你的字段是怎么存储的。只存储了年份与月分的话。就自己拼字符串啊

select cast(year(DATEADD(MONTH,-1,GETDATE())) as varchar) + '-' + right('00'+ cast(MONTH(DATEADD(MONTH,-1,GETDATE())) as varchar),2)

2021-09

select * from t where datefield = cast(year(DATEADD(MONTH,-1,GETDATE())) as varchar) + '-' + right('00'+ cast(MONTH(DATEADD(MONTH,-1,GETDATE())) as varchar),2)