IF-THEN-ELSE statements in postgresql
Solution 1:
As stated in PostgreSQL docs here:
The SQL CASE expression is a generic conditional expression, similar to if/else statements in other programming languages.
Code snippet specifically answering your question:
SELECT field1, field2,
CASE
WHEN field1>0 THEN field2/field1
ELSE 0
END
AS field3
FROM test
Solution 2:
case when field1>0 then field2/field1 else 0 end as field3