concatenate recursive cross join - sql

Question

I need to concatenate the name in a recursive cross join way. I don't know how to do this, I have tried a CTE using WITH RECURSIVE but no success.
I have a table like this:
group_id | name
---------------
13 | A
13 | B
19 | C
19 | D
31 | E
31 | F
31 | G
Desired output:
combinations
------------
ACE
ACF
ACG
ADE
ADF
ADG
BCE
BCF
BCG
BDE
BDF
BDG
Of course, the results should multiply if I add a 4th (or more) group.

Answer 1

Native Postgresql Syntax:
SqlFiddleDemo
WITH RECURSIVE cte1 AS
(
SELECT *, DENSE_RANK() OVER (ORDER BY group_id) AS rn
FROM mytable
),cte2 AS
(
SELECT
CAST(name AS VARCHAR(4000)) AS name,
rn
FROM cte1
WHERE rn = 1
UNION ALL
SELECT
CAST(CONCAT(c2.name,c1.name) AS VARCHAR(4000)) AS name
,c1.rn
FROM cte1 c1
JOIN cte2 c2
ON c1.rn = c2.rn + 1
)
SELECT name as combinations
FROM cte2
WHERE LENGTH(name) = (SELECT MAX(rn) FROM cte1)
ORDER BY name;
Before:
I hope if you don't mind that I use SQL Server Syntax:
Sample:
CREATE TABLE #mytable(
ID INTEGER NOT NULL
,TYPE VARCHAR(MAX) NOT NULL
);
INSERT INTO #mytable(ID,TYPE) VALUES (13,'A');
INSERT INTO #mytable(ID,TYPE) VALUES (13,'B');
INSERT INTO #mytable(ID,TYPE) VALUES (19,'C');
INSERT INTO #mytable(ID,TYPE) VALUES (19,'D');
INSERT INTO #mytable(ID,TYPE) VALUES (31,'E');
INSERT INTO #mytable(ID,TYPE) VALUES (31,'F');
INSERT INTO #mytable(ID,TYPE) VALUES (31,'G');
Main query:
WITH cte1 AS
(
SELECT *, rn = DENSE_RANK() OVER (ORDER BY ID)
FROM #mytable
),cte2 AS
(
SELECT
TYPE = CAST(TYPE AS VARCHAR(MAX)),
rn
FROM cte1
WHERE rn = 1
UNION ALL
SELECT
[Type] = CAST(CONCAT(c2.TYPE,c1.TYPE) AS VARCHAR(MAX))
,c1.rn
FROM cte1 c1
JOIN cte2 c2
ON c1.rn = c2.rn + 1
)
SELECT *
FROM cte2
WHERE LEN(Type) = (SELECT MAX(rn) FROM cte1)
ORDER BY Type;
LiveDemo
I've assumed that the order of "cross join" is dependent on ascending ID.
cte1 generate DENSE_RANK() because your IDs contain gaps
cte2 recursive part with CONCAT
main query just filter out required length and sort string

Answer 2

The recursive query is a bit simpler in Postgres:
WITH RECURSIVE t AS ( -- to produce gapless group numbers
SELECT dense_rank() OVER (ORDER BY group_id) AS grp, name
FROM tbl
)
, cte AS (
SELECT grp, name
FROM t
WHERE grp = 1
UNION ALL
SELECT t.grp, c.name || t.name
FROM cte c
JOIN t ON t.grp = c.grp + 1
)
SELECT name AS combi
FROM cte
WHERE grp = (SELECT max(grp) FROM t)
ORDER BY 1;
The basic logic is the same as in the SQL Server version provided by #lad2025, I added a couple of minor improvements.
Or you can use a simple version if your maximum number of groups is not too big (can't be very big, really, since the result set grows exponentially). For a maximum of 5 groups:
WITH t AS ( -- to produce gapless group numbers
SELECT dense_rank() OVER (ORDER BY group_id) AS grp, name AS n
FROM tbl
)
SELECT concat(t1.n, t2.n, t3.n, t4.n, t5.n) AS combi
FROM (SELECT n FROM t WHERE grp = 1) t1
LEFT JOIN (SELECT n FROM t WHERE grp = 2) t2 ON true
LEFT JOIN (SELECT n FROM t WHERE grp = 3) t3 ON true
LEFT JOIN (SELECT n FROM t WHERE grp = 4) t4 ON true
LEFT JOIN (SELECT n FROM t WHERE grp = 5) t5 ON true
ORDER BY 1;
Probably faster for few groups. LEFT JOIN .. ON true makes this work even if higher levels are missing. concat() ignores NULL values. Test with EXPLAIN ANALYZE to be sure.
SQL Fiddle showing both.