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 with rollup 聚合函数取名称

在使用 MySQL 的 `WITH ROLLUP` 语句进行聚合查询时，可以使用 `GROUPING()` 函数来判断当前行是否是汇总行。但是，`GROUPING()` 函数只返回 0 或 1，无法返回对应的名称。

如果需要在汇总行上显示名称，可以使用 `CASE` 语句结合 `GROUPING()` 函数来实现。例如：

SELECT 
    CASE WHEN GROUPING(column_name) = 1 THEN '总计' ELSE column_name END AS column_name,
    SUM(amount) AS total_amount
FROM table_name
GROUP BY column_name WITH ROLLUP;

在上述查询中，如果当前行是汇总行，则将名称设置为 '总计'，否则使用列名作为名称。这样就可以在汇总行上显示对应的名称了。