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Legal Studies如何使用單個 MySQL 查詢來計算忽略空值的列值？
為此，可以使用 COUNT() 方法，該方法不包括 NULL 值。我們首先建立一個表 -
mysql> create table DemoTable -> ( -> Name varchar(100), -> CountryName varchar(100) -> ); Query OK, 0 rows affected (0.49 sec)
使用 insert 命令在表中插入一些記錄 -
mysql> insert into DemoTable values('John',null);
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable values('Chris','US');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable values('Robert',null);
Query OK, 1 row affected (0.19 sec)
mysql> insert into DemoTable values('Bob','UK');
Query OK, 1 row affected (0.57 sec)使用 select 語句從表中顯示所有記錄 -
mysql> select *from DemoTable;
輸出
這將產生以下輸出 -
+--------+-------------+ | Name | CountryName | +--------+-------------+ | John | NULL | | Chris | US | | Robert | NULL | | Bob | UK | +--------+-------------+ 4 rows in set (0.00 sec)
以下是使用 COUNT() 並忽略 NULL 的查詢 -
mysql> select count(Name) AS TotalName,count(CountryName) AS CountryWhichIsNotNull from DemoTable;
輸出
這將產生以下輸出 -
+-----------+-----------------------+ | TotalName | CountryWhichIsNotNull | +-----------+-----------------------+ | 4 | 2 | +-----------+-----------------------+ 1 row in set (0.00 sec)
更新時間： 30-6-2020
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