Python 检查 SQLite3 中是否存在

Question

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...")

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

What is wrong with this code?

Answer 1

Try this instead:

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

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

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

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

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, ):
    ...

or

if None:
    ...

Answer 2

Let's prepare a database to test it.

import sqlite3
c = sqlite3.connect(":memory:")
c.execute("CREATE TABLE airports (ICAO STRING, col2 STRING, col3 STRING)")
c.execute("INSERT INTO airports (ICAO, col2, col3) VALUES (?, ?, ?)", ('EHAM', 'value2', 'value3'))

Since your SELECT 1 FROM airports WHERE ICAO = 'EHAM' already serves the purpose of checking existence, let's use it directly, without the redundant SELECT EXISTS()

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

the result is

Found!

Let's check a non-existent case

if c.execute("SELECT 1 FROM airports WHERE ICAO = 'NO-SUCH'").fetchone():
    print("Found!")
else:
    print("Not found...")

the result is

Not found...

If you just want to fix your code, you can try

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

the result is

Found!

Answer 3

Thanks for the answer from zero323, although the code snippet is wrong, as fetchone() does not return True or False. It only returns 1 for True and 0 for False. (binary) The following code works without problems in Python3:

response = self.connection.execute("SELECT EXISTS(SELECT 1 FROM invoices WHERE id=?)", (self.id, ))
fetched = response.fetchone()[0]
if fetched == 1:
    print("Exist")
else:
    print("Does not exist")