Altering a column: null to not null

First, make all current NULL values disappear:

UPDATE [Table] SET [Column]=0 WHERE [Column] IS NULL

Then, update the table definition to disallow "NULLs":

ALTER TABLE [Table] ALTER COLUMN [Column] INTEGER NOT NULL

I had the same problem, but the field used to default to null, and now I want to default it to 0. That required adding one more line after mdb's solution:

ALTER TABLE [Table] ADD CONSTRAINT [Constraint] DEFAULT 0 FOR [Column];

You will have to do it in two steps:

1. Update the table so that there are no nulls in the column.

UPDATE MyTable SET MyNullableColumn = 0
WHERE MyNullableColumn IS NULL

2. Alter the table to change the property of the column

ALTER TABLE MyTable
ALTER COLUMN MyNullableColumn MyNullableColumnDatatype NOT NULL