how to understand __table__ of following code

[l1t1]

I copied it from chdb homepage. but I don't understand why table refers the dataframe ret_tbl?

>>> import chdb.dataframe as cdf
>>> import pandas as pd
>>> # Join 2 DataFrames
>>> df1 = pd.DataFrame({'a': [1, 2, 3], 'b': ["one", "two", "three"]})
>>> df2 = pd.DataFrame({'c': [1, 2, 3], 'd': ["①", "②", "③"]})
>>> ret_tbl = cdf.query(sql="select * from __tbl1__ t1 join __tbl2__ t2 on t1.a = t2.c",
...                   tbl1=df1, tbl2=df2)
>>> print(ret_tbl)
   a         b  c                d
0  1    b'one'  1  b'\xe2\x91\xa0'
1  2    b'two'  2  b'\xe2\x91\xa1'
2  3  b'three'  3  b'\xe2\x91\xa2'
>>> # Query on the DataFrame Table
>>> print(ret_tbl.query('select b, sum(a) from __table__ group by b'))
          b  sum(a)
0    b'one'       1
1    b'two'       2
2  b'three'       3

[l1t1]

I tried to rename it to __t__, and learned from the error messages.

>>> print(ret_tbl.query('select b, sum(a) from __t__ group by b'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.10/dist-packages/chdb/dataframe/query.py", line 120, in query
    self._validate_sql(sql)
  File "/usr/local/lib/python3.10/dist-packages/chdb/dataframe/query.py", line 148, in _validate_sql
    raise ValueError("SQL should always contain `FROM __table__`")
ValueError: SQL should always contain `FROM __table__`

[auxten]

If you want to refer ret_tbl itself, you must use __table__ which is something like self
Like:

ret_tbl.query('select b, sum(a) from __table__ group by b')

Otherwise, if you want to query on dataframe vars you can query with

cdf.query(sql="select * from __tbl1__ t1 join __tbl2__ t2 on t1.a = t2.c",
                   tbl1=df1, tbl2=df2)

where __tbl1__ refer to tbl1, __tbl2__ refer to tbl2

[l1t1]

thanks. the syntax of Python module duckdb seems simpler

>>> import duckdb as dd
>>> df1=dd.sql("select 1 a").df()
>>> df2=dd.sql("select 2 b union select 3").df()
>>> dd.sql("select * from df1,df2")