feat: add sql solution to lc problem: No.0619 (doocs#747)

No.0619. Biggest Single Number

Author: Dhoni77

solution/0600-0699/0619.Biggest Single Number/README.md

@@ -96,7 +96,13 @@ MyNumbers table:
 ### **SQL**
 
 ```sql
-
+SELECT MAX(a.num) AS num
+FROM (
+	SELECT num
+	FROM MyNumbers
+	GROUP BY num
+	HAVING count(*) = 1
+) a;
 ```
 
 <!-- tabs:end -->

solution/0600-0699/0619.Biggest Single Number/README_EN.md

@@ -84,7 +84,13 @@ MyNumbers table:
 ### **SQL**
 
 ```sql
-
+SELECT MAX(a.num) AS num
+FROM (
+	SELECT num
+	FROM MyNumbers
+	GROUP BY num
+	HAVING count(*) = 1
+) a;
 ```
 
 <!-- tabs:end -->

solution/0600-0699/0619.Biggest Single Number/Solution.sql

@@ -0,0 +1,7 @@
+SELECT MAX(a.num) AS num
+FROM (
+	SELECT num
+	FROM MyNumbers
+	GROUP BY num
+	HAVING count(*) = 1
+) a;