根据SQL Server中的列值联接表

Question

Item table

id  key code    description
------------------------------
1   1   misc    miscellaneous
2   1   med     medicine    

Miscellaneous table:

id  code    description
------------------------
1   misc1   miscellaneous
2   misc1   miscellaneous

Medicine table:

id  code        description
---------------------------
1   medicine1   medicine
2   medicine1   medicine

I have this table structure; my main table is the Item table and I want to JOIN the main table with other table based on the column value in main table. The column that determines the table to be joined is code . If code is misc join with misc table if value is med join with medicine table .

I know the basic JOIN of table like

SELECT * 
FROM item 
INNER JOIN miscellaneous ON item.key = miscellaneous.id

But I don't know how to join when there is a condition that will point to which table to JOIN

Answer 1

You can use left join . Your question is vague on what should be joined with what, but it is something like this:

select i.*,
       coalesce(mi.code, me.code) as code_1,
       coalesce(mi.description, me.description) as description_1
from item i left join
     miscellaneous mi
     on mi.code = i.key and i.code = 'misc' left join
     medicine me
     on me.code = i.key and i.code = 'med';

Answer 2

You can try a LEFT JOIN , that will be the easiest method to implement this. But if you want both the table result to come under one column name, use UNION ALL

Using UNION ALL

SELECT *
FROM item i
INNER JOIN miscellaneous m on m.code=i.code
UNION ALL
SELECT *
FROM item i
INNER JOIN medicine  me on me.code=i.code

Using LEFT JOIN

SELECT *
FROM item i
LEFT JOIN miscellaneous m on m.code=i.code
LEFT JOIN medicine  me on me.code=i.code

Answer 3

Since Item is your 'base' / main table, you could match other tables with LEFT JOIN so that all rows from Item tables are present.

SELECT * 
FROM Item AS i
LEFT JOIN Miscellaneous AS mi ON (mi.[id] = i.[key] AND i.code = 'misc')
LEFT JOIN Medicine AS me ON (me.[id] = i.[key] AND i.code = 'med');

You could also combine Miscellaneous & Medicine tables columns using ISNULL() function with some pre-condition

SELECT i.*
  , ISNULL(mi.id, me.id) AS m_id
  , ISNULL(mi.code, me.code) AS m_code
  , ISNULL(mi.description, me.description) AS m_description
FROM Item AS i
LEFT JOIN Miscellaneous AS mi ON (mi.id = i.[key] AND i.code = 'misc')
LEFT JOIN Medicine AS me ON (me.id = i.[key] AND i.code = 'med');

SqlFiddle Demo