In dbt, i want to append table if exists. Basically, i am building an incremental model where i find out lagged values from previous day, for the first run, i want to union today’s data along with {{this}}, i know that for the first day {{this}} has no values, so i am trying to skip it in the union.

I am using the below query but it is returning an error:

{{
    config(
        materialized='incremental',
        unique_key='date_run',
        full_refresh=true
    )
}}

with subs_count as (
    select 
        current_date() as date_run,
        count(distinct email_address) as distinct_email,
        subscriber
    from {{ source('fan_table_sandbox', 'international_fan_data') }}
    group by date_run,subscriber
),

 aggregated_data as (
    select
        *
    from subs_count

    union all

    if(exists( (select 
        *
    from {{this}} if exists {{this}}
    order by date_run desc limit 1))
    
    
),

lag_values as (
    select  
        *,
        lag(distinct_email) over (partition by nba_subscriber order by  date_run asc) as previous_day_count
    from aggregated_data
)
select * from lag_values

how can this be resolved?