python - Python/Sqlite3:通过GROUP_CONCAT左联接

我有2张桌子：

Tasks:                     Predecessors:

ID | Task | Team           ID | TaskID | PredecessorID
---+------+------          ---+--------+--------------
1  | A    | 1              1  | 1      | None
2  | B    | 1              2  | 2      | 1
3  | C    | 2              3  | 3      | 1
4  | D    | 2              4  | 3      | 2
                           5  | 4      | None

我想从小组2中选择所有任务，并为每个任务添加前置任务，并以该形式用逗号分隔：

ID | Task | Team | Predecessors
---+------+------+-------------
3  | C    | 2    | 1, 2
4  | D    | 2    | None

因此，我使用以下SQL查询：

SELECT Tasks.*, GROUP_CONCAT(Predecessors.PredecessorID)
FROM Tasks LEFT JOIN Predecessors ON Tasks.ID = Predecessors.TaskID
WHERE Tasks.Team = 2 GROUP BY Predecessors.TaskID

ID | Task | Team | Predecessors
---+------+------+-------------
3  | C    | 2    | 1, 2

我该怎么办才能获得无值行？

这是我的测试代码：

# -*- coding: utf-8 -*-

import sqlite3


con = sqlite3.connect('test.db')
cur = con.cursor()

# Drop old tables if they already exits and create new ones. (For testing).
cur.execute("""DROP TABLE IF EXISTS Tasks""")
cur.execute("""DROP TABLE IF EXISTS Predecessors""")

# Create tables.
cur.execute("""CREATE TABLE Tasks (ID INTEGER PRIMARY KEY AUTOINCREMENT,
               Task TEXT, Team INTEGER)""")
cur.execute("""CREATE TABLE Predecessors (ID INTEGER PRIMARY KEY AUTOINCREMENT,
               TaskID INTEGER, PredecessorID INTEGER)""")

# Insert some values.
sql = """INSERT INTO Tasks (Task, Team) VALUES (?, ?)"""
for task in [('A', 1), ('B', 1), ('C', 2), ('D', 3)]:
    cur.execute(sql, task)

sql = """INSERT INTO Predecessors (TaskID, PredecessorID) VALUES (?, ?)"""
for predecessor in [(1, None), (2, 1), (3, 1), (3, 2), (4, None)]:
    cur.execute(sql, predecessor)

con.commit()

sql = """SELECT Tasks.*, GROUP_CONCAT(Predecessors.PredecessorID)
FROM Tasks LEFT JOIN Predecessors ON Tasks.ID = Predecessors.TaskID
WHERE Tasks.Team = 2 GROUP BY Predecessors.TaskID"""
cur.execute(sql)
data = cur.fetchall()
print data

con.close()

感谢帮助！！

最佳答案

问题是GROUP BY Predecessors.TaskID;对于没有匹配项的行，所有Predecessors列均为NULL。

只需使用GROUP BY Tasks.ID即可。

关于python - Python/Sqlite3:通过GROUP_CONCAT左联接，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/44745035/

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python - Python/Sqlite3:通过GROUP_CONCAT左联接