Uploaded Question 1.SQL

Author: AlexanderConnelly

SQL Query/Question 1.sql

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+/*Data Integrity Checking & Cleanup
+
+- Alphabetically list all of the country codes in the continent_map table that appear more than once. Display any values where country_code is null as country_code = "FOO" and make this row appear first in the list, even though it should alphabetically sort to the middle. Provide the results of this query as your answer.
+
+- For all countries that have multiple rows in the continent_map table, delete all multiple records leaving only the 1 record per country. The record that you keep should be the first one when sorted by the continent_code alphabetically ascending. Provide the query/ies and explanation of step(s) that you follow to delete these records.
+*/
+CREATE DATABASE BrainTree
+
+CREATE TABLE `braintree`.`continent_map` (`country_code` text, `continent_code` text);
+PREPARE stmt FROM 'INSERT INTO `braintree`.`continent_map` (`continent_code`,`country_code`) VALUES(?,?)'
+DEALLOCATE PREPARE stmt
+
+CREATE TABLE `braintree`.`continents` (`continent_code` text, `continent_name` text)
+PREPARE stmt FROM 'INSERT INTO `braintree`.`continents` (`continent_name`,`continent_code`) VALUES(?,?)'
+DEALLOCATE PREPARE stmt
+
+CREATE TABLE `braintree`.`countries` (`country_code` text, `country_name` text)
+PREPARE stmt FROM 'INSERT INTO `braintree`.`countries` (`country_name`,`country_code`) VALUES(?,?)'
+DEALLOCATE PREPARE stmt
+
+CREATE TABLE `braintree`.`per_capita` (`country_code` text, `year` int, `gdp_per_capita` double)
+PREPARE stmt FROM 'INSERT INTO `braintree`.`per_capita` (`gdp_per_capita`,`country_code`,`year`) VALUES(?,?,?)'
+DEALLOCATE PREPARE stmt
+
+/* Replace '' empty strings with NULL*/
+USE braintree;
+UPDATE continent_map
+    
+SET
+    country_code = CASE country_code WHEN '' THEN NULL ELSE country_code END,
+    continent_code = CASE continent_code WHEN '' THEN NULL ELSE continent_code END;
+
+/* Select Statement To Pull Up Duplicate Country Codes, FOO on top*/
+SELECT 
+    COALESCE(country_code, 'FOO')
+FROM
+    braintree.continent_map
+GROUP BY country_code
+HAVING COUNT(*) > 1
+ORDER BY country_code;
+
+/*Results from first part of question selecting duplicates and brining NULL up top.
+
+country_code:
+FOO
+ARM
+AZE
+CYP
+GEO
+KAZ
+RUS
+TUR
+UMI
+
+Part 2:
+
+ Create a temporary table with a new column ID as a row_number on the table after order by contry_code, continent_code*/
+ CREATE TABLE t1 (
+ SELECT row_number() over (order by country_code, continent_code asc) as 'ID',country_code
+      ,continent_code
+  FROM braintree.continent_map );
+CREATE TABLE t2 (Select MIN(ID) as ID from t1 group by country_code );
+ 
+/*Delete the rows that dont have a min ID number after group by country_code*/
+Delete From t1 where ID NOT IN(select ID from t2) ;
+
+/*Reset continent_map table*/
+Delete From continent_map;
+
+/*Refill continent_map from temp_table*/
+insert into continent_map
+  select country_code, continent_code from t1;
+ 
+ /*drop temporary tables*/
+ DROP TABLE t1;
+ DROP TABLE t2;
+ 
+ 
+/*Results in Table continent_map:
+
+country_code	continent_code
+NULL	AS
+ABW	NA
+AFG	AS
+AGO	AF
+AIA	NA
+ALA	EU
+ALB	EU
+AND	EU
+ANT	NA
+ARE	AS
+ARG	SA
+ARM	AF
+ASM	OC
+....... etc */