619. Biggest Single Number

Table: MyNumbers

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:

Output:

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:

Output:

Explanation: There are no single numbers in the input table so we return null.

SELECT Max(num) AS num
FROM (
    SELECT num 
    FROM MyNumbers
    HAVING COUNT(num) = 1
     )
     AS SingleNumber

學習點：
1.使用FROM子查詢的表，一定要AS成新的表格不然會語法錯誤
2.HAVING用在聚合函數上