[英]Select parts of one table and count rows of another
我有2个MySQL表:人和连接
我想从人员中选择所有列,并计算他们在连接表中有多少连接。
-------------------- ----------------------
| People | | Connections |
-------------------- ----------------------
| person_id | name | | cnt_id | person_id |
-------------------- ----------------------
我想从人员中选择所有人,并选择该人的person_id在第二个表中出现的次数。
我一直想弄明白,但不能。 希望你们都能提供帮助。
谢谢!
如果有人没有连接,这将给出一个零计数的行:
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
尝试这个
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
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