sql - postgresql 中的 IF-THEN-ELSE 语句

Question

我正在寻找编写一个 postgresql 查询来执行以下操作：

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

我试过这个查询，但它不起作用

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

谢谢你

Answer 1

如 PostgreSQL 文档中所述：

SQL CASE 表达式是一个通用的条件表达式，类似于其他编程语言中的 if/else 语句。

专门回答您的问题的代码段：

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

Answer 2

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

Answer 3

通常，替代case when ...is coalesce(nullif(x,bad_value),y)（不能在 OP 的情况下使用）。例如，

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

给出：

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