Converting Data Type of a full Column/Field

Question

I have a primary key ID column I want to convert to a string, any type of string. Now, I know how to do this for a specific value, by using either CAST or CONVERT. But how do I go about doing it for the whole field? I guess we use some type of Transact SQL , but I would appreciate some pointers on how to do this. Thanks.

Answer 1

I'm not sure what you mean by 'whole field", but this is how you'd convert an Integer to a varchar string. The 'second' select proves it's a string because you can concatenate brackets around it.

You'd apply the same technique when selecting rows from a table.

DECLARE @IntColumn int = 5
select convert(varchar(20),@IntColumn)
select '[' + convert(varchar(20),@IntColumn) + ']'

Answer 2

Changing the data type of a field used in the primary key of a table is a task not to be entered into lightly!

First you would need to drop the primary key constraint, then change the data type, then add a new primary key constraint. This will surely result in an outage and probably lots of other potential gotchas, so you'll need to think through all those implications.

Alter Table my_table Drop Constraint pk_my_table;
Alter Table my_table Alter Column ID varchar(11) Not Null;
Alter Table my_table Add Constraint pk_my_table Primary Key Clustered (ID);

(Note, this assumes that you don't have IDENTITY specified for the column. If you do then see https://stackoverflow.com/questions/8230257/remove-identity-from-a-column-in-a-table).

An alternative which may meet your needs is to add a computed column to the table (or to a view over the table) that contains the same value as the ID field but converted to a character type. This is less likely to interfere with other things, but still might cause trouble if there are SELECT * FROM my_table statements being used in applications.

Alter Table my_table Add ID_string As Convert(varchar(11), ID);