mysql case when if
时间: 2024-06-23 17:02:33 浏览: 6
在MySQL中,`CASE WHEN` 和 `IF` 都是用于条件逻辑判断的语句,它们在某些场景下可以互换使用,但`CASE` 提供了更灵活的选项和更清晰的语句结构。
`CASE WHEN` 语句主要用于根据给定的条件返回一个值。其基本语法如下:
```sql
CASE
WHEN condition THEN result_1
WHEN condition2 THEN result_2
[ ... ]
ELSE default_result
END
```
例如,如果你想根据用户分数计算等级:
```sql
SELECT
CASE
WHEN score >= 90 THEN 'A'
WHEN score >= 80 THEN 'B'
ELSE 'C'
END AS grade
FROM users;
```
`IF` 则是一个更简单的三元表达式,只支持两个条件:
```sql
IF (condition, true_result, false_result)
```
例如:
```sql
SELECT
IF(score >= 80, 'Good', 'Poor') AS performance
FROM users;
```
相关问题
mysql if 和case when
MySQL中的IF和CASE WHEN是两种常见的条件判断语句,可以用于根据某个条件来决定执行的语句或返回的结果。
IF语句的语法格式如下:
```
IF(condition, true_value, false_value)
```
其中,condition是一个条件表达式,true_value和false_value分别是满足和不满足条件时返回的值。如果condition为真,则返回true_value,否则返回false_value。
CASE WHEN语句的语法格式如下:
```
CASE expression
WHEN value1 THEN result1
WHEN value2 THEN result2
...
ELSE default_result
END
```
其中,expression是需要进行判断的表达式,value1、value2等是不同的取值,result1、result2等是对应的结果。如果expression的值等于某个value,则返回对应的result值;如果都不匹配,则返回default_result。
举个例子,假设我们有一个用户表user,其中有一个字段age表示用户的年龄。现在我们需要查询用户的信息,并根据年龄来判断用户的类别(年轻、中年、老年),则可以使用如下SQL语句:
```
SELECT name, age,
CASE
WHEN age < 30 THEN 'young'
WHEN age >= 30 AND age < 60 THEN 'middle-aged'
ELSE 'old'
END AS category
FROM user;
```
这条SQL语句会查询用户表中所有用户的姓名、年龄和类别,并根据不同的年龄段来分为年轻、中年和老年三类,并通过AS关键字指定类别字段的别名为category。
mysql case when
MySQL CASE WHEN statement is a conditional statement that evaluates multiple conditions and returns a value based on the first condition that is true.
The basic syntax for the MySQL case when statement is as follows:
```
CASE
WHEN condition1 THEN result1
WHEN condition2 THEN result2
...
WHEN conditionN THEN resultN
ELSE default_result
END
```
Here is an example of how to use the MySQL case when statement:
```
SELECT first_name, last_name,
CASE
WHEN age < 18 THEN 'Minor'
WHEN age BETWEEN 18 AND 65 THEN 'Adult'
ELSE 'Senior'
END AS age_group
FROM customers;
```
In this example, we are selecting the first name, last name, and age group of customers from the customers table. We use the MySQL case when statement to evaluate the age of each customer and assign them to an age group based on their age. If the customer is under 18, they are assigned to the 'Minor' group, if they are between 18 and 65, they are assigned to the 'Adult' group, and if they are over 65, they are assigned to the 'Senior' group.
The resulting output would look something like this:
```
+------------+-----------+-----------+
| first_name | last_name | age_group |
+------------+-----------+-----------+
| John | Doe | Adult |
| Jane | Smith | Senior |
| Bob | Johnson | Minor |
| Sarah | Lee | Adult |
+------------+-----------+-----------+
```
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