Transaction within IF THEN ELSE doesn't commit

Question

In my TSQL script I have an IF THEN ELSE structure that checks if a column already exists.
If not it creates the column and updates it.

IF NOT EXISTS(
    SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS
    WHERE TABLE_NAME = 'tableName' AND COLUMN_NAME = 'columnName'))
BEGIN
    BEGIN TRANSACTION
        ALTER TABLE tableName
        ADD columnName int NULL
    COMMIT
    BEGIN TRANSACTION
        update tableName
        set columnName = [something]
        from 
            [subquery]
    COMMIT
END

This doesn't work because the column doesn't exist after the commit.
Why doesn't the COMMIT commit?

Answer 1

I'm guessing you are getting an error at parse stage, rather than at execute stage. The COMMIT will indeed commit, but the query parser isn't as clever as the query execution engine, and all the parser knows is that it can see you referring to tableName.columnName, which at parse time doesn't exist.

Wrap the whole update statement in an EXEC:

EXEC ('
        update tableName
        set columnName = [something]
        from 
            [subquery]
')

and you should be OK. Bear in mind that you will need to double up 's within the 's of the EXEC.