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错误：You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'not null, add column username varchar not null' at line 1
原因：没有给varchar设定大小
1、java.sql.SQLException: Can not issue data manipulation statements with executeQuery().
at com.mysql.jdbc.Statement.checkForDml(Statement.java:298)
at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:950)
错误原因： executeUpdate和executeQuery看看sql的执行语句，是查询还是更新