SQL Server Join – How to Reveal Unmapped Records

join;sql servert-sql

I have two tables and I am having trouble figuring out how to join them to meet my projection needs. Here is a scenario that illustrates the problem

1. The first table has a fixed set of values/rows; in this case Room Types

Room Types

Id | RoomType
--------------
1  | Bed Room
2  | Kitchen
3  | Half-Bath

2. The second table maps the items from the first table to an instance of a Property. One Property can have 1 or more Room Types

Property Rooms

Property | RoomTypeId
---------------------
  ABC    |     1
  ABC    |     3
  EFG    |     1
  XYZ    |     1
  XYZ    |     2
  XYZ    |     3

Note: some of the above properties are not mapped to some Room Types

3. I would like to make a projection that joins the rows in such a way that I can easily account for Room Types that are mapped and not mapped to a Property. Here is a sample result:

Property | RoomTypeId | PropertyHasRoom
---------------------------------------
   ABC   |     1      |      Yes
   ABC   |     2      |   No or NULL
   ABC   |     3      |      Yes
   EFG   |     1      |      Yes
   EFG   |     2      |   No or NULL
   EFG   |     3      |   No or NULL
   XYZ   |     1      |      Yes
   XYZ   |     2      |      Yes
   XYZ   |     3      |      Yes

Can anyone explain how this can be accomplished in T-SQL.

Best Answer

You need to create a multiplication first - every combination of Property + RoomType. In SQL Server we use a CROSS JOIN for that. Then you need to outer join to the junction table, and conditionally display Yes/No based on whether the junction table had a matching row. This should provide the output you want:

SELECT 
  p.Property, 
  RoomTypeID = t.Id, -- why isn't this just called RoomTypeID? 
  PropertyHasRoom = CASE
    WHEN pr.Property IS NOT NULL THEN 'Yes' 
    ELSE 'No' END -- or just END if you want NULL
FROM dbo.Properties AS p
CROSS JOIN dbo.RoomTypes AS t
LEFT OUTER JOIN dbo.PropertyRooms AS pr
ON p.Property = pr.Property
AND t.Id = pr.RoomTypeID
ORDER BY p.Property, t.Id;

Related Solutions

SQL INNER JOIN and OUTER JOIN in Same Table – How to Implement

The query is joining 2 tables (receipts, sales) that both have a many-to-one relationship with product. This creates a kind of cartesian (cross) product and will give wrong results in the SUM() calculations. To avoid that, you need to do the summations for the two tables in two different subqueries to avoid errors.

Something like this will work:

SELECT 
    m.month, 
    p.name, 
    COALESCE(r.receipts_quantity, 0) AS receipts_quantity, 
    COALESCE(s.sales_quantity, 0)    AS sales_quantity
FROM 
    product AS p
  CROSS JOIN
    ( VALUES (1),(2),(3),(4),(5),(6) ) AS m(month)     -- months wanted in the report
  LEFT OUTER JOIN 
    ( SELECT month, product_id, SUM(quantity) AS receipts_quantity
      FROM receipts
      GROUP BY month, product_id
    ) AS r
    ON  p.id_product = r.product_id
    AND m.month = r.month
  LEFT OUTER JOIN  
    ( SELECT month, product_id, SUM(quantity) AS sales_quantity
      FROM sales
      GROUP BY month, product_id
    ) AS s
    ON  p.id_product = s.product_id
    AND m.month = s.month ;

If you want to avoid having results for products that did not have any activity in a month, you can add the condition:

WHERE r.month IS NOT NULL 
   OR s.month IS NOT NULL

SQL Server – Running Updates and Schema Changes in 24/7 Environment with AlwaysOn

1.When and how do we install Microsoft Updates?

Using AOAGs could potentially help with this, depending on how much downtime is considered to be "too much". There is still a blip associated with switching the primary to a synchronous secondary as the clients will be disconnected (this may or may not be an issue in your environment) as the move happens. They will need to reconnect using the listener name (so no connection string changes should be required) and be using a library that supports this.

1.For Windows Updates, bring one server down, install windows updates, bring it back up, and then update the second instance.

That's the basics of how it works. Remove automatic failover (synchronous), install the updates/patches, reboot (in this scenario).

2.How do we apply schema changes?

AOAGs aren't going to help you in this respect. The transaction log is basically replayed from the primary to all of the secondary's. The secondary's could be readable (if you allow them to be) but won't be able to have any write activity (including ddl changes) made to them. A synchronous secondary is a mirror copy of the primary.

Could these changes be made when the application changes are made to minimize downtime? It may make more sense to use a load balanced front end using something such as an F5 and keeping the data in sync through something such as peer to peer replication than use AOAGs. I'm not a replication expert, but it definitely sounds like you will need two distinct systems that have some type of load balanced front end so that changes can be made to a single system at a time to move resources in and out while keeping them in sync.

2.For schema changes, bring one server down, apply schema changes, bring it back up, and then apply same changes to the second instance.

This can't be done this way (or any way, really) with AOAGs.

Best Answer

Related Solutions

SQL INNER JOIN and OUTER JOIN in Same Table – How to Implement

SQL Server – Running Updates and Schema Changes in 24/7 Environment with AlwaysOn

Related Question