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法學避免顯示重複值的 MySQL 查詢？
為此，可以使用 GROUP BY 並使用 COUNT 僅獲取非重複值。以下是語法 −
select yourColumnName from yourTableName group by yourColumnName having count(*)=1;
讓我們首先建立一個表 −
mysql> create table DemoTable -> ( -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, -> Subject varchar(100) -> ); Query OK, 0 rows affected (0.70 sec)
使用 insert 命令在表中插入一些記錄 −
mysql> insert into DemoTable(Subject) values('MySQL');
Query OK, 1 row affected (0.21 sec)
mysql> insert into DemoTable(Subject) values('MongoDB');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable(Subject) values('Java');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable(Subject) values('MongoDB');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable(Subject) values('C');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(Subject) values('C++');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable(Subject) values('Java');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable(Subject) values('C');
Query OK, 1 row affected (0.23 sec)使用 select 語句顯示錶中的所有記錄 −
mysql> select *from DemoTable;
輸出
+----+---------+ | Id | Subject | +----+---------+ | 1 | MySQL | | 2 | MongoDB | | 3 | Java | | 4 | MongoDB | | 5 | C | | 6 | C++ | | 7 | Java | | 8 | C | +----+---------+ 8 rows in set (0.00 sec)
以下是可以避免顯示重複項的查詢 −
mysql> select Subject from DemoTable group by Subject having count(*)=1;
輸出
+---------+ | Subject | +---------+ | MySQL | | C++ | +---------+ 2 rows in set (0.00 sec)
更新於：30-7-2019
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