发布于 2017-06-24 13:34:42 | 123 次阅读 | 评论: 0 | 来源: 网友投递
Mysql关系型数据库管理系统
MySQL是一个开放源码的小型关联式数据库管理系统，开发者为瑞典MySQL AB公司。MySQL被广泛地应用在Internet上的中小型网站中。由于其体积小、速度快、总体拥有成本低，尤其是开放源码这一特点，许多中小型网站为了降低网站总体拥有成本而选择了MySQL作为网站数据库。
核心代码
/*--------------------------------
求2个或以上字段为NULL 的记录
t1:
id, id1, id2, id3, id4, id5, id6
在t1 表中有个字段；
其中id是主键；
怎样打印其中个字段或以上为NULL 的记录id？
另外，存储过程中怎么实现按顺序一条一条读取记录最方便？
注：主键id 是没有顺序的，也可能是字符串的；
-----------------------------------------*/
drop table if exists t1;
create table t1(id int,id1 int,id2 int,id3 int,id4 int,id5 int,id6 int);
insert t1 select
1,1,1,1,1,null,null union all select
2,null,null,null,1,2,3 union all select
3,1,2,3,4,5,6 union all select
4,1,2,3,4,5,null union all select
5,null,3,4,null,null,null ;
delimiter $$
create procedure usp_c_null()
begin
declare n_c int;
declare idd int;
declare cur cursor for
select id,case char_length(concat(ifnull(id1,'@'),ifnull(id2,'@'),ifnull(id3,'@'),ifnull(id4,'@'),ifnull(id5,'@'),ifnull(id6,'@')))
-char_length(replace(concat(ifnull(id1,'@'),ifnull(id2,'@'),ifnull(id3,'@'),ifnull(id4,'@'),ifnull(id5,'@'),ifnull(id6,'@')),'@','') )
when 6 then 6 when 5 then 5 when 4 then 4 when 3 then 3 when 2 then 2 when 1 then 1 else 0 end as c from t1;
declare exit HANDLER for not found close cur ;
open cur;
repeat
fetch cur into idd,n_c;
if(n_c>=2) then
select * from t1 where id=idd;
end if ;
until 0 end repeat;
close cur;
end ;
$$
delimiter ;
/*
+------+------+------+------+------+------+------+
| id | id1 | id2 | id3 | id4 | id5 | id6 |
+------+------+------+------+------+------+------+
| 1 | 1 | 1 | 1 | 1 | NULL | NULL |
+------+------+------+------+------+------+------+
1 row in set (0.10 sec)
+------+------+------+------+------+------+------+
| id | id1 | id2 | id3 | id4 | id5 | id6 |
+------+------+------+------+------+------+------+
| 2 | NULL | NULL | NULL | 1 | 2 | 3 |
+------+------+------+------+------+------+------+
1 row in set (0.14 sec)
+------+------+------+------+------+------+------+
| id | id1 | id2 | id3 | id4 | id5 | id6 |
+------+------+------+------+------+------+------+
| 5 | NULL | 3 | 4 | NULL | NULL | NULL |
+------+------+------+------+------+------+------+
1 row in set (0.17 sec)
*/