Add: Biggest Single Number

problems/big-countries/big_countries.sql

@@ -1,11 +1,11 @@
 # Write your MySQL query statement below
 
 SELECT
-	`name`,
-	`population`,
-	`area`
+  `name`,
+  `population`,
+  `area`
 FROM
-	`World`
+  `World`
 WHERE
-	`area` > 3000000
-	OR `population` > 25000000;
+  `area` > 3000000
+  OR `population` > 25000000;

problems/biggest-single-number/README.md

@@ -1,2 +1,25 @@
 ## 619. Biggest Single Number
 
+<p>
+Table <code>number</code> contains many numbers in column <b>num</b> including duplicated ones.<br>
+Can you write a SQL query to find the biggest number, which only appears once.<br>
+</p>
+<pre>+---+
+|num|
++---+
+| 8 |
+| 8 |
+| 3 |
+| 3 |
+| 1 |
+| 4 |
+| 5 |
+| 6 | 
+</pre>
+For the sample data above, your query should return the following result:
+<pre>+---+
+|num|
++---+
+| 6 |
+</pre>
+<b>Note:</b><br> If there is no such number, just output <b>null</b>.<p></p>

problems/biggest-single-number/biggest_single_number.sql

@@ -0,0 +1,17 @@
+# Write your MySQL query statement below
+
+# Method 1
+SELECT num
+FROM
+  `number`
+GROUP BY num
+HAVING COUNT(num) = 1;
+
+# Method 2
+SELECT MAX(num) AS num
+FROM
+  (SELECT num
+   FROM
+     number
+   GROUP BY num
+   HAVING COUNT(num) = 1) AS t;

problems/duplicate-emails/duplicate_emails.sql

@@ -1,10 +1,10 @@
 # Write your MySQL query statement below
 
 SELECT
-	Email
+  Email
 FROM
-	Person
+  Person
 GROUP BY
-	Email
+  Email
 HAVING
-	count( 1 ) > 1;
+  count(1) > 1;

problems/swap-salary/swap_salary.sql

@@ -1,8 +1,10 @@
 # Write your MySQL query statement below
 
+# Method 1
 UPDATE salary
 SET sex = IF(sex = 'm', 'f', 'm');
 
+# Method 2
 UPDATE salary
 SET sex = CASE sex
           WHEN 'm'