Conditional expressions allow developers to control the action of the function, based on a defined criteria. PostgreSQL provides the CASE and IF statements to execute different commands based on conditions. The following is an example of the usage of a CASE statement to control how a string is treated based on its value. If the value is null or contains a zero-length string, it is treated the same as null:
CREATE OR REPLACE FUNCTION format_us_full_name(
prefix text, firstname text,
mi text, lastname text,
suffix text)
RETURNS text AS
$$
DECLARE
fname_mi text;
fmi_lname text;
prefix_fmil text;
pfmil_suffix text;
BEGIN
fname_mi := CONCAT_WS(' ',
CASE trim(firstname)
WHEN ''
THEN NULL
ELSE firstname
END,
CASE trim(mi)
WHEN ''
THEN NULL
ELSE mi
...