选择一个表的一部分并计算另一个表的行

Question

I have 2 MySQL tables: people & connections

I want to select all columns from people and also count up how many connections they have in the connections table.

--------------------   ----------------------
| People           |   | Connections        |
--------------------   ----------------------
| person_id | name |   | cnt_id | person_id |
--------------------   ----------------------

I want to select all from people and select a count of how many times the person_id for that person appears in the second table.

I have been trying to figure it out, but can't. Hopefully you all can help.

Thanks!

Answer 1

If there is a person with no connections, this will give a row with zero count:

SELECT
    People.person_id,
    People.name,
    COUNT(Connections.person_id) AS number_of_connections
FROM People
LEFT JOIN Connections
ON People.person_id = Connections.person_id 
GROUP BY People.person_id

Answer 2

try this

SELECT name,count(Connection.person_id) as cnt FROM 
      People LEFT JOIN Connection ON People.person_id = Connection.person_id 
      GROUP BY Connection.person_id