NULL 值

1. 先繼續在資料表 dcount 新增兩筆紀錄且其 dcount 都是 NULL。

   mariadb root@localhost:dyw> insert into dcount(author,dcount) values('jenny',nul
                               l),('sara',null);                                   
   Query OK, 2 rows affected
   Time: 0.006s
   

2. 要查詢資料表 dcount 中欄位 dcount 是空的紀錄， 不能使用運算子 =, !=。

   mysql> select * from dcount_tbl where dyw_count = null;
   mysql> select * from dcount_tbl where dyw_count != null;
   

3. 要查詢資料表 dcount 中欄位 dcount 是空的紀錄，必須使用 IS NULL, IS NOT NULL 等語法。

   mariadb root@localhost:dyw> select * from dcount where dcount is null;          
   +--------+--------+
   | author | dcount |
   +--------+--------+
   | jenny  | <null> |
   | sara   | <null> |
   +--------+--------+
   2 rows in set
   Time: 0.025s
   mariadb root@localhost:dyw> select * from dcount where dcount is not null;      
   +--------+--------+
   | author | dcount |
   +--------+--------+
   | dywang | 4      |
   | linda  | 3      |
   | peter  | 2      |
   | rita   | 1      |
   +--------+--------+
   4 rows in set
   Time: 0.021s
   

4. <=> 運算子等效 IS NULL 語法。

   mariadb root@localhost:dyw> select * from dcount where dcount <=> null;         
   +--------+--------+
   | author | dcount |
   +--------+--------+
   | jenny  | <null> |
   | sara   | <null> |
   +--------+--------+
   2 rows in set
   Time: 0.021s