为什么这段代码没有给出预期的输出？

I was experimenting with database operations. For example,

我正在尝试数据库操作。例如,

begin
   db_con = PG.connect :dbname => 'sureshkumar', :user => 'sureshkumar'
   db_con.prepare 'get_result', "SELECT * from users;"
   users_name = db_con.exec_prepared 'get_result'
   if users_name != nil
       puts users_name.values
   else
       puts "There is no users available in the table users"
   end
rescue PG::Error => e
        puts e.message
ensure
        db_con.close if db_con
end

The table users contains only two columns. They are,

表用户只包含两列。他们是,

user_id
user_name

If the table does not contain any row, the exec_prepared statement does not return any row. In this case, why does the if condition not become false? How do I check if the exec_prepared method doesn't return any row?

如果表不包含任何行,则exec_prepared语句不返回任何行。在这种情况下,为什么if条件不会变错?如何检查exec_prepared方法是否不返回任何行?

1 个解决方案

#1

users_name = db_con.exec_prepared 'get_result' will never return nil to users_name.

users_name = db_con.exec_prepared'get_result'永远不会返回nil到users_name。

According to the documentation of PG::Connection#exec_prepared it will return PG::Result object. So, users_name object can never be nil. It will always be an instance of PG::Result.

根据PG :: Connection#exec_prepared的文档,它将返回PG :: Result对象。因此,users_name对象永远不能为零。它将永远是PG :: Result的一个实例。

So, to check whether you have any rows returned from the query execution you need to check if this PG::Result object has any values in it. In your case, users_name variable contains the PG::Result. So you will have to call ntuples on it so see how many rows are returned from the query.

因此,要检查是否从查询执行返回任何行,您需要检查此PG :: Result对象中是否包含任何值。在您的情况下,users_name变量包含PG :: Result。因此,您必须在其上调用ntuples,以便查看从查询返回的行数。

so change your if clause to something like:

所以将你的if子句改为:

if users_name.ntuples > 0
  puts "There are users"
else
  puts "There is no users available in the table users"
end

See PG::Result's documentation to know what other methods you can call on it to access the result.

请参阅PG :: Result的文档,了解可以调用的其他方法来访问结果。

#1