Question

I have two tables: States, and Items.

States:

+----+------+-------+----------+
| id | name | state | priority |
+----+------+-------+----------+
|  1 |   AA |    10 |        1 |
|  2 |   AB |    10 |        2 |
|  3 |   AC |    10 |        3 |
|  4 |   BA |    20 |        1 |
|  5 |   BB |    20 |        5 |
|  6 |   BC |    20 |       10 |
|  7 |   BD |    20 |       50 |
+----+------+-------+----------+

Items:

+----+--------+-------+
| id | item   | state |
+----+--------+-------+
|  1 |   Blue |    10 |
|  2 |    Red |    20 |
|  3 |  Green |    20 |
|  4 | Yellow |    10 |
|  5 |  Brown |    10 |
+----+--------+-------+

The priority column is not used in the Items table, but complicates getting the data I need, as shown below.

What I want is a list of the rows in the Items table, replacing the state.id value in each row with the name of the highest priority state.

Results would look like this:

+----+--------+-------+
| id | item   | state |
+----+--------+-------+
|  1 |   Blue |    AC |
|  2 |    Red |    BD |
|  3 |  Green |    BD |
|  4 | Yellow |    AC |
|  5 |  Brown |    AC |
+----+--------+-------+

Here’s the tiny monster I’ve come up with. Is this the best way, or can I be more efficient / less verbose? (Sub-sub-selects make my palms itch. 😛 )

SELECT *
FROM
  Items AS itm
  INNER JOIN (SELECT sta.name, sta.state
              FROM (SELECT state, MAX(priority) [highest]
                    FROM States
                    GROUP BY state) AS pri
                INNER JOIN States AS sta
                  ON sta.state = pri.state
                     AND sta.priority = pri.highest) AS nam
    ON item.state = name.state

Update: I’m using MS-SQL 2005 and MS-SQL 2008R2

Answer 1

You did not post your version of SQL-Server. Assuming you are on 2005 or later you can use the ROW_NUMBER() function together with a cross apply like this:

 CREATE TABLE dbo.States(id INT, name NVARCHAR(25), state INT, priority INT);
 INSERT INTO dbo.States
 VALUES
(  1 ,'AA',    10 ,        1 ),
(  2 ,'AB',    10 ,        2 ),
(  3 ,'AC',    10 ,        3 ),
(  4 ,'BA',    20 ,        1 ),
(  5 ,'BB',    20 ,        5 ),
(  6 ,'BC',    20 ,       10 ),
(  7 ,'BD',    20 ,       50 );

CREATE TABLE dbo.Items( id INT ,item NVARCHAR(25), state INT );
INSERT INTO dbo.Items
VALUES
(  1 ,'Blue',    10 ),
(  2 ,'Red',    20 ),
(  3 ,'Green',    20 ),
(  4 ,'Yellow',    10 ),
(  5 ,'Brown',    10 );


SELECT  i.id,
        i.item,
        s.name,
        s.priority
FROM dbo.Items i
CROSS APPLY (
SELECT *,ROW_NUMBER()OVER(ORDER BY priority DESC) rn FROM dbo.States si WHERE si.state = i.state 
)s
WHERE s.rn = 1;

The cross apply works like a join but allows to reference columns on the left side in the right side as you can see in the where clause. The ROW_NUMBER() function numbers all rows in the states table that match the current state value in reverse priority order so that the row with the highest priority always gets the number 1. The final where clause is filtering out just those rows.

EDIT:

I just started a blog series about joins: A Join A Day
The Cross Apply will be topic of day 8 (12/8/2012).