SQL 检查是否存在条件

Question

I have the following scenario:

I want to select a mission from a table(missions) for a user (userID=?) , or I will select all missions if user is not assigned to any mission.

This can be achieved in 2 queries:

   Select * from missions where userID = :ID

If (result is empty)

   Select * from missions 

I want these 2 SQL in one SQL if possible any Idea

I other words. User can eather select messions he assigned to by admin or he can work with all messions.

Answer 1

You may try:

SELECT *
FROM missions
WHERE
    userID = :ID OR
    NOT EXISTS (SELECT 1 FROM missions WHERE userID = :ID);

Should one or more records exist with userID =:ID , the first condition would be true for those records, and the exists condition would also be false, and therefore ignored. Should no records exist with userID =:ID , then the first condition would always be false, and therefore ignored, while the second condition would always be true, thereby returning the entire table.