MySQL 8.0

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Create table If Not Exists Employee (id int, salary int);
insert into Employee (id, salary) values (1, 100), (2,100);

Records: 2  Duplicates: 0  Warnings: 0


WITH sub AS (
    SELECT id,
           salary,
           DENSE_RANK() OVER (ORDER BY salary DESC) AS num
    FROM Employee 
)
SELECT id, salary
FROM sub
WHERE num = 2
UNION ALL
SELECT NULL, NULL
WHERE 0 = ( SELECT COUNT(*)
            FROM sub 
            WHERE num = 2 );


id
salary


null
null


insert into Employee (id, salary) values (3, 200);
WITH sub AS (
    SELECT id,
           salary,
           DENSE_RANK() OVER (ORDER BY salary DESC) AS num
    FROM Employee 
)
SELECT id, salary
FROM sub
WHERE num = 2
UNION ALL
SELECT NULL, NULL
WHERE 0 = ( SELECT COUNT(*)
            FROM sub 
            WHERE num = 2 );


id
salary


1
100

2
100