怎么新建MySQL数据库

在MariaDB中，UPDATE语句用于通过更改表中的值来修改现有字段。

语法：

UPDATE table_name SET field=new_value, field2=new_value2,...  
[WHERE ...]

或者，UPDATE语句可以与WHERE，ORDER BY和LIMIT子句一起使用。

UPDATE table  
SET column1 = expression1,  
    column2 = expression2,  
    ...  
[WHERE conditions]  
[ORDER BY expression [ ASC | DESC ]]  
[LIMIT number_rows];

1. 更新单个列

假设我们有一个表 - students，并具有以下数据记录：

MariaDB [testdb]> select * from students;
+------------+--------------+-----------------+----------------+
| student_id | student_name | student_address | admission_date |
+------------+--------------+-----------------+----------------+
|          1 | Maxsu        | Haikou          | 2017-01-07     |
|          3 | JMaster      | Beijing         | 2016-05-07     |
|          4 | Mahesh       | Guangzhou       | 2016-06-07     |
|          5 | Kobe         | Shanghai        | 2016-02-07     |
|          6 | Blaba        | Shengzheng      | 2016-08-07     |
+------------+--------------+-----------------+----------------+
5 rows in set (0.00 sec)

现在，更改student_id列的值为 6的行记录，把student_address列的值更新为：Shenzhen。

UPDATE Students  
SET student_address = 'Shenzhen'
WHERE student_id = '6';

执行上面更新语句后，查询更新的结果 -

MariaDB [testdb]> UPDATE Students
    -> SET student_address = 'Shenzhen'
    -> WHERE student_id = '6';
Query OK, 1 row affected (0.21 sec)
Rows matched: 1  Changed: 1  Warnings: 0

MariaDB [testdb]> select * from students where student_id=6;
+------------+--------------+-----------------+----------------+
| student_id | student_name | student_address | admission_date |
+------------+--------------+-----------------+----------------+
|          6 | Blaba        | Shenzhen        | 2016-08-07     |
+------------+--------------+-----------------+----------------+
1 row in set (0.08 sec)

2. 更新多列

还可以使用MariaDB数据库中的UPDATE语句来更新多个列。 在以下示例中，将更新表students中student_name为Kobe的两列 - student_name和student_address的值。参考以下更新语句 -

UPDATE Students  
SET student_name = '科比', student_address = 'Haikou'
WHERE student_name = 'Kobe';

执行上面语句，得到以下结果 -

MariaDB [testdb]> UPDATE Students
    -> SET student_name = '科比', student_address = 'Haikou'
    -> WHERE student_name = 'Kobe';
Query OK, 1 row affected (0.09 sec)
Rows matched: 1  Changed: 1  Warnings: 0

-- 查询更新的结果
MariaDB [testdb]> select * from students;
+------------+--------------+-----------------+----------------+
| student_id | student_name | student_address | admission_date |
+------------+--------------+-----------------+----------------+
|          1 | Maxsu        | Haikou          | 2017-01-07     |
|          3 | JMaster      | Beijing         | 2016-05-07     |
|          4 | Mahesh       | Guangzhou       | 2016-06-07     |
|          5 | 科比         | Haikou          | 2016-02-07     |
|          6 | Blaba        | Shenzhen        | 2016-08-07     |
+------------+--------------+-----------------+----------------+
5 rows in set (0.02 sec)

注意事项

在执行语句时，经常要指定更新的条件，如果忘记了指定了WHERE子句中的条件，那么将会更新所有行记录。想象一下，以下两个语句执行的效果 -

-- 语句1
UPDATE Students  
SET student_name = '科比', student_address = 'Haikou'
WHERE student_name = 'Kobe';

-- 语句2
UPDATE Students  
SET student_name = '科比', student_address = 'Haikou';除数据