[sql] IF-THEN-ELSE statements in postgresql

I'm looking to write a postgresql query to do the following :

if(field1 > 0,  field2 / field1 , 0)

I've tried this query, but it's not working

if (field1 > 0)
then return field2 / field1 as field3
else return 0 as field3

thank youu

case when field1>0 then field2/field1 else 0 end as field3

In general, an alternative to case when ... is coalesce(nullif(x,bad_value),y) (that cannot be used in OP's case). For example,

select coalesce(nullif(y,''),x), coalesce(nullif(x,''),y), *
from (     (select 'abc' as x, '' as y)
 union all (select 'def' as x, 'ghi' as y)
 union all (select '' as x, 'jkl' as y)
 union all (select null as x, 'mno' as y)
 union all (select 'pqr' as x, null as y)
) q

gives:

 coalesce | coalesce |  x  |  y  
----------+----------+-----+-----
 abc      | abc      | abc | 
 ghi      | def      | def | ghi
 jkl      | jkl      |     | jkl
 mno      | mno      |     | mno
 pqr      | pqr      | pqr | 
(5 rows)