SQLITE根据索引获取列的子字符串

Question

我想根据sqlite中的索引获取列的子字符串。例如，我有一个名为 ID 的列，如 100P_10 。我想在＆＃39; _＆＃39; 之后获取数字。目前我正在使用以下查询来获取此信息。但我确信这不是正确的做法。请帮我解决这个问题。

select max(CAST( (case when substr(ID,1,1)  is '_' then  substr(ID,1+1) when substr(ID,2,1)  is '_' then  substr(ID,2+1) when substr(ID,3,1)  is '_' then  substr(ID,3+1) when substr(ID,4,1)  is '_' then  substr(ID,4+1) when substr(ID,5,1)  is '_' then  substr(ID,5+1)when substr(ID,6,1)  is '_' then  substr(ID,6+1) when substr(ID,7,1)  is '_' then  substr(ID,7+1) else ID end) as integer)) from student

Answer 1

select cast(substr(a, instr(a, '_') + 1) as integer) from A;