Python检查SQLite3中是否存在 [英] Python check if exists in SQLite3

问题描述

我正在尝试检查SQLite3数据库中是否存在变量.不幸的是，我似乎无法使其正常工作.机场表包含3个列，国际民航组织为第一列.

I'm trying to check whether a variable exists in an SQLite3 db. Unfortunately I can not seem to get it to work. The airports table contains 3 colums, with ICAO as the first column.

if c.execute("SELECT EXISTS(SELECT 1 FROM airports WHERE ICAO='EHAM')") is True:
    print("Found!")
else:
    print("Not found...")

代码运行无任何错误，但结果始终相同(未找到).

The code runs without any errors, but the result is always the same (not found).

此代码有什么问题?

推荐答案

尝试以下方法:

c.execute("SELECT EXISTS(SELECT 1 FROM airports WHERE ICAO='EHAM')")

if c.fetchone():
    print("Found!")

else:
    print("Not found...")

cursor.execute的返回值是游标(或更精确地引用其自身)，并且独立于查询结果.您可以轻松地检查以下内容:

Return value of cursor.execute is cursor (or to be more precise reference to itself) and is independent of query results. You can easily check that:

 >>> r = c.execute("SELECT EXISTS(SELECT 1 FROM airports WHERE ICAO='EHAM')")
 >>> r is True
 False
 >>> r is False
 False
 >>> r is None
 False

 >>> r is c
 True

另一方面，如果您调用cursor.fetchone结果元组，或者如果没有行通过查询条件，则为None.因此，在您的情况下，if c.fetchone():表示以下之一:

From the other hand if you call cursor.fetchone result tuple or None if there is no row that passes query conditions. So in your case if c.fetchone(): would mean one of the below:

if (1, ):
    ...

或

if None:
    ...

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