# 简单的 UPDATE 语句

我们在查询教师表`teachers`的时候发现，教师姓名`name`为'Linghu Chong'的老师邮箱`email`信息为NULL，即没有该部分信息，我们现在希望更新邮箱信息，这时候就需要用到`UPDATE`语句。

UPDATE 语句用于更新表中已存在的记录。

**语法**

```sql
UPDATE table_name
SET column1=value1,column2=value2,...
WHERE some_column=some_value;
```

> 请注意 SQL UPDATE 语句中的 WHERE 子句！WHERE 子句规定哪条记录或者哪些记录需要更新。如果您省略了 WHERE 子句，所有的记录都将被更新！

## UPDATE实例

假设我们要把 教师名`name`为"Linghu Chong" 的邮箱`email`更新为 '<LinghuChong@lintcode.com>'

```sql
UPDATE teachers
SET email='LinghuChong@lintcode.com'
WHERE name = 'Linghu Chong';
```

执行输出结果

```bash
mysql> UPDATE teachers
    -> SET email='LinghuChong@lintcode.com'
    -> WHERE name = 'Linghu Chong';
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT * FROM teachers WHERE name='Linghu Chong';
+----+--------------+--------------------------+-----+---------+
| id | name         | email                    | age | country |
+----+--------------+--------------------------+-----+---------+
|  5 | Linghu Chong | LinghuChong@lintcode.com |  18 | CN      |
+----+--------------+--------------------------+-----+---------+
1 row in set (0.01 sec)
```

**注意**

在更新记录时要格外小心！在上面的实例中，如果我们省略了 WHERE 子句，如下所示：

```sql
UPDATE teachers
SET email='LinghuChong@lintcode.com';
```

执行以上代码会将 `teachers`表中所有数据的 `email`改为 '<LinghuChong@lintcode.com>'。

执行没有 WHERE 子句的 UPDATE 要慎重，再慎重。

## 练习题: 更新表中信息

题目描述: 因为最近人工智能发展迅速，越来越多的人希望学习、了解人工智能课程，lintcode的人工智能课程人数也有大幅度增加，我们希望更新课程表`courses`中的人工智能课程(即课程名`name`为'Artificial Intelligence')的学生人数为500，请写出对应的SQL语句

```sql
-- write your sql here
```


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