Merge pull request #376 from daKeshra/resolve-conflict

Understanding sql sargability

Author: maibin

sql-queries-10/understanding-sql-sargability/data-type-mismatch.sql

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+-- Non-Sargable (if id is INT) 
+EXPLAIN
+SELECT * FROM customers 
+WHERE id = '1001';
+
+-- Sargable 
+EXPLAIN
+SELECT * FROM customers 
+WHERE id = 1001;

sql-queries-10/understanding-sql-sargability/function-on-a-column.sql

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+-- Non-Sargable 
+EXPLAIN 
+SELECT * FROM Student
+WHERE YEAR(birth_date) = 2001;
+
+-- Sargable 
+EXPLAIN 
+SELECT * FROM Student
+WHERE birth_date >= '2001-01-01' AND birth_date < '2002-01-01';

sql-queries-10/understanding-sql-sargability/leading-wildcard-in-like.sql

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+-- Non-sargable
+EXPLAIN 
+SELECT * FROM Student 
+WHERE name LIKE '%Po';
+
+-- Sargable
+EXPLAIN 
+SELECT * FROM Student 
+WHERE name LIKE 'Po%';

sql-queries-10/understanding-sql-sargability/main

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+-- non-sargable 
+EXPLAIN 
+SELECT * FROM Student
+WHERE national_id = 123345566 OR name = 'John Liu';
+
+-- Sargable
+EXPLAIN 
+SELECT * FROM Student WHERE national_id = 123345566 
+UNION 
+SELECT * FROM Student WHERE name='John Liu';

sql-queries-10/understanding-sql-sargability/or-conditions-across-different-columns.sql

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+-- non-sargable JOINs 
+EXPLAIN 
+SELECT c.name, e.semester, e.grade
+FROM Course c
+JOIN Exam e
+        ON LOWER(c.id) = LOWER(e.course_id)
+WHERE c.credits > 4;
+
+
+-- Sargable 
+EXPLAIN 
+SELECT c.name, e.semester, e.grade 
+FROM Course c 
+JOIN Exam e 
+         ON c.id = e.course_id WHERE c.credits > 4;