MySQL で rownum

SELECT

@rownum := 0 AS rownum

, c1

, c2

UNION ALL

SELECT

@rownum := @rownum + 1 AS rownum

, c1

, c2

FROM

tbl1

ORDER BY

c1

とすると、なぜかNULLの行が含まれる...

wnum	c1	c2
0	NULL	NULL
1	1	2
2	1	3
3	1	4
4	1	5