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While running the below code i get an error saying invalid input syntax for type timestamp from admission_datetime.

    UPDATE ccsm.stg_demographics_baseline
    SET xx_los_days =
    (CASE WHEN admission_datetime IS NULL OR 
    date_trunc('day',admission_datetime) = ''
    THEN NULL
    WHEN discharge_datetime IS NULL OR 
    date_trunc('day',discharge_datetime) = ''
    THEN date_diff('day', admission_datetime, CURRENT_DATE)
    date_diff('day', admission_datetime, discharge_datetime)
    END);
enter code here
  
The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).
So you can not compare it with an empty string:
date_trunc('day', admission_datetime) = ''
The invalid input syntax for type timestamp error message concerns the empty string (''), not the admission_datetime column.
Furthermore, there is no date_diff function in PostgreSQL. Just subtract one timestamp from another and you will get an interval result:
SELECT timestamp '2001-09-29 03:00' - timestamp '2001-09-27 12:00'
You'll get
interval '1 day 15:00:00'
If you need the difference in days, try this:
SELECT DATE_PART('day', timestamp '2001-09-29 03:00' - timestamp '2001-09-27 12:00')
The result is 1.
See here for examples of DATEDIFF-like expressions in PostgreSQL.
        
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