Add 57 new questions and exercises

exercises/sql/improve_query.md

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+## Comparisons vs. Functions
+
+1. Improve the following query
+
+```
+SELECT count(*)
+FROM shawarma_purchases
+WHERE
+  YEAR(purchased_at) == '2017'
+```

exercises/sql/solutions/improve_query.md

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+## Comparisons vs. Functions - Solution
+
+```
+SELECT count(*)
+FROM shawarma_purchases
+WHERE
+  purchased_at >= '2017-01-01' AND
+  purchased_at <= '2017-31-12'
+```