Join table based on column value in SQL Server - sql

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. 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

Answer 4

You can do this in a dynamic way
preparation of the data set for testing
IF (OBJECT_ID('item') IS NULL)
BEGIN
CREATE TABLE item (id int, [key] nvarchar(128), code nvarchar(128),
description nvarchar(512))
INSERT INTO item (id, [key] , code, description)
SELECT 1, 1, 'misc', 'miscellaneous' UNION ALL
SELECT 2, 1, 'med', 'medicine'UNION ALL
SELECT 3, 2, 'test not existing table', 'not_existing_table';
END
IF (OBJECT_ID('miscellaneous') IS NULL)
BEGIN
CREATE TABLEe miscellaneous (id int, code nvarchar(128), description
nvarchar(512))
INSERT INTO miscellaneous (id, code, description)
SELECT 1, 'misc1', 'miscellaneous' UNION ALL
SELECT 2, 'misc2', 'miscellaneous_xxx';
END
IF (OBJECT_ID('medicine') IS NULL)
BEGIN
CREATE TABLE medicine (id int, code nvarchar(128), description
nvarchar(512))
INSERT INTO medicine (id, code, description)
SELECT 1, 'medicine1', 'medicine' UNION ALL
SELECT 2, 'medicine2', 'medicine_xxx';
END
SQL Script
DECLARE #joins nvarchar(max) ='',#sql NVARCHAR(MAX) = ' SELECT * FROM item';
SELECT #joins = STUFF((SELECT ( CHAR(13)+CHAR(10) + ' LEFT JOIN ' +
description + ' ' + description + ' ON ' + description +'.id = item.[key] AND item.description = ''' + description + '''' )
FROM item i WHERE ISNULL(description,'') != '' AND
(select OBJECT_ID(description) ) IS NOT NULL
GROUP BY description
ORDER BY description
FOR XML PATH(''), TYPE ).value('.', 'NVARCHAR(MAX)') ,1,1,'');
IF(ISNULL(#joins,'') != '' )
BEGIN
SET #sql = #sql + #joins;
EXECUTE sp_executesql #sql;
PRINT #sql
END;