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database/notes/09-subqueries-views.md
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@ -13,6 +13,7 @@
- [Index Range scan](#index-range-scan) - [Index Range scan](#index-range-scan)
- [Index seek](#index-seek) - [Index seek](#index-seek)
- [Some guidelines for optimizing MySQL queries](#some-guidelines-for-optimizing-mysql-queries) - [Some guidelines for optimizing MySQL queries](#some-guidelines-for-optimizing-mysql-queries)
- [Practice questions](#practice-questions)
## Sub-queries ## Sub-queries
@ -223,4 +224,12 @@ This happens when you specify a condition in WHERE clause like searching an empl
* Avoid unnecessary columns in SELECT clause * Avoid unnecessary columns in SELECT clause
Instead of using ‘SELECT *’, always specify columns in the SELECT clause to improve MySQL performance. Because unnecessary columns cause additional load on the database, slowing down its performance as well whole systematic process. Instead of using ‘SELECT *’, always specify columns in the SELECT clause to improve MySQL performance. Because unnecessary columns cause additional load on the database, slowing down its performance as well whole systematic process.
* Pagination * Pagination
* Avoid SELECT DISTINCT * Avoid SELECT DISTINCT
## Practice questions
* [Sub-queries - I](https://leetcode.com/problems/customers-who-never-order/)
* [Sub-queries - II](https://leetcode.com/problems/sales-person/)
* [Sub-queries - III](https://leetcode.com/problems/consecutive-available-seats/)
* [Sub-queries - IV](https://leetcode.com/problems/sales-analysis-ii/)
* [Sub-queries - V](https://leetcode.com/problems/students-with-invalid-departments/)
* [Sub-queries - VI](https://leetcode.com/problems/customer-who-visited-but-did-not-make-any-transactions/)