Question

I’m having a problem when converting all table columns with ntext type in my database. I wrote this query in order to alter my columns, but there is a syntax error:

ALTER TABLE mytable ALTER COLUMN mycolumn 
VARCHAR(SELECT MAX(DATALENGTH(mycolumn)) FROM  mytable);

Even though SELECT MAX(DATALENGTH(mycolumn)) FROM mytable is returning the correct number, the query cannot be executed.

The syntax error is:

Incorrect syntax near the keyword ‘select’.(which is inside the
varchar)

How can I solve the problem?

Answer 1

You will need to execute this as dynamic sql because the size of the column cannot be a variable.

DECLARE @Length int = SELECT MAX(DATALENGTH(mycolumn)) FROM mytable
DECLARE @MyTable varchar(100) = 'mytable'
DECLARE @MyColumn varchar(100) = 'mycolumn'
DECLARE @SQL varchar(8000) = 'ALTER TABLE ' + @MyTable +' ALTER COLUMN '+ @MyColumn +' VARCHAR(' + CONVERT(varchar, @Length) + ')'
EXEC(@SQL)

The benefit of this is you could loop over sys.objects and sys.columns to find all ntext columns and convert them to varchar.