[MySQL] case问题

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本文介绍两个实用的SQL技巧：一是通过单条更新语句批量转换表中性别字段；二是通过SQL查询判断给定边长是否能构成三角形。这两种方法能够帮助开发者高效地解决实际问题。

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Swap Salary

Given a table salary, such as the one below, that has m=male and f=female values. Swap all f and m values (i.e., change all f values to m and vice versa) with a single update query and no intermediate temp table.

For example:

| id | name | sex | salary |
|----|------|-----|--------|
| 1  | A    | m   | 2500   |
| 2  | B    | f   | 1500   |
| 3  | C    | m   | 5500   |
| 4  | D    | f   | 500    |

After running your query, the above salary table should have the following rows:

| id | name | sex | salary |
|----|------|-----|--------|
| 1  | A    | f   | 2500   |
| 2  | B    | m   | 1500   |
| 3  | C    | f   | 5500   |
| 4  | D    | m   | 500    |

update salary 
set sex=case
when sex='m' then 'f'
else 'm'
end

Triangle Judgement

A pupil Tim gets homework to identify whether three line segments could possibly form a triangle.

However, this assignment is very heavy because there are hundreds of records to calculate.

Could you help Tim by writing a query to judge whether these three sides can form a triangle, assuming table triangle holds the length of the three sides x, y and z.

| x  | y  | z  |
|----|----|----|
| 13 | 15 | 30 |
| 10 | 20 | 15 |

For the sample data above, your query should return the follow result:

| x  | y  | z  | triangle |
|----|----|----|----------|
| 13 | 15 | 30 | No       |
| 10 | 20 | 15 | Yes      |

select x,y,z,                   
case 
  when x+y>z and x+z>y and y+z>x then 'Yes' else 'No'
end as triangle
from triangle

方法二

select x,y,z,
if (x+y>z and x+z>y and y+z>x,'Yes', 'No') as triangle
from triangle