Client APIs
Column Expression
This expression references a column by name.  
```python
import duckdb
import pandas as pd

df = pd.DataFrame({
'a': [1, 2, 3, 4],
'b': [True, None, False, True],
'c': [42, 21, 13, 14]
})

# selecting a single column
col = duckdb.ColumnExpression('a')
res = duckdb.df(df).select(col).fetchall()
print(res)
# [(1,), (2,), (3,), (4,)]

# selecting multiple columns
col_list = [
duckdb.ColumnExpression('a') * 10,
duckdb.ColumnExpression('b').isnull(),
duckdb.ColumnExpression('c') + 5
]
res = duckdb.df(df).select(*col_list).fetchall()
print(res)
# [(10, False, 47), (20, True, 26), (30, False, 18), (40, False, 19)]
```