如何检查结果集是否为空? [英] How to check if a result set is empty?

问题描述

我有一个不返回任何命中的 sql 语句.例如，'select * from TAB where 1 = 2'.

I have a sql statement that returns no hits. For example, 'select * from TAB where 1 = 2'.

我想查看返回了多少行，

I want to check how many rows are returned,

cursor.execute(query_sql)

rs = cursor.fetchall()

这里我已经得到了异常:(0，'无结果集')"

Here I get already exception: "(0, 'No result set')"

如何防止这个异常，检查结果集是否为空?

How can I prevend this exception, check whether the result set is empty?

推荐答案

cursor.rowcount 通常会设置为 0.

但是，如果您运行的语句从不返回结果集(例如 INSERT 没有 RETURNING，或 SELECT ... INTO)，则不需要调用.fetchall();这样的语句不会有结果集.调用 .execute() 足以运行该语句.

If, however, you are running a statement that would never return a result set (such as INSERT without 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.

请注意，如果数据库适配器无法确定确切的受影响计数，也允许数据库适配器将行数设置为 -1.请参阅 PEP 249 Cursor.rowcount 规范:

Note that database adapters are also allowed to set the rowcount to -1 if the database adapter can't determine the exact affected count. See the PEP 249 Cursor.rowcount specification:

属性为-1，以防没有.execute*()对游标执行过或者接口无法确定上次操作的行数.

The attribute is -1 in case no .execute*() has been performed on the cursor or the rowcount of the last operation is cannot be determined by the interface.

sqlite3 库 倾向于这样做.在所有此类情况下，如果您必须预先知道受影响的行数，请先在同一事务中执行 COUNT() 选择.

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