从MySQL字段中分隔几天

Question

美好的一天，我遇到了一个问题，其中天数的值用MTWThF，ThF，TTh等格式化。我想转换成列。这是我的代码：

SELECT 
dys,
CASE WHEN dys LIKE 'M%' THEN 'M' ELSE '' END AS M,
CASE WHEN (dys LIKE '%T%' AND NOT '%Th%') THEN 'T' ELSE '' END AS T,
CASE WHEN dys LIKE '%W%' THEN 'W' ELSE '' END AS W,
CASE WHEN dys LIKE '%Th%' AND NOT 'T' THEN 'Th' ELSE '' END AS th,
CASE WHEN dys LIKE '%F%' THEN 'F' ELSE '' END AS F,
CASE WHEN (dys LIKE '%S%' AND NOT 'Su') THEN 'S' ELSE '' END AS S,
CASE WHEN (dys LIKE '%Su%' AND NOT 'S') THEN 'Su' ELSE '' END AS Su
FROM sched
GROUP BY dys

，结果是

Tb result

直到在来自SSu的ThF相同的S上检测到T.我错过了什么吗？谢谢