Question

这是我想要实现的目标：根据联系类型选择一个人或公司的名称。

表格：

CONTACTS :               PEOPLE              FIRMS
- id                     - contact_id        - contact_id    
- type (person/firm)     - name              - name

我想要这种魔力：

SELECT type 
FROM contacts 
WHERE contacts.id = 42;

SELECT model.name 
FROM contacts LEFT JOIN model ON model.contact_id = contacts.id 
WHERE contacts_id = 42 
AND model = CASE (WHEN type = "person" THEN "people" 
                  WHEN type = "firm" THEN "Firms" END); (Not sure this one works)

一站式查询。谢谢！

Answer 1

您可以执行以下操作，加入两个表，然后根据contacts.type决定使用哪个列。

SELECT 
  (CASE WHEN t1.type = 'person' THEN t2.name WHEN t1.type = 'firm' THEN t3.name) AS contact_name
FROM contacts t1
LEFT JOIN people t2 ON t1.id = t2.contact_id
LEFT JOIN firms t3 ON t1.id = t3.contact_id

Answer 2

您可以像这样使用动态SQL：

declare 
    @table_name nvarchar(20),
    @query nvarchar(max)

SELECT 
    @table_name = 
                    case type 
                        when 'person' then 'people'
                        when 'firm' then 'firms'
                    end
FROM contacts  
WHERE contacts.id = 42;


set @query = N'SELECT ' + @table_name + '.name ' +
                'FROM contacts LEFT JOIN model ON ' + @table_name + '.contact_id = contacts.id 
                WHERE contacts_id = 42 '

exec sp_executesql @query

Answer 3

简单地离开加入两个表;其中一个将匹配id和类型。然后使用COALESCE显示找到的一个名称。

select coalesce(p.name, f.name) as name
from contacts c
left join people p on p.contacts_id = c.id and c.type = 'person'
left join firms f on f.contacts_id = c.id and c.type = 'firm'
where c.id = 42;

Answer 4

以下是没有联接的替代方法：

select 
  case when c.type = 'person' then
    (select name from people p where p.contacts_id = c.id)
  else
    (select name from firms f on f.contacts_id = c.id)
  end as name
from contacts c
where c.id = 42;

你知道，有很多方法可以解决这个问题。只需选择您认为最具可读性的那个。

SELECT * FROM TABLE，TABLE取决于另一个查询 - SQL

4 个答案: