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Behind the Scenes: SQL Query Execution Order

What is SQL Order of Execution mean?

The order of Execution in SQL is a step- by- step process that a database follows to perform a Query which involves different clauses and operations within a Query i.e., identifying data, filtering, combining data, calculation, sorting, limiting.


Is it Important to understand the sequence of execution within a SQL Query?

YES, it's a fundamental skill and essential for anyone working with databases and SQL queries to know the order of execution for the following reason:


a) Query optimization - Knowing the sequence of query will help you to design it to minimize the amount of work the database needs to perform, which in turn improves the query speed.


b) Debugging - Knowing the sequence helps you to identify accurately where the error occurred in your query and rectify it.


c) Efficient Data retrieval - It plays a vital role in fetching the necessary data required when dealing with large datasets using clause like filtering and limiting data early in the query process and retrieve data efficiently.


d) Accurate Results - Knowing the sequence of operation helps to ensure you get the desired result.


e) Optimizing Resources -Thoroughly understanding the sequence of execution process helps to optimize the memory usage and minimize the network traffic placing the clause at the right spot in an efficient manner.



Breakdown of sequence of operation in orderly wise:




Example: SQL Query Statement with Data Output




The above query gets executed in the following order:

  1. Retrieve Data from the film Table.

  2. Combines data from 2 tables - film & film_category.

  3. Filter the rows based on the condition replacement cost > 9.99 in where clause.

  4. Group the filtered rows based on the specified column (title & replacement_cost) and perform the calculation Sum (rental_rate).

  5. Then the above grouping is filtered based on the specified condition: sum (rental_rate) > 2.99 in having clause.

  6. The select determines the title,replacement_cost & total as result set.

  7. Sort the result set based on replacement_cost in descending order.

  8. Finally, the first 5 rows from the above sorted data set are skipped out based on the comment "offset 5 rows" and next 10 rows are displayed based on the "LIMIT 10" comment.


Highlights / Points to be noted:

  1. WHERE clause is used to filter the individual rows based on the specified condition using various logical & comparison operators like AND, OR, NOT,>, < etc. you can also combine multiple conditions using logical operators.

  2. GROUP clause is often used with aggregate functions like Sum,Count,Average etc.

  3. HAVING clause perform the filtering operation based on aggregate functions and groupings on specified condition, so it is generally used after GROUP BY clause. whereas the ORDER BY clause is placed after HAVING clause (if any).

  4. SELECT clause is Versatile tool which allows you to retrieve specific columns, apply calculations, use aliases for clarity and aggregate data to obtain meaningful results.

  5. Its highly advisable to use Limit/offset & fetch clauses in conjunction with the ORDER BY clause else it may lead to inconsistent results.

Conclusion:

Understanding the order of execution is essential for writing SQL queries efficiently in order to retrieve accurate and optimized results as well as it helps you in debugging your scripts.




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