Python，如何检查结果集是否为空？
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Python, how to check if a result set is empty?
提问by Tao Venzke
I have a sql statement that returns no hits. For example, 'select * from TAB where 1 = 2'.
我有一个不返回任何命中的 sql 语句。例如，'select * from TAB where 1 = 2'。
I want to check how many rows are returned,
我想检查返回了多少行，
cursor.execute(query_sql)
rs = cursor.fetchall()
Here I get already exception: "(0, 'No result set')"
在这里我已经得到异常：“（0，'无结果集'）”
How can I prevend this exception, check whether the result set is empty?
如何防止此异常，检查结果集是否为空？
采纳答案by Martijn Pieters
cursor.rowcountwill usually be set to 0.
cursor.rowcount通常会设置为 0。
If, however, you are running a statement that would neverreturn a result set (such as INSERTwithout RETURNING, or SELECT ... INTO), then you do not need to call .fetchall(); there won't be a result set for such statements. Calling .execute()is enough to run the statement.
但是，如果您正在运行一个永远不会返回结果集的语句（例如INSERTwithout RETURNING, or SELECT ... INTO），那么您不需要调用.fetchall(); 这样的语句不会有结果集。调用.execute()足以运行该语句。
Note that database adapters are also allowed to set the rowcount to -1if the database adapter can't determine the exact affected count. See the PEP 249 Cursor.rowcountspecification:
请注意，-1如果数据库适配器无法确定确切的受影响计数，则还允许数据库适配器将行计数设置为。请参阅PEP 249Cursor.rowcount规范：
The attribute is
-1in case no.execute*()has been performed on the cursor or the rowcount of the last operation is cannot be determined by the interface.
该属性用于在游标上未执行
-1任何.execute*()操作或接口无法确定最后一次操作的行数的情况下。
The sqlite3libraryis prone to doing this. In all such cases, if you must know the affected rowcount up front, execute a COUNT()select in the same transaction first.
该sqlite3库是容易发生这样做。在所有这些情况下，如果您必须预先知道受影响的行数，COUNT()请先在同一事务中执行选择。
回答by thavan
MySQLdb will not raise an exception if the result set is empty. Additionally cursor.execute() function will return a long value which is number of rows in the fetched result set. So if you want to check for empty results, your code can be re-written as
如果结果集为空，MySQLdb 不会引发异常。此外， cursor.execute() 函数将返回一个 long 值，它是获取的结果集中的行数。所以如果你想检查空结果，你的代码可以重写为
rows_count = cursor.execute(query_sql)
if rows_count > 0:
rs = cursor.fetchall()
else:
// handle empty result set
回答by Rockallite
Notice:This is for MySQLdb module in Python.
注意：这是针对 Python 中的 MySQLdb 模块。
For a SELECTstatement, there shouldn't be an exception for an empty recordset. Just an empty list ([]) for cursor.fetchall()and Nonefor cursor.fetchone().
对于SELECT语句，不应该有空记录集的例外。只是一个空列表 ( []) forcursor.fetchall()和Nonefor cursor.fetchone()。
For any other statement, e.g. INSERTor UPDATE, that doesn't return a recordset, you can neither call fetchall()nor fetchone()on the cursor. Otherwise, an exception will be raised.
对于不返回记录集的任何其他语句，例如INSERTor UPDATE，您既不能调用fetchall()也不能fetchone()在游标上。否则，将引发异常。
There's one way to distinguish between the above two types of cursors:
有一种方法可以区分上述两种类型的游标：
def yield_data(cursor):
while True:
if cursor.description is None:
# No recordset for INSERT, UPDATE, CREATE, etc
pass
else:
# Recordset for SELECT, yield data
yield cursor.fetchall()
# Or yield column names with
# yield [col[0] for col in cursor.description]
# Go to the next recordset
if not cursor.nextset():
# End of recordsets
return
回答by Ryan Tuck
if you're connecting to a postgres database, the following works:
如果您要连接到 postgres 数据库，则以下操作有效：
result = cursor.execute(query)
if result.returns_rows:
# we got rows!
return [{k:v for k,v in zip(result.keys(), r)} for r in result.rows]
else:
return None
回答by Anand Kumar Singh
You can do like this :
你可以这样做：
count = 0
cnxn = pyodbc.connect("Driver={SQL Server Native Client 11.0};"
"Server=serverName;"
"Trusted_Connection=yes;")
cursor = cnxn.cursor()
cursor.execute(SQL query)
for row in cursor:
count = 1
if true condition:
print("True")
else:
print("False")
if count == 0:
print("No Result")
Thanks :)
谢谢 ：）
回答by Piney
I had issues with rowcount always returning -1 no matter what solution I tried.
无论我尝试什么解决方案，我都遇到了 rowcount 总是返回 -1 的问题。
I found the following a good replacement to check for a null result.
我发现以下一个很好的替代品来检查空结果。
c.execute("SELECT * FROM users WHERE id=?", (id_num,))
row = c.fetchone()
if row == None:
print("There are no results for this query")
