Question

我正在尝试执行此查询以获取所有用户，组和电子邮件的列表，但是我的问题是，当用户与任何组都不相关时，所以radcheck.id_group列= NULL，但是我仍然要列出它。

ENDPOINT_URL = 'https://example.com'
headers = {'Api-key': '123abc'}

import threading
with open("student.csv", "r") as csv_ledger:
    r = csv.DictReader(csv_ledger)
    data = [dict(d) for d in r ]
    groups = {}

    for k, g in groupby(data, lambda r: (r['name'])):
        #My data mapping
        t = threading.Thread(target=requests.post,
                             args=(ENDPOINT_URL, groups, headers=headers))
        t.setDaemon(True)
        t.start()

如何正确书写？

Answer 1

使用显式LEFT JOIN：

SELECT 
    r.username AS username, 
    g.name AS groupname, 
    g.id as gid, 
    u.email
FROM useremail u
LEFT JOIN radcheck r ON r.username = u.username
LEFT JOIN group g ON g.id = r.id_group

使用表别名通常也是一个好习惯：使用表别名可以使查询更易于阅读和维护（并在需要时允许自联接表...）。

Answer 2

使用左联接：

SELECT 
  radcheck.username AS username, 
  group.name AS groupname, 
  group.id as gid, 
  useremail.email as email 
FROM useremail left join radcheck
ON radcheck.username = useremail.username
left join group
ON group.id = radcheck.id_group

当列为NULL时如何获取此查询？

2 个答案: