Pull Request For Draft Under Review (#365)

* Create direct-sub-query.sql

* Create using-concat.sql

* Create using-two-columns-mysql.sql

* Create count-distinct-national-id.sql

* Create count-distinct-position.sql

Author: kdqed

sql-queries-11/count-distinct-multiple-columns/count-distinct-national-id.sql

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+SELECT COUNT(DISTINCT national_id)
+FROM Faculty;

sql-queries-11/count-distinct-multiple-columns/count-distinct-position.sql

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+SELECT COUNT(DISTINCT position)
+FROM Faculty;

sql-queries-11/count-distinct-multiple-columns/direct-sub-query.sql

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+SELECT COUNT(*) 
+FROM (
+  SELECT DISTINCT national_id, position 
+  FROM Faculty 
+) AS subQuery;

sql-queries-11/count-distinct-multiple-columns/using-concat.sql

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+SELECT COUNT(DISTINCT CONCAT(national_id, position))
+FROM Faculty;

sql-queries-11/count-distinct-multiple-columns/using-two-columns-mysql.sql

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+SELECT COUNT(DISTINCT national_id, position)
+FROM Faculty;