Question

考虑以下这段代码：

for count in range(28, -1, -1):
    crsr.execute("SELECT board, win1, win2 FROM positions WHERE balls = ?", (count,))
    response = crsr.fetchall()
    print count, len(response)
    for possibility in response:
        internal = possibility[0]
        player = count & 1
        victor = 1 - player
        opponent = 2 - player
        victory = possibility[opponent]
        if victory:
            crsr.execute("UPDATE positions SET result = ? WHERE board = ?", (victor, internal))
        else:
            subsequent = derive((internal, count))
            for derived in subsequent:
                external = reduce(derived[0])
                crsr.execute("SELECT result FROM positions WHERE board = ?", (external,))
                colour = crsr.fetchall()
                if colour[0][0] == player:
                    victor = player
                    break
            crsr.execute("UPDATE positions SET result = ? WHERE board = ?", (victor, internal))

考虑一下：

response = crsr.fetchall()

每当response中有多达10个⁷行时，上述语句就会返回内存错误，即使在具有8 GB RAM的系统上也是如此。

所以，我决定用以下代码改变：

for count in range(28, -1, -1):
    crsr.execute("SELECT board, win1, win2 FROM positions WHERE balls = ?", (count,))
    response = crsr.fetchall()
    print count, len(response)
    for possibility in response:
        internal = possibility[0]

为：

for count in range(28, -1, -1):
    crsr.execute("SELECT COUNT(board) FROM positions WHERE balls = ?", (count,))
    sum = crsr.fetchall()
    total = sum[0][0]
    print count, total
    crsr.execute("SELECT board, win1, win2 FROM positions WHERE balls = ?", (count,))
    for possibility in range(total):
        response = crsr.fetchone()
        internal = response[0]

现在就行：

response = crsr.fetchone()

利用crsr变量为possibility中range(total)的每次迭代执行SQLite3选择查询。

在'for'循环中已经存在其他crsr个语句：

crsr.execute("UPDATE positions SET result = ? WHERE board = ?", (victor, internal))

该声明发生两次，

crsr.execute("SELECT result FROM positions WHERE board = ?", (external,))。

该声明发生一次。

因此，只要crsr行中的response = crsr.fetchall()变量随着possibility中range(total)的每次迭代而变化，它就不会与其他crsr冲突}语句已经在同一个'for'循环中？

我们无法使用其他游标变量创建用于执行不同的SQLite3查询，因为crsr是通过使用crsr = connection.cursor()为特定数据库文件定义的，只要它被初始化（以spline.db为准），这个特例）。

所以，我想知道，如果有任何其他替代解决方案可供选择，那么非常有效。

Answer 1

结果集是游标对象的一部分，因此无论何时调用execute()，都会中止对同一游标对象的任何先前查询。避免这种情况的唯一方法是在执行下一个查询之前使用fetchall()读取所有结果行。

为了能够执行多个同时查询，您必须使用多个游标。只需多次致电connection.cursor()。

请注意，您不得修改您仍在阅读的表格（即使您使用的是多个游标）;读取光标可能会跳过或读取两次更改的行。如果您无法使用fetchall()，请将第一个查询的结果放入临时表中：

crsr1.execute("CREATE TEMP TABLE temp_pos(board, win1, win2)")

for count in ...:
    crsr1.execute("INSERT INTO temp_pos SELECT board, win1, win2 ...")

    crsr1.execute("SELECT board, win1, win2 FROM temp_pos")
    for row in crsr1:
        if ...:
            crsr2.execute("UPDATE positions ...")
        else:
            crsr2.execute("SELECT ... FROM positions ...")
            ...
    crsr1.execute("DELETE FROM temp_pos")

crsr1.execute("DROP TABLE temp_pos")

Python SQLite3 / Nested cursor.execute

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