Question

Given the following (heavily simplified) tables:

create table Tags (
   TagId int Primary Key
)

create table OrderLines (
   Branch int,
   Station int,
   TransNo int,
   TagId int foreign key references Tags,
   primary key (Branch, Station, TransNo)
)

I need a list of Tags along with an OrderLine which references each Tag. I am expecting zero or one OrderLines to reference each Tag, but there is nothing in the database constraints to enforce this.

Given input like this:

 OrderLines                                     Tags
 Branch Station TransNo TagId                   TagId
 1      100     2345    1                       1
 1      100     2346    1                       2
 1      101     5223    2                       3
 3      100     6677    4                       4

I want to get output like this:

  TagId     Branch    Station    TransNo  
  1         1         100        2345     <-- it could list 2346 here, don't care
  2         1         101        5223     
  3         null      null       null
  4         3         100        6677

Note that although TagId 1 is referenced twice, my output only contains one of it. It doesn’t matter which OrderLine is listed alongside it, but there must be only one instance of each tag in the output.

What is the most efficient way to do this?

I’m not able to alter the database schema.

Answer 1

You’d have to look at the execution plan to gauge efficiency

;WITH O AS
(
SELECT Branch, Station, TransNo, TagId,
ROW_NUMBER() OVER (PARTITION BY TagId ORDER BY TagId ) AS RN
FROM OrderLines
)
    SELECT T.TagID, O.Branch, O.Station, O.TransNo
        FROM Tags T
        LEFT JOIN O ON T.TagID = O.TagID and RN=1