问题
Possible Duplicate:
combinations (not permutations) from cross join in sql
I've currently got a table with the following records:
A1
A2
A3
B1
B2
C1
C2
Where the same letter denotes some criteria in common (e.g. a common value for the column 'letter'). I do a self join on the criteria as follows:
SELECT mytable.*, self.* FROM mytable INNER JOIN mytable AS self
ON (mytable.letter = self.letter and mytable.number != self.number);
This join gives something like the following:
A1 A2
A2 A1
A1 A3
A3 A1
A2 A3
A3 A2
B1 B2
B2 B1
C1 C2
C2 C1
However, I only want to include each pair once (a combination instead of a permutation). How would I get the following:
A1 A2
A1 A3
A2 A3
B1 B2
C1 C2
回答1:
Changing the JOIN condition slightly will achieve what you want..
Instead of:
ON (mytable.letter = self.letter and mytable.number != self.number)
use
ON (mytable.letter = self.letter and mytable.number > self.number)
This will only include combinations where self.number is greater than mytable.number which in effect restricts the results to one valid ordering of each combination...
来源：https://stackoverflow.com/questions/7452981/sql-self-join-using-each-rows-only-once