SQL Section: Only Names, that are exists on one entry-CodePudding

I have an DB with two columns names and day. I want to find all names that exists only once at day x. Really simple, but my brain doesn't work .

I thought a simple self join would do this but..

SELECT d1.id 
FROM day AS d1 
JOIN day AS d2 ON d1.id = d2.id 
AND d1.day=3 AND d1.day != d2.day

Example:
| name     | day          |
| -------- | ------------ |
| First    | 1            |
| First    | 2            |
| First    | 3            |
| Second   | 3            |

and want only return Second

CodePudding user response：

Does the following get your expected results? Select the desired rows and check rows for other days do not exist

Select d.name
from Day d
where d.day=3
and not exists (
    select * from Day d2
    where d2.name = d.name and d2.day != d.day
)