Question

我正在寻找比较两个表的最佳方法，并返回包含两个表之间差异的行。

//表A

CREATE TABLE `mydb`.`a` (
  `id` int(10) unsigned NOT NULL,
  `color` varchar(45) DEFAULT NULL,
  `animal` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `mydb`.`a` 
    (id,color,animal) 
VALUES 
    (1,"red",""),
    (2,"blue","cat"),
    (3,"green","dog");
    (4,"orange","fish");

//表B

CREATE TABLE `mydb`.`b` (
  `id` int(10) unsigned NOT NULL,
  `color` varchar(45) DEFAULT NULL,
  `animal` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `mydb`.`b` 
    (id,color,animal) 
VALUES 
    (1,"red",""),
    (2,"blue","cat"),
    (3,"green","bird");
    (4,"brown","fish");

该语句将返回表B的第3,4行，因为bird和brown与dog和orange不同。

Answer 1

NULL表示缺少信息。它永远不会匹配其他值（包括其他NULL）。因此，例如，零等于零：

SQL> select case when 0 = 0 then 'true' else 'false' end ;
case when 0 = 0 then 'true' else 'false' end
--------------------------------------------
true

NULL 不等于NULL：

SQL> select case when null = null then 'true' else 'false' end ;
case when null = null then 'true' else 'false' end
--------------------------------------------------
false

如果你想匹配它们，你必须将 NULL转换为其他值（例如零）：

SQL> select case when ifnull(null, 0) = ifnull(null, 0) then 'true' else 'false' end ;
case when ifnull(null, 0) = ifnull(null, 0) then 'true' else 'false' end
------------------------------------------------------------------------
true

修改

假设你有：

mysql> select * from a; +----+--------+--------+ | id | color | animal | +----+--------+--------+ | 1 | red | | | 2 | blue | cat | | 3 | green | dog | | 4 | orange | fish | +----+--------+--------+ 4 rows in set (0.00 sec) mysql> select * from b; +----+-------+--------+ | id | color | animal | +----+-------+--------+ | 1 | red | | | 2 | blue | cat | | 3 | green | bird | | 4 | brown | fish | +----+-------+--------+ 4 rows in set (0.00 sec)

我会用：

mysql> select b.* from b left outer join a on a.id = b.id and a.animal = b.animal and a.color = b.color where a.id is null and a.animal is null and a.color is null ; +----+-------+--------+ | id | color | animal | +----+-------+--------+ | 3 | green | bird | | 4 | brown | fish | +----+-------+--------+ 2 rows in set (0.00 sec)

查找B中的行，与A中的相应行有所不同。和

mysql> select a.* from a left outer join b on a.id = b.id and a.animal = b.animal and a.color = b.color where b.id is null and b.animal is null and b.color is null ; +----+--------+--------+ | id | color | animal | +----+--------+--------+ | 3 | green | dog | | 4 | orange | fish | +----+--------+--------+ 2 rows in set (0.00 sec)

查找A中与B中相应行不同的行。

表与NULL值比较

1 个答案: