Client APIs
Constant Expression
This expression contains a single value.  
```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]
})

const = duckdb.ConstantExpression('hello')
res = duckdb.df(df).select(const).fetchall()
print(res)
# [('hello',), ('hello',), ('hello',), ('hello',)]
```