Output:
A = [ d   x    y    z
      0   1    2    5
      1   2    1    9
      2   8    1    2
      3   3    40   7
      4   6    1    7
      5   4    30   3]

Output:
A = [ d   x    y    z
      0   1    2    5
      1   2    2    9
      2   8    2    2
      3   3    40   7
      4   6    40   7
      5   4    30   3]

import csv
import numpy as np
import numpy
import pandas as pd

conn = sqlite3.connect('data.db')
conn.text_factory = str 
cur = conn.cursor()
A = cur.execute("SELECT * FROM CH1_data")
data = cur.execute('UPDATE CH2_data t1 SET y = (SELECT t2.y FROM CH2_data t2 WHERE t2.d < t1.d AND t2.y <> 1 ORDER BY t2.d DESC LIMIT 1) WHERE y = "" ')

conn.commit()


with open('output_data1001.csv', 'w') as f: 
  writer = csv.writer(f)
  writer.writerow(['d', 'x','y','z'])
  writer.writerows(A)