mysql case when效率
时间: 2023-08-25 13:16:31 浏览: 55
MySQL 中的 CASE WHEN 语句可以用于在查询结果中根据条件进行值的转换。这种语句通常会稍微降低查询的效率,因为它需要在查询过程中进行额外的计算。
然而,这种效率降低通常是可以接受的,尤其是在数据集较小或者查询不太频繁的情况下。如果你的查询需要用到复杂的条件判断和值转换,CASE WHEN 语句是一个非常方便和可读性高的解决方案。
如果你担心查询效率问题,可以尝试优化查询,例如创建索引、使用合适的数据类型、避免使用通配符等等,以提高查询效率。
相关问题
mysql case when CASE WHEN
CASE WHEN语句用于计算条件列表并返回多个可能结果表达式之一。它有两种格式:简单CASE函数和搜索CASE函数。
简单CASE函数将某个表达式与一组简单表达式进行比较以确定结果。它的语法格式如下:
CASE input_expression
WHEN when_expression THEN result_expression
[ELSE else_result_expression]
END
搜索CASE函数计算一组布尔表达式以确定结果。它的语法格式如下:
CASE
WHEN [Boolean_expression] THEN [result1]
...
ELSE [default]
END
在使用简单CASE函数时,when_expression后面必须接case列的值,不能是表达式。而在使用搜索CASE函数时,可以使用任意的布尔表达式。
以下是一个示例,以人员表为例,展示了使用CASE WHEN语句的两种用法:
1. 使用简单CASE函数:
SELECT
CASE sex
WHEN '1' THEN '男'
WHEN '2' THEN '女'
ELSE '其他'
END AS '性别'
FROM people;
2. 使用搜索CASE函数:
SELECT
CASE
WHEN sex = '1' THEN '男'
WHEN sex = '2' THEN '女'
ELSE '其他'
END AS '性别'
FROM people;
在这个案例中,由于在第一个示例中使用了错误的语法,所以会导致结果不符合预期。正确的写法是在when后面直接写表达式,而不是case列的值。
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 |
+------------+-----------+-----------+
```