java - 如何防止重复。 Java、SQL

Question

我尝试复制username列中的条目，但结果始终在else语句中，即数据已成功插入到不应重复的表中基于 if 语句。顺便说一下String value1=username1.getText();。

代码如下:

String sq = "SELECT username FROM login where username = '"+value1+"'";

if (sq.equals(value1)){ 
JOptionPane.showMessageDialog(null,"username is already existed! please 
create new username.");   
}
else{
int k=st.executeUpdate("insert into 
login(username,password,firstname,lastname,address,contactno) 


values('"+value1+"','"+value2+"','"+value3+"','"+value4+"',
'"+value5+"','"+value6+"')");
JOptionPane.showMessageDialog(null,"Data is successfully saved");
}

Answer 1

您只需创建 SQL 字符串并从不运行它(相反，您将其与用户名进行比较，而用户名始终为 false)。

您应该实际执行它并检查结果。例如。 (为简洁起见，省略了错误处理和资源清理):

Connection conn = // Connect to the database... 
PreparedStatement ps = 
    conn.prepareStatement("SELECT username FROM login where username = ?";
ps.setString(1, value1);
ResultSet rs = ps.executeQuery();
if (rs.next()) {
    JOptionPane.showMessageDialog
        (null,"username is already existed! please create new username.");   
}