'''
https://stackoverflow.com/questions/3850022/how-to-load-existing-db-file-to-memory-in-python-sqlite3
The followinng python program reads an existing sqlite3 database from disk
to memory. It first reads the entire table by select *, then executes
the sequence of sqlite3 instructions to create an in-memory database.

Note: The memory version of a sqlite db can't simply do queries such as

cur.execute("select * from stocks")

it must be done using replacement query such as
cur.execute("select * from stocks where trans_text=:type", {"type":"BUY"})

Xiannong Meng
2021-10-29
'''
import sqlite3

source = sqlite3.connect('example.db')
dest = sqlite3.connect(':memory:')
#dest = sqlite3.connect('copy.db')
source.backup(dest)

cur = dest.cursor()

# in the following line, the reference 'type' is replacing the field
# 'trans_text' in the table 'stocks'
cur.execute("select * from stocks where trans_text=:type", {"type":"BUY"})
#cur.execute("select * from stocks") # can't do this for in-memory db

print(cur.fetchall())

# We can also close the connection if we are done with it.
# Just be sure any changes have been committed or they will be lost.
source.close()
dest.close()