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CTE vs Temp Tables vs Subquery - SQL.pdf
This document provides the CTE for SQL
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CTE vs Temp Tables vs Subquery - SQL.pdf
- 1.
- 2. Complex queries means multiplesteps. Subqueries, temporary tables, and common table expressions (CTEs) can help. But what are they? WHY YOU SHOULD KNOW THIS...
- 3. WHAT IT IS Aquery inside a query In the FROM, WHERE, or HAVING clause
- 4. Subquery runs Results ofthe subquery replace the subquery Outer query executes HOW IT WORKS
- 5. Subquery in parenthesis Subquery needsan alias AN EXAMPLE
- 6. PROs Simplest option forfiltering or performing calculations on data within a query Often have good performance CONs Can make complex queries harder to read and understand Not reusable in the same query PROS & CONS
- 7. Stores a temporaryresult set that you can reuse Exists within a single MySQL session Can only be used by the person who created it WHAT IT IS
- 8. CREATE TEMPORARY TABLE statement Writequery with results you want stored Use the temp table like you would any table HOW IT WORKS
- 9. Temp table needsa ; Create temp table first AN EXAMPLE Use the temp table in following queries
- 10. ANOTHER EXAMPLE Same syntaxfor each temp table Use the temp table in following queries Create temp table first
- 11. PROs Act like regulartables for storing intermediate results Can be accessed by multiple queries within the same session Useful for complex data manipulation with multiple steps CONs Disappear after the session ends Data is physically stored PROS & CONS
- 12. A named temporary resultset Used in SELECT, INSERT, UPDATE, or DELETE Exists for the duration of a single query WHAT IT IS
- 13. Open with WITH( ) clause Write query whose result set you want to use Select data from the CTE in a following query HOW IT WORKS
- 14. AN EXAMPLE Use theCTE in following queries Create CTE using WITH clause No ; after CTE, closed with parenthesis
- 15. ANOTHER EXAMPLE Create first CTE using WITHclause Following CTEs are separated by a comma
- 16. PROs Improve readability bybreaking complex queries into named result sets Can be reused multiple times within a single query Support recursive queries CONs Limited to a single query (not accessible by other queries in the session) PROS & CONS
- 17. Subqueries, temp tables, andCTEs all have similar functionality You can often use whichever one you prefer Each has pros and cons WHAT IT ALL MEANS