MySQL中遇到的问题以及解决方法（一）出现负值

SELECT
    p.PatientID ,p.SBP, p.SBP2,p.SBP-p.SBP2
FROM
    ppg_data AS p 
    where p.SBP2 is not null 
    and p.SBP is not null

更改为

SELECT

p.PatientID ,p.SBP, p.SBP2,cast(p.SBP as signed)-cast(p.SBP2 as signed) FROM ppg_data AS p where p.SBP2 is not null and p.SBP is not null