Can't assign value to column that was just created in table - sql

Question

For some reason when I run this, it says Invalid column name 'col3'.:
IF NOT EXISTS (SELECT name FROM SYS.COLUMNS WHERE name = 'col3' AND object_id IN (SELECT object_id
FROM SYS.TABLES WHERE name = 'table1')) BEGIN
ALTER TABLE table1 ADD col3 int
UPDATE table1 SET col3=col1+col2
END
But if I alter the table first and after the END of the IF I try to update the value of col3 like this, it just works:
IF NOT EXISTS (SELECT name FROM SYS.COLUMNS WHERE name = 'col3' AND object_id IN (SELECT object_id
FROM SYS.TABLES WHERE name = 'table1')) BEGIN
ALTER TABLE table1 ADD col3 int
END
UPDATE table1 SET col3=col1+col2
Why can't I update it when I create it?

Answer 1

This is a parse-time error - SQL Server is trying to validate the entire batch as a single, atomic operation. It doesn't see that you are going to add a column with that name, it just knows that there isn't a column with that name now - it evaluates this independently from all of the other statements in the batch. For the same reason you can't do this:
IF 1 = 1
CREATE TABLE #t(i INT);
ELSE
CREATE TABLE #t(y INT);
Obviously you and I know that only one of those branches will ever execute, but the error message you get from SQL Server (there is already an object named #t) hints that it doesn't understand branching or sequencing.
Two ways to circumvent this:
Issue the two commands in separate batches. If you are using Management Studio, simply put a GO between the ALTER and the UPDATE. This will force Management Studio to evaluate the batches in dependency order. Or even more simply - highlight the ALTER, and run that, then highlight the UPDATE, and run that.
Execute the update using dynamic SQL.
IF NOT EXISTS (blah blah)
BEGIN
ALTER TABLE dbo.table1 ADD col3 INT;
END
EXEC dbo.sp_executesql N'UPDATE dbo.table1 SET col3 = col1 + col2;';