where if postgresql code example

Example 1: select if then postgresql

SELECT field1, field2,
  CASE
    WHEN field1>0 THEN field2/field1
    ELSE 0
  END 
  AS field3
FROM test

Example 2: postgress if

-- use postgress case

SELECT name, continent, indep_year,
    CASE WHEN indep_year < 1900 THEN 'before 1900'
         WHEN indep_year <= 1930 THEN 'between 1900 and 1930'
         ELSE 'after 1930' END
         AS indep_year_group
FROM countries
ORDER BY indep_year_group;