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,,) in ('','')   /*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,,)
else '' + substring(PersonalId,,)
end as datetime),getdate())
FROM Student
WHERE
datediff(yy,cast(case
when substring(PersonalId,,) in ('','')
then substring(PersonalId,,)
else '' + substring(PersonalId,,)
end as datetime),getdate()) <