Question

我正在使用psycopg2通过Python 3访问PostgreSQL数据库，并且我正在尝试查询我想要选择名称在列表中的所有用户的查询，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，而不管他们的名字。

我已经尝试了以下三个调用：

# 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':()}, 
) 

但他们都抛出一个无效的语法错误，从一个点或另一个：

# 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

可选参数，你想要一个SQL where子句如：

where column = :parameter or :parameter is null 

与上面的那样当th e参数is null将返回所有行，否则仅返回符合条件的行。

Psycopg将Python list改编为Postgresql array。为了检查是否有任何PostgreSQL的array值等于某一值：

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

要获得一个Python None，其适于一个PostgreSQL null，从空的Python list：

parameter = [] or None 

传递一个dictionary到cursor.execute方法避免了在参数参数参数重复：

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：

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