如何在mysql中使用if else子查询

Question

SELECT IF{ 
    (select sum(amount) 
     from bank_credit 
     where bank_name='IDBI') - 
    (select sum(amount) 
     from bank_debit 
     where bank_name='IDBI') as idbi == NULL} 
 else {
     select sum(amount) 
     from bank_credit 
     where bank_name='IDBI')
} as idbi

如何用mysql中的if else解决这个子查询... plz帮助一些

Answer 1

您可以使用function ifnull()将NULL转换为0，然后进行计算。

SELECT 
  IFNULL(
    ( SELECT SUM(amount) 
        FROM bank_credit 
        WHERE bank_name='IDBI'
    ), 0
  )
  - 
  IFNULL(
    (
      SELECT SUM(amount) 
        FROM bank_debit 
        WHERE bank_name='IDBI' 
    ), 0
  ) AS idbi;

另见https://stackoverflow.com/a/1441358/1331451。

---

使用此查询查看isnull()的行为：

select 
  ifnull( (select 1), 0)    - ifnull( (select 1), 0)    as 1_1,
  ifnull( (select 1), 0)    - ifnull( (select 0), 0)    as 1_0,
  ifnull( (select 1), 0)    - ifnull( (select NULL), 0) as 1_NULL,
  ifnull( (select 0), 0)    - ifnull( (select 1), 0)    as 0_1,
  ifnull( (select NULL), 0) - ifnull( (select 1), 0)    as NULL_1,
  ifnull( (select NULL), 0) - ifnull( (select NULL), 0) as NULL_NULL
;