Error when adding a new column and change the value in the same SQL script

Question

In one SQL script (SQL Server 2016) I want to add a column by using ALTER TABLE and change the values by using UPDATE:

IF NOT EXISTS (SELECT * FROM sys.all_columns  
               WHERE object_id = OBJECT_ID(N'MyTable') and NAME = 'MyAttribute')
BEGIN
    ALTER TABLE MyTable 
        ADD MyAttribute BIT NOT NULL DEFAULT(0);

    UPDATE MyTable 
    SET MyAttribute = 1;
END

but I get an error

Invalid column name 'MyAttribute'

for line 5 (the update statement).

The background for the code is:

• The script is running every time a new program version for the database is installed (I can't change this behaviour)
• MyAttribute should only be added at the first time of script execution
• The value for all existing records should be 1, but the default value for all new records must be 0

Answer 1

They need to run in different batches:

declare @new bit = 0;

IF NOT EXISTS (SELECT * FROM sys.all_columns  
               WHERE object_id = OBJECT_ID(N'MyTable') and NAME = 'MyAttribute')
BEGIN
    set @new = 1;
    ALTER TABLE MyTable ADD MyAttribute BIT NOT NULL DEFAULT(0);

END
Go
if @new = 1
begin
    UPDATE MyTable SET MyAttribute = 1;
end

EDIT:

IF NOT EXISTS (SELECT * FROM sys.all_columns  
               WHERE object_id = OBJECT_ID(N'MyTable') and NAME = 'MyAttribute')
BEGIN
    ALTER TABLE MyTable 
        ADD MyAttribute BIT NOT NULL 
        CONSTRAINT df_attr DEFAULT(1);
    ALTER TABLE MyTable
        DROP CONSTRAINT df_attr; 
    ALTER TABLE MyTable
        ADD CONSTRAINT df_attr DEFAULT 0 FOR MyAttribute; 
END