis?

    - by Jian Lin
    On Ruby on Rails, say, if the Actor model object is Tom Hanks, and the "has_many" fans is 20,000 Fan objects, then actor.fans gives an Array with 20,000 elements. Probably, the elements are not pre-populated with values? Otherwise, getting each Actor object from the DB can be extremely time consuming. So it is on a "need to know" basis? So does it pull data when I access actor.fans[500], and pull data when I access actor.fans[0]? If it jumps from each record to record, then it won't be able to optimize performance by doing sequential read, which can be faster on the hard disk because those records could be in the nearby sector / platter layer -- for example, if the program touches 2 random elements, then it will be faster just to read those 2 records, but what if it touches all elements in random order, then it may be faster just to read all records in a sequential way, and then process the random elements. But how will RoR know whether I am doing only a few random elements or all elements in random?

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  • Thinking of an Inner Join as a Cross Join and then satisfying some condition(s)?

    - by Jian Lin
    It seems like the safest way to think of an Inner Join is to think of it as a Cross Join and then satisfying some condition(s)? Because the equi-join can be obvious, but the non-equi-join can be a bit confusing. But if we always use the Cross Join, and then filter out the ones satisfying the condition, then we get the resulting table. In other words, we can always analyze it by using the first record on the left table, and then go through every single records on the right, and then repeat that for 2nd record on the left, and for the 3rd, 4th, ... etc. So in our mind, we can analyze it using this way, and it is like O(n^2), although what happens in the DBMS maybe that it is a lot faster (when an index is present). Is there another good way to think of it besides this method?

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  • It seems like the safest way to think of an Inner Join is to think of it as a Cross Join and then sa

    - by Jian Lin
    It seems like the safest way to think of an Inner Join is to think of it as a Cross Join and then satisfying some condition(s)? Because the equi-join can be obvious, but the non-equi-join can be a bit confusing. But if we always use the Cross Join, and then filter out the ones satisfying the condition, then we get the resulting table. In other words, we can always analyze it by using the first record on the left table, and then go through every single records on the right, and then repeat that for 2nd record on the left, and for the 3rd, 4th, ... etc. So in our mind, we can analyze it using this way, and it is like O(n^2), although what happens in the DBMS maybe that it is a lot faster (when an index is present). Is there another good way to think of it besides this method?

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  • Why is the ( ) mandatory in the SQL statement select * from gifts INNER JOIN sentgifts using (giftID

    - by Jian Lin
    Why is the ( ) mandatory in the SQL statement select * from gifts INNER JOIN sentgifts using (giftID); ? The ( ) usually is for specifying grouping of something. But in this case, are we supposed to be able to use 2 or more field names... in the example above, it can be all clear that it is 1 field, is it just that the parser is not made to bypass the ( ) when it is all clear? (such as in the language Ruby).

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  • In SQL, in what situation do we want to Index a field in a table, or 2 fields in a table at the same

    - by Jian Lin
    In SQL, it is obvious that whenever we want to do a search on millions of record, say CustomerID in a Transactios table, then we want to add an index for CustomerID. Is another situation we want to add an index to a field when we need to do inner join or outer join using that field as a criteria? Such as Inner join on t1.custumerID = t2.customerID. Then if we don't have an index on customerID on both tables, we are looking at O(n^2) because we need to loop through the 2 tables sequentially. If we have index on customerID on both tables, then it becomes O( (log n) ^ 2 ) and it is much faster. Any other situation where we want to add an index to a field in a table? What about adding index for 2 fields combined in a table. That is, one index, for 2 fields together?

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  • In Ruby, how does coerce() actually work?

    - by Jian Lin
    It is said that when we have a class Point and knows how to perform point * 3 like the following: class Point def initialize(x,y) @x, @y = x, y end def *(c) Point.new(@x * c, @y * c) end end point = Point.new(1,2) p point p point * 3 Output: #<Point:0x336094 @x=1, @y=2> #<Point:0x335fa4 @x=3, @y=6> but then, 3 * point is not understood: Point can't be coerced into Fixnum (TypeError) So we need to further define an instance method coerce: class Point def coerce(something) [self, something] end end p 3 * point Output: #<Point:0x3c45a88 @x=3, @y=6> So it is said that 3 * point is the same as 3.*(point) that is, the instance method * takes an argument point and invoke on the object 3. Now, since this method * doesn't know how to multiply a point, so point.coerce(3) will be called, and get back an array: [point, 3] and then * is once again applied to it, is that true? point * 3 which is the same as point.*(3) and now, this is understood and we now have a new Point object, as performed by the instance method * of the Point class. The question is: 1) who invokes point.coerce(3) ? Is it Ruby automatically, or is it some code inside of * method of Fixnum by catching an exception? Or is it by case statement that when it doesn't know one of the known types, then call coerce? 2) Does coerce always need to return an array of 2 elements? Can it be no array? Or can it be an array of 3 elements? 3) And is the rule that, the original operator (or method) * will then be invoked on element 0, with the argument of element 1? (element 0 and element 1 are the two elements in that array returned by coerce) Who does it? Is it done by Ruby or is it done by code in Fixnum? If it is done by code in Fixnum, then it is a "convention" that everybody follows when doing a coerce? So could it be the code in * of Fixnum do something like this: if (something.typeof? ...) else if ... # other type else if ... # other type else # if it is not a type I know array = something.coerce(self) return array[0].*(array[1]) end

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  • How can this closure test be written in other languages?

    - by Jian Lin
    I wonder how the following closure test can be written in other languages, such as C and Java. Can the same result be expected also in Perl, Python, and PHP? Ideally, we don't need to make a new local variable such as x and assign it the value of i inside the loop, but just so that i has a new copy in the new scope each time. (if possible). (some discussion is in this question.) The following is in Ruby, the "1.8.6" on the first line of result is the Ruby version which can be ignored. p RUBY_VERSION $foo = [] (1..5).each do |i| $foo[i] = lambda { p i } end (1..5).each do |j| $foo[j].call() end the print out is: [MacBook01:~] $ ruby scope.rb "1.8.6" 1 2 3 4 5 [MacBook01:~] $ Contrast that with another test, with i defined outside: p RUBY_VERSION $foo = [] i = 0 (1..5).each do |i| $foo[i] = lambda { p i } end (1..5).each do |j| $foo[j].call() end the print out: [MacBook01:~] $ ruby scope2.rb "1.8.6" 5 5 5 5 5 [MacBook01:~] $

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