mysql - MYSQL中如何生成子集？

Question

我有一封表信。一列名为“letter”，类型为 VARCHAR。 例如

---------
|letter|
|a     |
|b     |
|c     |
--------

我想要的输出是:

-----------------
|NULL|NULL|NULL|
|a   |NULL|NULL|
|b   |NULL|NULL|
|c   |NULL|NULL|
|a   |b   |NULL|
|a   |c   |NULL|
|b   |c   |NULL|
|a   |b   |c   |
-----------------

如何生成这样的表格....

Answer 1

CREATE TABLE letters (letter char);
INSERT INTO letters VALUES ('a');
INSERT INTO letters VALUES ('b');
INSERT INTO letters VALUES ('c');

SELECT
   t1.letter,
   t2.letter,
   t3.letter
 FROM 
   (SELECT NULL AS letter UNION SELECT letter FROM letters) t1
   JOIN (SELECT NULL AS letter UNION SELECT letter FROM letters) t2 ON t2.LETTER IS NULL OR t2.letter > t1.letter
   JOIN (SELECT NULL AS letter UNION SELECT letter FROM letters) t3 ON t3.LETTER IS NULL OR t3.letter > t2.letter

SQL fiddle :http://sqlfiddle.com/#!9/a8232b9/2