sql 根据身份证判断年龄是否小于18岁

SELECT *, 
Age=
datediff(yy,cast(case 
when substring(PersonalId,7,1) in ('1','2')   /*若第7位不是'1'或'2'则表示是15位身份证编码规则*/
then substring(PersonalId,7,8) 
else '19' + substring(PersonalId,7,6) 
end as datetime),getdate())
FROM Student
WHERE
datediff(yy,cast(case 
when substring(PersonalId,7,1) in ('1','2') 
then substring(PersonalId,7,8) 
else '19' + substring(PersonalId,7,6) 
end as datetime),getdate()) <18