查询可选参数

Question

Suppose I have a user table with age and name columns. I would like to write a function to query this table, either selecting everything, or selecting based on age. Naively I might do

def query(age=''):
    query_args = ' WHERE {}'.format(age) if age else ''
    db.execute('SELECT * FROM users' + query_args)

obviously this is a horrible idea, but I'm not sure what the better way to deal with this situation is - it seems pretty ugly to write a separate query if age is passed as an argument or it's not, especially in more complicated examples where I might have multiple parameters to the query.

Answer 1

You are right in that building a query string is a terrible idea. This problem is not a driver one. It is just SQL:

query = """
    select *
    from users
    where
        (%(age)s is null or %(age)s = age)
        and
        (%(name)s is null or %(name)s = name)
"""
parameters = dict(name = None, age = 35)
cursor.execute(query, parameters)

If a certain parameter is passed as null it will not be filtered.