how to save a file in a relative directory using pyinstaller built exe. file - pyinstaller

Question

My packed code looks like this
import sqlite3
import os
main_path = os.path.join(os.path.dirname(__file__))
conn = sqlite3.connect(main_path+'\\db\\data.db')
c = conn.cursor()
c.execute("""CREATE TABLE
IF NOT EXISTS
personel(
大风车 text,
service text,
work text,
item text)
""")
c.execute("INSERT INTO personel VALUES('B座','建筑','竣工工程','测量资料')")
conn.commit()
conn.close()
import sqlite3
import os
main_path = os.path.join(os.path.dirname(__file__))
conn = sqlite3.connect(main_path+'\\db\\data.db')
c = conn.cursor()
c.execute("""CREATE TABLE
IF NOT EXISTS
personel(
大风车 text,
service text,
work text,
item text)
""")
c.execute("INSERT INTO personel VALUES('B座','建筑','竣工工程','测量资料')")
conn.commit()
conn.close()
And my spec file looks like this
# -*- mode: python ; coding: utf-8 -*-
block_cipher = None
a = Analysis(['sql_trial.py'],
pathex=['./db'],
binaries=[],
hiddenimports=[],
hookspath=[],
runtime_hooks=[],
excludes=[],
win_no_prefer_redirects=False,
win_private_assemblies=False,
cipher=block_cipher,
noarchive=False)
pyz = PYZ(a.pure, a.zipped_data,
cipher=block_cipher)
exe = EXE(pyz,
a.scripts,
a.binaries,
a.zipfiles,
a.datas,
[],
name='abc',
debug=True,
bootloader_ignore_signals=False,
strip=False,
upx=True,
upx_exclude=[],
runtime_tmpdir=None,
console=False,
icon='app_icon.ico' )
What I want to do is to save the data.db file inside a folder "db" which is under the same directory as the exe.file. I tried pathex=['./db'],but it still returns error message:"sqlite3.OperationalError: unable to open database file". So how to fix this problem?