SQL Server 2019

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create table MyData
(
  Item_Cost nvarchar(100)
);

insert into MyData (Item_Cost) values ('1.25000000000000000000');

select * from MyData;

Item_Cost

1.25000000000000000000

ALTER TABLE MyData Alter Column Item_Cost DECIMAL(10, 3);
--> decimal type with room for 10 numbers of which 3 numbers are available after the decimal place
select * from MyData;

Item_Cost

1.250