What is CROSS JOIN (SELECT 0 as a UNION ALL SELECT 1 UNION ALL SELECT 2 UNION…)


What is CROSS JOIN (SELECT 0 as a UNION ALL SELECT 1 UNION ALL SELECT 2 UNION…)



I came across an odd bit of legacy code while going back through old code for cleanup, and I'm trying to pin down exactly what it does...


CROSS JOIN (SELECT 0 as a UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3) as b



It's a technique I haven't seen before, including multiple union selects of numbers inside a cross join, and I'm honestly really confused about what it's doing and why someone would do it.





Possible duplicate of SQL Server: What is the difference between CROSS JOIN and FULL OUTER JOIN?
– Alessandro Da Rugna
Jun 30 at 23:31




1 Answer
1



Cross join creates a Cartesian product.


Cross join



Assuming the from clause looks like:


from


FROM t CROSS JOIN
( . . . )



This creates four rows for every one row in t. The value of b.a in these rows varies from 0 to 3.


t


b.a





So, basically, it turns [column1:foo, column2:bar] into [column1:foo, column2:bar, b:0], [column1:foo, column2:bar, b:1], [column1:foo, column2:bar, b:1], [column1:foo, column2:bar, b:3]?
– liljoshu
Jun 30 at 22:23






Just tested on a different server... yea, that's exactly what it does... weird, not sure the point of it though. Thanks!
– liljoshu
Jun 30 at 22:41






By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

-

Popular posts from this blog

How to input without newline? (Python)

How to input without newline? (Python) I am trying to create a simple file editor in Python via the console (called PyCons). Here is my current program: def PyCons(f): file = open(f, "r") appe = open(f, "a") print("n=====nPyCons Editor, v1.2nPython 3.6.1n") global i i = 0 for line in file: i += 1 print(" {}| ".format(i) + line, end='') for k in range(10000000000000): print("n {}| ".format(i+1), end='') inp = input("") if inp == "@PyCons.save()": print("n=====") break else: i += 1 appe.write("n" + inp) I used the end method to make sure that the existing code in the file was printed properly in its line format, but the input function does not have an end attribute. So, when I go to enter code in the editor: end input end PyCons("magic.html") ... ===== PyCons Editor, v1.2 Python 3.6.1 1| <p>Hello!...
Read more

C++ thread error: no type named ‘type’ MINGW

C++ thread error: no type named ‘type’ MINGW I'm currently doing a project for my operational systems class and I have to make a program to find prime numbers to run in threads. So I did this: #include <iostream> #include <cmath> #include <thread> #define THREADNUMBER 100 using namespace std; int CONTADOR = 0; int CONTADORTHREADEXECUTADA = 0; int primeRange(int v1, int v2) { int a, limit; bool isprime; for (int i = v1; i <= v2; i++) { if (i % 2 != 0 && i % 3 != 0) { isprime = true; limit = i / 2; if(i == 1) isprime =false; limit = (int)sqrt(i); //General case for(a=2; a <= limit; a++){ if(i % i == 0 && i != 2){ isprime = false; break; } } if (isprime) { CONTADOR++; } } } CONTADORTHREADEXECUTADA++; return 1; } int main(int argc, char *argv[ ] ) { int number1 = atoi(argv[1]); int num...
Read more

Analog for TagView in flutter

Image
Analog for TagView in flutter I want to have UI like this: With native Android it can be done with libraries like https://github.com/whilu/AndroidTagView, how can it be done with flutter? 2 Answers 2 you can make the same UI by combining the Wrap and Chip widget as @wasyl montioned . but this is a full example about what you need Wrap Chip Notes : spacing deleteIcon avatar Chip Shape new Column( crossAxisAlignment: CrossAxisAlignment.stretch, children: <Widget>[ new Wrap( children: <Widget>[ Chip( label: new Text("Java"), onDeleted: () {}, labelPadding: EdgeInsets.all(2.0), deleteIcon: Icon(Icons.clear), ), Chip( label: new Text("C++"), onDeleted: () {}, labelPadding: EdgeInse...
Read more