我有两张桌子:
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
我想选择来自第2组的所有任务,并在该表单中为每个以逗号分隔的任务添加前置任务:
ID | Task | Team | Predecessors
---+------+------+-------------
3 | C | 2 | 1, 2
4 | D | 2 | None
因此我使用以下SQL-Query:
SELECT Tasks.*, GROUP_CONCAT(Predecessors.PredecessorID)
FROM Tasks LEFT JOIN Predecessors ON Tasks.ID = Predecessors.TaskID
WHERE Tasks.Team = 2 GROUP BY Predecessors.TaskID
返回:
ID | Task | Team | Predecessors
---+------+------+-------------
3 | C | 2 | 1, 2
我该怎样做才能获得无值的行?
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()
感谢您的帮助!!
答案 0 :(得分:2)
问题是GROUP BY Predecessors.TaskID
;对于没有匹配的行,所有Predecessors
列都为NULL。
只需使用GROUP BY Tasks.ID
。
答案 1 :(得分:1)
你应该仔细检查你的代码。你的SQL中没有错,你只是写错了测试数据。
your test data[('A', 1), ('B', 1), ('C', 2), ('D', 3)]
and it should be [('A', 1), ('B', 1), ('C', 2), ('D', 2)]