如何避免多次加入同一个表

Question

In the simplified example: The idea is to get all (player, coach, and ref) names into the final query, but the only way I can think to do that is to join 3 times on the respective id. What is a better way?

Team

...|Coachid | Playerid | Refid|
    --------------------------
...|   98    |  23    |  77    |

Name

Id    | Name      |
--------------------
  98  |  Andy     |
  23  |  Charlie  |

SELECT [t].[Id],
       [t].[TeamName],
       [c].[Name] AS CoachName,
       [p].[Name] AS PlayeName,
       [r].[Name] as RefName
FROM Team [t]
JOIN Name [c]
ON c.id = t.Coachid
JOIN Name [p]
ON p.id = t.PlayerId
JOIN Name [r] 
ON r.id = t.RefId

Answer 1

As it has been commented, your approach is the best way to adress your case and should have good performance.

Alternatives would include:

1) A series of correlated subqueries - this is OK because there is just one value to return per relation:

SELECT 
    t.Id,
   t.TeamName,
   (SELECT n.Name FROM AS Name n WHERE n.id = t.CoachId) CoachName,
   (SELECT n.Name FROM AS Name n WHERE n.id = t.PlayerId) PlayerName,
   (SELECT n.Name FROM AS Name n WHERE n.id = t.RefId) RefName
FROM Team t 

2) Conditional aggregation - makes the query more cumbersome:

SELECT 
    t.Id,
    t.TeamName,
    MAX(CASE WHEN n.id = t.CoachId THEN n.Name END) CoachName,
    MAX(CASE WHEN n.id = t.PlayerId THEN n.Name END) PlayerName,
    MAX(CASE WHEN n.id = t.RefId THEN n.Name END) RefName
FROM Team t
INNER JOIN Name n ON n.id IN (t.CoachId, t.PlayerId, t.RefId)
GROUP BY t.Id, t.TeamName

Answer 2

With the way your table is structured, the way you provided is the best way to do it.

It is odd though, the way you have structured the Team table.

You can simplify the whole query just to one join, if you had made your Team table just have the id's of each person, and in your Name table, you would have the id and the role / position of that id.

I guess this is a good example of how important it is to structure your tables correctly.