Joining Two SQL Tables on Different Databases [duplicate] - sql

Question

This question already has answers here:
Can we use join for two different database tables?
(2 answers)
Closed 4 years ago.
I have two tables on two separate databases. We will call them Table1 and Table2.
Table1:
+----------+----------+----------+---------+------+----------+
| UniqueID | Date1 | Date2 | Fruit | Cost | Duration |
+----------+----------+----------+---------+------+----------+
| 1 | 09/10/18 | 10/20/18 | Apples | 1.50 | 7 |
| 2 | 09/18/18 | 10/25/18 | Oranges | 1.75 | 10 |
| 3 | 10/01/18 | 10/30/18 | Bananas | 2.00 | 10 |
+----------+----------+----------+---------+------+----------+
Table2:
+----------+---------+------+----------+-----------+
| Date1 | Fruit | Cost | Duration | New Price |
+----------+---------+------+----------+-----------+
| 09/10/18 | Savory | 1.50 | 7 | 1.90 |
| 09/18/18 | Citrusy | 1.75 | 10 | 2.50 |
| 10/01/18 | Mealy | 2.00 | 10 | 2.99 |
| 10/20/18 | Savory | 1.50 | 7 | 3.90 |
| 10/25/18 | Citrusy | 1.75 | 10 | 4.50 |
| 10/30/18 | Mealy | 2.00 | 10 | 5.99 |
+----------+---------+------+----------+-----------+
What I need the output to look like:
+----------+----------+--------------------+----------+--------------------+
| UniqueID | Date1 | New Price on Date1 | Date2 | New Price on Date2 |
+----------+----------+--------------------+----------+--------------------+
| 1 | 09/10/18 | 1.90 | 10/20/18 | 3.90 |
| 2 | 09/18/18 | 2.50 | 10/25/18 | 4.50 |
| 3 | 10/01/18 | 2.99 | 10/30/18 | 5.99 |
+----------+----------+--------------------+----------+--------------------+
I need to first convert table1.fruit to the representation of table2.fruit (apples-->savory, oranges-->citrusy, bananas-->mealy) then join on table1.fruit = table2.fruit, table1.duration = table2.duration, table1.cost = table2.cost, table1.date1 = table2.date1, and table1.date2 = table2.date1.
I don't know where to start on writing the statement. I looked over some previous questions posted here, but they really just go over the basics of linking two tables from different databases. Do I convert the table1.fruit first in the select statement, then join, or do I convert table1.fruit in the join statement? How do I join table2.date1 on both table1.date1 and table1.date2 to get the price associated with both dates?
If I can provide any more information for you, please let me know.
I am on SQL Server 2017 using Management Studio.
Thanks for any help in advance!

Answer 1

Create a mapping table to bridge between the different codes for fruits.
IF OBJECT_ID('tempdb..#FruitMappings') IS NOT NULL
DROP TABLE #FruitMappings
CREATE TABLE #FruitMappings (
Table1Fruit VARCHAR(100),
Table2Fruit VARCHAR(100))
INSERT INTO #FruitMappings (
Table1Fruit,
Table2Fruit)
VALUES
('Apples', 'Savory'),
('Oranges', 'Citrusy'),
('Bananas', 'Mealy')
SELECT
T1.*
--, whichever columns you need
FROM
Database1.Schema1.Table1 AS T1
INNER JOIN #FruitMappings AS F ON T1.Fruit = F.Table1Fruit
INNER JOIN Database2.Schema2.Table2 AS T2 ON
F.Table2Fruit = T2.Fruit AND
T1.Cost = T2.Cost AND
T1.Duration = T2.Duration
-- AND any other matches you need
You can use LEFT JOIN or even FULL JOIN, depending if you might have some fruits on a table that aren't available on the other (careful with NULL values if FULL JOIN).

Answer 2

If both databases are on the same SQL Server instance and your SQL Server login has access to both databases you can just use the full form of the object names:
select * -- Whatever...
from Database1.dbo.Table1 t1
inner join Database2.dbo.Table2 t2
on t1,UniqueId = t2.UniqueId -- Or whatever your join condition is
(adding where etc. clauses as required.)
This assumes both databases are using the default schema, otherwise replace dbo as necessary.
If the databases are on different servers you can use linked servers, but there are performance implications (the whole remote table may be read because the optimiser can't do much to filter it).