Question

嗨，大家好我想尝试做像

这样的事情

SELECT * from accounttable, peopletable, companytable 
WHERE if accounttable.account_type = company 
JOIN companytable 
WHERE companytable.id = accounttable.company_id 
ELSE IF accounttable.account_type = = person 
JOIN peopletable 
  WHERE peopletable.id = accounttable.person_id

对不起它有点sqlenglish但我真的不知道如何写出来！

Answer 1

如下：

SELECT * 
from accounttable
left join peopletable ON (accounttype = 'person' AND peopletable.id = accounttable.person_id)
left join companytable ON (accounttype = 'company' AND companytable.id = accounttable.company_id)

我将加入两个表，因此您将在输出中包含所有三个表的字段。如果peopletable，NULL的字段将为accounttype != 'person'，companytable的字段将为NULL accounttype != 'company'。

您可以在here ...

等地方找到有关LEFT JOIN语法的更多信息

Answer 2

SELECT a.*, p.*, c.* from accounttable a
LEFT JOIN peopletable p ON (a.person_id = p.id AND a.account_type = 'person')
LEFT JOIN companytable c ON (a.company_id = c.id AND a.account_type = 'company')

请注意，a，p，c是完整表名的别名，这可以节省输入。

此查询将为行中的所有null提供p。*或c。* 您可以像这样重写选择部分：

SELECT 
  a.id, a.accounttype
  , COALESCE(p.name, c.name) as name
  , COALESCE(p.address, c.address) as address
....
FROM .....

请参阅：http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#function_coalesce

Answer 3

必须如下（据我所知）

SELECT a.*,p.*,c.* 
FROM accounttable a
LEFT JOIN companytable c ON c.id = a.company_id AND a.account_type = 'company'
LEFT JOIN peopletable p ON p.id = a.person_id AND  a.account_type = 'person'

MySQL if语句条件连接

3 个答案: