BiggestSingleNumber.sql

-- Table: MyNumbers

-- +-------------+------+
-- | Column Name | Type |
-- +-------------+------+
-- | num         | int  |
-- +-------------+------+
-- This table may contain duplicates (In other words, there is no primary key for this table in SQL).
-- Each row of this table contains an integer.
 
-- A single number is a number that appeared only once in the MyNumbers table.

-- Find the largest single number. If there is no single number, report null.

-- The result format is in the following example.

-- Example 1:

-- Input: 
-- MyNumbers table:
-- +-----+
-- | num |
-- +-----+
-- | 8   |
-- | 8   |
-- | 3   |
-- | 3   |
-- | 1   |
-- | 4   |
-- | 5   |
-- | 6   |
-- +-----+
-- Output: 
-- +-----+
-- | num |
-- +-----+
-- | 6   |
-- +-----+
-- Explanation: The single numbers are 1, 4, 5, and 6.
-- Since 6 is the largest single number, we return it.
-- Example 2:

-- Input: 
-- MyNumbers table:
-- +-----+
-- | num |
-- +-----+
-- | 8   |
-- | 8   |
-- | 7   |
-- | 7   |
-- | 3   |
-- | 3   |
-- | 3   |
-- +-----+
-- Output: 
-- +------+
-- | num  |
-- +------+
-- | null |
-- +------+
-- Explanation: There are no single numbers in the input table so we return null.

-- Write your PostgreSQL query statement below

select max(num) as num from 
(
    select num,
        count(num) as num_times 
    from MyNumbers group by num
)
where num_times = 1