示例：
若没有变量很简单：
String sql = "SELECT * FROM user WHERE name= ‘Jack‘ and password = ‘123456‘ ";
若有变量进行动态拼接，写法如下：
String sql = "SELECT * FROM user WHERE name= ‘"+name+"‘ and password = ‘"+password+"‘ ";
步骤：
SELECT * FROM user WHERE name= ‘‘ and password = ‘‘ ;
先在‘‘ 中添加 ""
SELECT * FROM user WHERE name= ‘""‘ and password = ‘""‘ ;
然后在 " " 中添加两个+号，然后在 ++ 中添加 要动态传入的字符串，然后把语句 复制到 String sql = **" "**; 中的引号中
String sql = "SELECT * FROM user WHERE name= ‘"+name+"‘ and password = ‘"+password+"‘ ";