SQL ELSE block still throws error when IF condition is true - sql-server

Question

I have the following T-SQL script that copies the value of an old column into a new one, then drops the old column. See here:
--step 1: create new column
IF NOT EXISTS(SELECT 1 from sys.columns
WHERE Name = N'UserColumn2'
AND Object_ID = Object_ID(N'Account'))
BEGIN
ALTER TABLE Account
ADD UserColumn2 int null
;
END
GO
;
--step 2: copy and drop
IF NOT EXISTS(SELECT 1 from sys.columns
WHERE Name = N'UserColumn1'
AND Object_ID = Object_ID(N'Account'))
BEGIN
PRINT 'Column ''UserColumn1'' does not exist.';
END
ELSE
BEGIN
UPDATE Account
SET UserColumn2 = UserColumn1
WHERE UserColumn1 is not null
;
BEGIN TRY
Declare #count int;
SELECT #count = Count(AccountID)
FROM Account
WHERE UserColumn2 <> UserColumn1
;
IF #count > 0
BEGIN
--PRINT 'Not all records were properly updated. UserColumn1 has not been dropped.';
THROW 50000,'Not all records were properly updated. UserColumn1 has not been dropped.',1
;
END
ELSE
BEGIN
ALTER TABLE Account
DROP Column UserColumn1
;
END
END TRY
BEGIN CATCH THROW; END CATCH
END
GO
;
The first step runs correctly but the second step still throws an error in the ELSE block even if the UserColumn1 column doesn't exist:
(note: this actually throws on line 24 for the code here. The code in my SSMS doesn't have the comments for 'step 1', etc.)
Why is this happening and how can I prevent it?
I've tried removing the NOT and moving the instructions out of the ELSE block but the behavior did not change. I've also tried writing the beginning of the second step like this:
IF (SELECT 1 from sys.columns
WHERE Name = N'UserColumn1'
AND Object_ID = Object_ID(N'Account')) <> null
and I get the same result.

Answer 1

The issue is that the entire sql text is parsed and compiled before it's executed, the error is being thrown at compile time.
You could workaround it by executing the update statement in its own process using dynamic sql - although there is nothing dynamic in this usage, it simply defers the compilation and execution of the update statement where it only happens in your else condition:
IF NOT EXISTS(SELECT 1 from sys.columns
WHERE Name = N'UserColumn1'
AND Object_ID = Object_ID(N'Account'))
BEGIN
PRINT 'Column ''UserColumn1'' does not exist.';
END
ELSE
BEGIN
EXEC sp_executesql N'
UPDATE Account
SET UserColumn2 = UserColumn1
WHERE UserColumn1 is not null;'
...
...