null比较需要用 is null 或者 is not null 判断
null遇见运算符 一律返回null =null 或 !=null 等等一律返回null ，并且效率不高，所以建表时候一般都会有not null default '';
mysql> select goods_name from goods where goods_name is not null limit 3; +--------------------+ | goods_name | +--------------------+ | KD876 | | htcN85原装充电器 | | 诺基亚原装5800耳机 | +--------------------+
3 rows in set (0.00 sec)
mysql> select goods_name from goods where goods_name is null ; Empty set (0.00 sec)
null 遇见运算符 一律返回null
mysql> select goods_name from goods where goods_name=null ; Empty set (0.00 sec)
mysql> select goods_name from goods where goods_name!=null ; Empty set (0.00 sec)下载本文
