declare @Medications table
(
[Client_ID] int,
[ClientName] varchar(50),
[EffectiveDate] date,
[MedConditionID] int,
[MedCondition] varchar(50)
);
insert into @Medications(Client_Id, ClientName, EffectiveDate, MedConditionId, MedCondition)
values
(100, 'jeffrey', '20190116', 123, 'Alcohol'),
(100, 'jeffrey', '20191016', 123, 'Alcohol'),
(100, 'jeffrey', '20191016', 267, 'Head'),
(100, 'jeffrey', '20191220', 123, 'Alcohol'),
(100, 'jeffrey', '20191220', 267, 'Head'),
(100, 'jeffrey', '20200121', 123, 'Alcohol'),
(100, 'jeffrey', '20200121', 267, 'Head');
select EffectiveDate,
dateadd(day, -1,
lead(EffectiveDate, 1, '20510101' ) over(partition by Client_id, MedConditionId order by EffectiveDate)
) as ExpirationDate,
*
from @Medications;