mysql case with

在MySQL中，`CASE` 语句是一种条件表达式，它用于根据给定的条件返回不同的值。`CASE` 通常与 `WHEN`, `ELSE` 和 `END` 关键字一起使用，用于执行简单的条件判断和结果映射。以下是一个基本的 `CASE` 结构的例子：

SELECT 
    CASE 
        WHEN condition1 THEN result1
        WHEN condition2 THEN result2
        ELSE default_result
    END AS case_expression
FROM table_name;

在这个结构中：
- `condition1`, `condition2` 是评估的条件，如果满足，会执行相应的 `result1` 或 `result2`。
- 如果所有 `WHEN` 条件都不满足，就会执行 `ELSE` 后面的 `default_result`。
- `AS case_expression` 是为了给结果指定一个别名。

例如，如果你想根据某个字段的值（如性别）显示不同的问候语：

SELECT 
    CASE 
        WHEN gender = 'Male' THEN 'Hello, Mr.'
        WHEN gender = 'Female' THEN 'Hello, Ms.'
        ELSE 'Hello'
    END AS greeting
FROM users;

相关问题

mysql with case

`CASE` is a conditional statement in MySQL that allows you to evaluate a series of conditions and return a value based on the first condition that is true. You can use `CASE` in conjunction with SQL statements like `SELECT`, `UPDATE`, and `DELETE`.

Here is an example of using `CASE` in a `SELECT` statement:

SELECT order_id,
       CASE
            WHEN order_total >= 1000 THEN 'High'
            WHEN order_total >= 500 THEN 'Medium'
            ELSE 'Low'
       END AS order_priority
FROM orders;

In this example, we are selecting the `order_id` column from the `orders` table and creating a new column called `order_priority`. The value of `order_priority` is determined by the `CASE` statement. If `order_total` is greater than or equal to 1000, then `order_priority` is set to 'High'. If `order_total` is greater than or equal to 500, then `order_priority` is set to 'Medium'. Otherwise, `order_priority` is set to 'Low'.

You can also use `CASE` in conjunction with other SQL statements, such as `UPDATE`. Here is an example of updating a table using `CASE`:

UPDATE employees
SET salary = CASE
                WHEN department = 'Sales' THEN salary * 1.1
                WHEN department = 'Marketing' THEN salary * 1.2
                ELSE salary
             END;

In this example, we are updating the `salary` column in the `employees` table. The new value of `salary` is determined by the `CASE` statement. If the `department` is 'Sales', then `salary` is multiplied by 1.1. If the `department` is 'Marketing', then `salary` is multiplied by 1.2. Otherwise, `salary` remains the same.

Overall, `CASE` is a useful tool in MySQL for creating conditional statements and controlling the flow of your SQL queries.

mysql中的case when then

### MySQL 中 CASE WHEN THEN 语句详解

#### 理解 CASE WHEN THEN 结构
在 MySQL 查询中，`CASE WHEN THEN` 提供了一种灵活的方式来实现条件判断逻辑。这种结构允许基于不同条件返回特定值，类似于编程中的 `if-else` 控制流。

#### 基本语法形式
最简单的 `CASE` 表达式可以写作：

CASE column_name
    WHEN value1 THEN result1
    WHEN value2 THEN result2
    ELSE default_result
END AS alias_column

此表达式的含义是在给定列等于指定值时返回相应结果；如果都不匹配，则返回默认结果[^1]。

对于更复杂的多条件判断场景，推荐使用带有布尔表达式的完整版 `CASE`:

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ELSE default_result
END AS alias_column

这里可以根据任意有效的 SQL 条件表达式来进行分支选择[^4]。

#### 实际应用案例分析

##### 场景一：简单 Case 函数转换性别描述
假设有一个名为 `users` 的表，其中包含用户的 ID 和性别编码（0表示女性, 1表示男性）。为了使报告更加友好易读，可以通过 `CASE` 将这些数值映射为人可理解的文字说明：

SELECT id,
       CASE gender_code
           WHEN 0 THEN 'Female'
           WHEN 1 THEN 'Male'
           ELSE 'Unknown'
       END as gender_description
FROM users;

这段代码会遍历每一行记录并根据 `gender_code` 字段的内容设置新的别名列 `gender_description`。

##### 场景二：解决排名问题
考虑一个销售数据集，想要计算每位销售人员在其部门内的相对业绩位置。通过组合窗口函数与 `CASE` 可以轻松完成这一任务：

WITH ranked_sales AS (
    SELECT employee_id,
           department,
           sales_amount,
           RANK() OVER (PARTITION BY department ORDER BY sales_amount DESC) rank_num
    FROM sales_data
)
SELECT *,
      CASE
          WHEN rank_num = 1 THEN 'Top Performer'
          WHEN rank_num BETWEEN 2 AND 5 THEN 'Good Performance'
          ELSE 'Needs Improvement'
      END performance_level
FROM ranked_sales;

上述查询首先利用窗口功能为各部门成员按销售额降序排列赋予等级编号，接着再依据该编号分配绩效评价标签[^2]。

##### 场景三：动态调整显示内容
有时可能希望根据不同业务规则改变某些字段的表现方式而不影响原始存储的数据。比如在一个订单系统里，针对状态码做个性化提示信息展示：

SELECT order_number,
       status_code,
       CASE
           WHEN status_code IN ('PENDING', 'PROCESSING') THEN CONCAT('Your Order ', order_number, ' is being processed.')
           WHEN status_code = 'SHIPPED' THEN CONCAT('Order ', order_number, ' has been shipped and will arrive soon!')
           ELSE 'Thank you for your purchase!'
       END message_to_customer
FROM orders;

这样就可以向客户发送定制化的通知消息而无需修改数据库内实际的状态标记。