Question

我正在使用psycopg2通過Python 3訪問PostgreSQL數據庫，並且我正在嘗試查詢我想要選擇名稱在列表中的所有用戶的查詢，如果列表不爲空。如果提供的列表爲空，我想忽略該條件，即選擇所有用戶，而不管他們的名字。

我已經嘗試了以下三個調用：

# 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':()}, 
) 

但他們都拋出一個無效的語法錯誤，從一個點或另一個：

# 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

可選參數，你想要一個SQL where子句如：

where column = :parameter or :parameter is null 

與上面的那樣當th e參數is null將返回所有行，否則僅返回符合條件的行。

Psycopg將Python list改編爲Postgresql array。爲了檢查是否有任何PostgreSQL的array值等於某一值：

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

要獲得一個Python None，其適於一個PostgreSQL null，從空的Python list：

parameter = [] or None 

傳遞一個dictionary到cursor.execute方法避免了在參數參數參數重複：

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}) 

輸出：

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

當列表爲空：

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