为什么此准备好的语句出现语法错误? [英] Why am i getting a syntax error with this prepared statement?

问题描述

我正在尝试学习如何执行PreparedStatement，如下所示:

I am trying to learn how to do a PreparedStatement as follows:

createConnection();
conn.setAutoCommit(false);
String sql = "SELECT MAX(?) FROM ?";
PreparedStatement stmt = conn.prepareStatement(sql);

但是，当我碰到最后一行时，它会抛出一个java.sql.SQLSyntaxErrorException，如下所示:

However, when I hit the last line, it throws a java.sql.SQLSyntaxErrorException as follows:

java.sql.SQLSyntaxErrorException: Syntax error: Encountered "?" at line 1, column 20.

我到处搜索过，但找不到任何失败的原因.我究竟做错了什么?稍后在代码中，我将设置?参数表示完整的字符串值，但是当我调试时，我从没有达到过这一点.它点击此conn.prepareStatement行并引发异常.谢谢您的帮助.我不知道它是否有作用，但是它在derby数据库中.

I have searched everywhere but cant find any reason for it to fail. What am I doing wrong? later on in the code I am setting the ? parameters as meaning full string values but when I debug I never get to that point. It hits this conn.prepareStatement line and throws the exception. Thanks for the help. I don't know if it makes a difference but it is on a derby database.

推荐答案

您认为呢?标记为sql变量.您无法在变量上执行MAX的选择，语法上是错误的.

Think of your ? mark as a sql variable. You can't do a select MAX on a variable, it's syntactically incorrect.

您不能执行以下操作:

declare @myvar int
select @myvar = 1

SELECT MAX(@myvar) from SomeTable

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