从一对多关系中仅检索一条记录

Question

I have 2 tables. One is Agent and the other one is Agent_Contact . The Agent_Contact table is there to maintain the contact information of Agent .

The relationship from Agent to Agent_Contact is One-To-Many which means an Agent can have many Agent_Contacts

The Agent_Contact is holding the Foreign Key of Agent .

There can be Agents that who do not have any Agent_Contact as well.

Now, please have a look at the below code.

SELECT Agent.*,
Agent_Contact.*
FROM Agent
LEFT JOIN Agent_Contact ON Agent.idAgent = Agent_Contact.idAgent

Now, this returns all the Agents with their Agent_Contact s( If they have any). However imagine one Agent got 15 Agent_Contact s, then it will return 15 rows.

This is not what I want. Once Agent_Contact per one Agent is enough. But how can I modify the above sql query to achieve this task?

Answer 1

Since there is no rule to which contact must be returned, it is enough to add a group by clause.

SELECT Agent.*,
Agent_Contact.*
FROM Agent
LEFT JOIN Agent_Contact ON Agent.idAgent = Agent_Contact.idAgent
GROUP Agent.idAgent

Answer 2

If you only want 1 agent_contact returned per agent returned then the question is "which agent_contact do you want returned?"

That's like selecting a classroom full of 20 students and saying "Please return 1 student." Well...which student do you want?

When it comes to selecting your Agent_Contact, you could select then alphabetically, or the one with the longest name, or something that will reduce the number of agent_contacts down to 1.

This query selects the agent_contact with the maximum name. You can use whatever agent_contact column you want, it just has to be put in an aggregate function that will return only one kind.

SELECT t.Id, ac.* FROM
(SELECT Agent.Id, Max(AgentContact.Name)
FROM Agent
LEFT JOIN Agent_Contact ON Agent.idAgent = Agent_Contact.idAgent
GROUP BY Agent.Id) as t
INNER JOIN Agent_Contact as ac ON t.Name = ac.Name