postgresql - 如何检查值是否在列表中或列表是否为空？

Question

我正在使用 psycopg2通过 Python 3 访问 PostgreSQL 数据库，我正在尝试进行查询，我想在其中选择名称在列表中的所有用户，如果列表不为空。如果提供的列表为空，我想忽略该条件，即选择所有用户而不考虑他们的名字。

我已经尝试过以下三个调用:

# Using list
cursor.execute(
    "SELECT age FROM user WHERE %(names) = '{}' OR user.name IN %(names)s",
    {'names': []},
)

# Using tuple
cursor.execute(
    "SELECT age FROM user WHERE %(names) = () OR user.name IN %(names)s",
    {'names': ()},
)

# Using both list and tuple
cursor.execute(
    "SELECT age FROM user WHERE %(names_l) = '{}' OR user.name IN %(names_t)s",
    {'names_l': [], 'names_t': ()},
)

但它们都从某一点或另一点引发无效语法错误:

# Using list
psycopg2.ProgrammingError: syntax error at or near "'{}'"
LINE 17:         user.name IN '{}'

# Using tuple
psycopg2.ProgrammingError: syntax error at or near ")"
LINE 16:         () == ()

# Using both list and tuple
psycopg2.ProgrammingError: syntax error at or near ")"
LINE 17:         user.name IN ()

Answer 1

对于可选参数，您需要一个 SQL where 子句，例如:

where column = :parameter or :parameter is null

当参数 为 null 时，将返回所有行，否则仅返回满足条件的行。

Psycopg 将 Python list 适配为 Postgresql array。检查是否有任何 Postgresql array 值等于某个值:

where column = any (array[value1, value2])

要从一个空的 Python list 中获取一个适用于 Postgresql null 的 Python None:

parameter = [] or None

将字典传递给cursor.execute方法避免参数参数中的参数重复:

names = ['John','Mary']

query = """
    select age
    from user
    where user.name = any (%(names)s) or %(names)s is null
"""
print (cursor.mogrify(query, {'names': names or None}).decode('utf8'))
#cursor.execute(query, {'names': names or None})

输出:

select age
from user
where user.name = any (ARRAY['John', 'Mary']) or ARRAY['John', 'Mary'] is null

当列表为空时:

select age
from user
where user.name = any (NULL) or NULL is null

http://initd.org/psycopg/docs/usage.html#passing-parameters-to-sql-queries