Question

SELECT Trainer.id AS idTrainer, surname, lastname, name,  
if(month = '', 'NO',month) as monthT, quarter, halfyear, year, id_Person, photo
FROM Trainer, Person
WHERE Trainer.id_Person = Person.id

phpmyadmin中的

if(month = '', 'NO',month) as monthT返回good或'no'或month

在C＃中返回byte[]，但如果month为空，则需要返回NO，否则返回month。

我的C＃代码如下所示：

string sql = @" SELECT Trainer.id AS idTrainer, surname, lastname, name,  if(month = '', 'NO',month) as monthT, quarter, halfyear, year, id_Person, photo
                FROM Trainer, Person
                WHERE Trainer.id_Person = Person.id";
using (MySqlConnection connection = ConnectToDataBase.GetConnection())
{
    MySqlCommand command = new MySqlCommand(sql, connection);
    MySqlDataAdapter adapter = new MySqlDataAdapter();

    connection.Open();
    adapter.SelectCommand = command;
}

Answer 1

在研究时我发现使用CAST存在VARCHAR的错误。解决方案是使用CHAR而不是VARCHAR。

证明链接：

Using Conversion Functions

Bug #34564 CAST does not accept varchar type

Cast Functions and Operators

Answer 2

我不是MySql专家，但为什么你不尝试演员？

CAST(if(month = '', 'NO',month) AS VARCHAR) AS monthT

更多信息： http://dev.mysql.com/doc/refman/5.0/en/cast-functions.html

如何将byte []转换为varchar？

2 个答案: