在SQL/Plus中默认的表示替代变量，也就是说，只要在命令中出现该符号，SQL/Plus就会要你输入替代。 这就意味着你无法将一个含有该符号的字符串输入到数据库中或赋给某一个变量。 如字符串“YouI”系统会理解为以“You”打头的字符串，它会提示你输入替代变量
在SQL/Plus中默认的"&"表示替代变量，也就是说，只要在命令中出现该符号，SQL/Plus就会要你输入替代值。
这就意味着你无法将一个含有该符号的字符串输入到数据库中或赋给某一个变量。
如字符串“You&I”系统会理解为以“You”打头的字符串，它会提示你输入替代变量I的值，如果你输入I，则最终字符串转化为“YouI”。
set define off 则关闭该功能，“&”将作为普通字符，如上例，最终字符就为“You&I”
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