MySQL Simple row num incremental counter - mysql

Question

I'm trying to produce an incremental counter (column 'rownum' in the query below) in an SQL select clause. the counter should start over each time a new user switches.
SELECT * FROM (
SELECT CONCAT(' ',g.node1,' ',g.node2),
#r:= CASE WHEN #g = g.`user` THEN #r +1 ELSE 1 END rownum,
#g:= g.`user` user_group
FROM sn.sn_graph_reduced g
CROSS JOIN (SELECT #g:=0,#r:=0) t2
ORDER BY `user` , RAND()
) t
WHERE rownum <= 100
However, the above code snippet returns the row number, and since the records are RANDOMLY sampled, the row numbers are not incremental.
What I need is a simple counter (1,2,3....) for each row returned.
thanks

Answer 1

Try putting the variable outside of the inline view:
SELECT t.*,
#r:= CASE WHEN #t = t.`user` THEN #r +1 ELSE 1 END rownum,
#t:= t.`user` user_group
FROM (
SELECT CONCAT(' ',g.node1,' ',g.node2), g.`user`
FROM sn.sn_graph_reduced g
ORDER BY `user` , RAND()
) t
CROSS JOIN (SELECT #t:=0,#r:=0) t2
where rownum <= 100

Answer 2

the method for incrementing row number doesn't cope with RAND() directly, so use values from rand() as a column. Also initiate #g as '' not zero and then you need a final ORDER BY.
SELECT
*
FROM (
SELECT
CONCAT(' ', g.node1, ' ', g.node2) AS node_concat
, #r:= IF(#g = g.`user`, #r + 1, 1) AS rownum
, #g:= g.`user` AS user_group
FROM (
SELECT *, rand() AS R FROM sn_graph_reduced
) g
CROSS JOIN ( SELECT #g:= '' ,#r:= 1 ) t2
ORDER BY
`user`
, R
) t
WHERE rownum <= 100
ORDER BY
user_group
, rownum
;
see: This SQLfiddle