检查条目存在于 sqlite3 python 中

Question

I checked some similar posts, but don't get my answer.

I want to check if specific username is in database or not.

import sqlite3
con = sqlite3.connect("session.db")
c = con.cursor()
a1 = 'jack'
c.execute("SELECT EXISTS(SELECT username FROM User WHERE username=?)", (a1))
if c.fetchone():
    print("Found!")

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

Answer 1

Your script has one obvious problem with it, namely that you are not actually passing a valid Python tuple to the call to execute() . Instead, you are passing a single string variable. You should be passing (a,) , with a trailing comma, to tell Python that you intend this to mean a tuple.

import sqlite3
con = sqlite3.connect("session.db")
c = con.cursor()
a1 = 'jack'
c.execute("SELECT EXISTS (SELECT 1 FROM User WHERE username = ?)", (a1,))
if c.fetchone():
    print("Found!")
else:
    print("Not found...")

To be clear here, ('jack') will be interpreted as a string, while ('jack',) will be interpreted as a tuple.

Answer 2

You should change your code a bit like this-

c.execute("SELECT EXISTS(SELECT username FROM User WHERE username=?)", (a1,))

Notice the comma after a1 . Without this change the code should throw an error like Incorrect number of bindings supplied as only a1 (in your case value jack ) will be treated as 4 different parameter provided to c.execute() . Turning it to tuple will bind the whole string jack to the appropriate placeholder.

After that check the result like -

found, = c.fetchone()
# found will be 1 in case the entry is present
if found: 
    print("Found")
else:
    print("Not found")

c.fetchone() returns a tuple. In your case tuple with one element indicating whether the entry exists or not (1 if present, 0 otherwise)