Error in postgresql function - postgresql

Question

Explain me please, why I'm getting error here:
djabase=# CREATE or REPLACE FUNCTION upset(domain_name varchar)
RETURNS TABLE (id int, domain varchar(50)) AS $$
BEGIN
SELECT domain from separser_domains where domain=$1;
EXCEPTION
when sqlstate 'no_data' then
INSERT into separser_domains(domain) VALUES (domain_name);
END; $$
LANGUAGE 'sql' STABLE;
ERROR: syntax error at or near "SELECT"
LINE 4: SELECT domain from separser_domains where domain=$1;

Answer 1

You are using PLPGSQL syntax but declaring the function as SQL. This the reason, why the function fails with a SYNTAX ERROR. The language of the function is declared with the statement LANGUAGE and you declared it in the last line:
LANGUAGE 'sql' STABLE;
SQL functions do not support the BEGIN - END statement and exceptions trapping.
To fix it, simple change LANGUAGE 'sql' in LANGUAGE 'plpgsql'.
Some other considerations:
Catching a exception in this place is probably not necessary. Use the FOUND variable instead.
With the STABLE keyword, the function can not perform INSERT. Change STABLEto VOLATILE.
You declared RETURNS TABLE but the function does not return anything. Use RETURNS VOID instead.
Using SELECT and discarding the result is not allowed. Use PERFORM instead.
A valid version of your version could be this:
DROP FUNCTION IF EXISTS upset(varchar);
CREATE or REPLACE FUNCTION upset(domain_name varchar)
RETURNS VOID AS $$
BEGIN
PERFORM domain from separser_domains where domain=domain_name;
IF NOT FOUND THEN
INSERT into separser_domains(domain) VALUES (domain_name);
END IF;
RETURN;
END;
$$
LANGUAGE 'plpgsql' VOLATILE;