How to select a single record in a left join

Question

I need to select a specific model from the Models table using its key ModelID. I also need to add a blurb of content from the Model_Content table. The Models_Content table, howe

Answer 1

 SELECT
   M.ModelID, M.Model, C.Content
 FROM
   Models M
 LEFT JOIN
   Models_Content C
     ON C.ContentID = (SELECT MIN(ContentID) FROM Models_Content WHERE ModelID = M.ModelID)
 WHERE
   M.ModelID = 5

Or

;WITH sorted_content AS
(
  SELECT
    ROW_NUMBER() OVER (PARTITION BY ModelID ORDER BY ContentID) AS itemID,
    *
  FROM
    Models_Content
)
 SELECT
   M.ModelID, M.Model, C.Content
 FROM
   Models M
 LEFT JOIN
   sorted_content C
     ON  C.ModelID = M.ModelID
     AND C.itemID  = 1
 WHERE
   M.ModelID = 5

Answer 2

Change your JOIN to:

LEFT JOIN (SELECT ModelID, MAX(Content) as Content FROM Models_Content GROUP BY ModelID)

This is assuming you don't care which Content you get.

Answer 3

you could select the MIN ( contentID )

Answer 4

Sean's answer is the best specific solution but just to add another "generalised" solution

SELECT M.ModelID,
       M.Model,
       C.Content
FROM   Models M
       OUTER APPLY (SELECT TOP 1 *
                    FROM   Models_Content C
                    WHERE  M.ModelID = C.ModelID
                    ORDER  BY C.ContentID ASC) C
WHERE  M.ModelID = 5  

Answer 5

This may be an expansion on Randy's answer, but it's not completely clear to me.

For performance reasons I wanted to minimize the number of SELECT statements in my query, so I used a MIN statement within the primary select instead of within the JOIN:

SELECT 
    table_1.id AS id, 
    table_1.name AS name,
    MIN(table_2.id) AS table_2_id 
FROM table_1
LEFT JOIN table_2 ON table_2.table_1_id = table_1.id
-- additional JOINs and WHERE excluded
GROUP BY table_1.id, table_1.name
LIMIT 5

There may be tradeoffs depending on the total amount of data. IDK. My query needs to tunnel into the data from a condition several steps removed.

Answer 6

SELECT  M.ModelID
      , M.Model
      , (SELECT TOP 1 Content 
         FROM Models_Content 
         WHERE ModelID = M.ModelID 
         ORDER BY ContentId ASC) as Content
FROM    Models M 
WHERE   M.ModelID = 5