This question popped in the microsoft.public.sqlserver.programming forum yesterday. A person wanted to change a column from varchar 500 to varchar 2000. This is actually pretty easy to do in SQL Server, you can use the alter table alter column command
Let’s take a quick look at how this works
First create this table

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CREATE TABLE [TestTable] (
[id] [int] IDENTITY (1, 1) NOT NULL ,
[itemdate] [datetime] NOT NULL ,
[title] [varchar] (500) NOT NULL ,
) ON [PRIMARY]
CREATE TABLE [TestTable] (
[id] [int] IDENTITY (1, 1) NOT NULL ,
[itemdate] [datetime] NOT NULL ,
[title] [varchar] (500) NOT NULL ,
) ON [PRIMARY]

Now insert one row of data

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insert [TestTable]
select getdate(),'bla'
insert [TestTable]
select getdate(),'bla'

Now do a simple select and verify that you have one row of data

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select * from [TestTable]
select * from [TestTable]

Results

id          itemdate                title
----------- ----------------------- ----------
1           2010-01-18 17:28:19.820 bla

We will change the column from varchar 500 to varchar 2000. All you have to do is alter table [table name] alter column [column name] [new data type and length]
So in this case the command would look like this

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alter table [TestTable] alter column [title] [varchar] (2000)
alter table [TestTable] alter column [title] [varchar] (2000)

To verify that indeed the column is now varchar 2000 run the following query

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select COLUMN_NAME,DATA_TYPE,CHARACTER_MAXIMUM_LENGTH
from INFORMATION_SCHEMA.COLUMNS
where table_name = 'TestTable'
select COLUMN_NAME,DATA_TYPE,CHARACTER_MAXIMUM_LENGTH
from INFORMATION_SCHEMA.COLUMNS
where table_name = 'TestTable'

Results

COLUMN_NAME    DATA_TYPE       CHARACTER_MAXIMUM_LENGTH
-------------- -----------------------------------------
id              int             NULL
itemdate        datetime        NULL
title           varchar         2000

Now let’s change the datetime column to a varchar, execute the following query

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alter table [TestTable] alter column [itemdate] [varchar] (49)
alter table [TestTable] alter column [itemdate] [varchar] (49)

Now verify that it was changed

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select COLUMN_NAME,DATA_TYPE,CHARACTER_MAXIMUM_LENGTH
from INFORMATION_SCHEMA.COLUMNS
where table_name = 'TestTable'
select COLUMN_NAME,DATA_TYPE,CHARACTER_MAXIMUM_LENGTH
from INFORMATION_SCHEMA.COLUMNS
where table_name = 'TestTable'

Results

COLUMN_NAME    DATA_TYPE       CHARACTER_MAXIMUM_LENGTH
-------------- -----------------------------------------
id              int             NULL
itemdate        varchar         49
title           varchar         2000

Insert a new row

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insert [TestTable]
select getdate(),'bla'
insert [TestTable]
select getdate(),'bla'

Now check what is in the table

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select * from [TestTable]
select * from [TestTable]

Results

id          itemdate                                          title
----------- ------------------------------------------------- -----------
1           Jan 18 2010  5:28PM                               bla
2           Jan 18 2010  5:30PM                               bla

Now we will add a column. The command to add a column is very similar to the one where you alter a column, the difference is that you don’t use the column keyword. Below is a query that will add an int column with a default of 0

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alter table [TestTable] add  [SomeInt] int default 0 not null
alter table [TestTable] add  [SomeInt] int default 0 not null

Run this query to see what is in the table now

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select * from [TestTable]
select * from [TestTable]

Results

id          itemdate                    title     SomeInt
----------- ---------------------------- ---------- -----------
1           Jan 18 2010  5:28PM         bla       0
2           Jan 18 2010  5:30PM         bla       0

As you can see the column was added and the default of 0 was used.

We can interrogate the INFORMATION_SCHEMA.COLUMNS view again to verify that the column is there

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select COLUMN_NAME,DATA_TYPE,CHARACTER_MAXIMUM_LENGTH
from INFORMATION_SCHEMA.COLUMNS
where table_name = 'TestTable'
select COLUMN_NAME,DATA_TYPE,CHARACTER_MAXIMUM_LENGTH
from INFORMATION_SCHEMA.COLUMNS
where table_name = 'TestTable'

Results

COLUMN_NAME    DATA_TYPE       CHARACTER_MAXIMUM_LENGTH
-------------- -----------------------------------------
id              int             NULL
itemdate        varchar         49
title           varchar         2000
SomeInt		int		NULL

Now let’s insert a row, we will use a value of 2 for the newly added SomeInt column

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insert [TestTable]
select getdate(),'bla',2
insert [TestTable]
select getdate(),'bla',2

When we run this query again

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select * from [TestTable]
select * from [TestTable]

We can see that the value 2 is there

Results

id          itemdate			title     SomeInt
----------- ------------------------------------------------- 
1           Jan 18 2010  5:28PM           bla       0
2           Jan 18 2010  5:30PM           bla       0
3           Jan 18 2010  5:33PM           bla       2

If you try to change a column to a datatype which is incompatible with the data that is stored you will get an error message

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alter table [TestTable] alter column [itemdate] int
alter table [TestTable] alter column [itemdate] int

Here is the error for the above command
Msg 245, Level 16, State 1, Line 1
Conversion failed when converting the varchar value ‘Jan 18 2010 5:12PM’ to data type int.
The statement has been terminated.

That is it for this post, as you can see it is pretty easy to change the data type of a column by using the alter table alter column command

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