我想对以下数据项进行排序，但有时他会将B放入A.（U8B-U8A，U11B-U11A，U15B-U15A和U17B-U17A） 使用的查询是：
SELECT * FROM trainers
ORDER BY CAST(SUBSTR(cat,2) AS SIGNED) AS
如何解决这个问题？
Cat fisrt last
U5 Steve Vangeel
U6 Steve Vangeel
U7A Davy Cums
U7B Gerbe Suttels
U8B Roel Tastenoye
U8A Davy Vlasselaerts
U9A Jordi Knops
U9B Piet Veulemans
U9C Tom Delie
U9C Patrick Uyttebroeck
U10A Jonas Delvaux
U10B Ruben Francois
U11B Peter Desmet
U11A Tom Delie
U12B Hans Smets
U12A Joeri Tits
U13 Patrick Vanuytrecht
U15B Ronny Vanderhoeft
U15A Gunther Roosen
U17B Dirk Knops
U17A Jonas Vanhelmont
U17A Tim Knops
U21A Patrick Uyttebroeck
U21B Luc Mathues