I understand the execution order of a SQL query

Generally, in the work environment, I usually give tips and explain some concepts to junior developers, so that they can, little by little, absorb notions about performance, good practices and security aspects . One of the points I like to highlight is the importance of the order of execution of a SQL query. I usually approach this subject when I see that they have overcome their initial difficulties with SQL and are starting to try to create more robust and complex queries.

Execution Order

1. FROM: Defines the main table of the data query.
2. JOIN: Establishes the join and relationship between tables.
3. ON: Defines the condition for joining tables, highlighting the relationship keys between them.
4. WHERE: Filters the queried data.
5. GROUP BY: Groups data that shares a specific value.
6. HAVING: Filters the data after grouping done by GROUP BY.
7. SELECT: Selects the columns of a table.
8. DISTINCT: Removes rows with duplicate data.
9. ORDER BY: Orders the set of queried data.
10. LIMIT: Restricts the number of rows returned in the final result.

There are several reasons to highlight the importance of the order of execution, and below I highlight some of them:

1. Query Optimization: An interesting example is the use of filters. Knowing that the JOIN and ON clauses are processed before the WHERE, it is possible to use them to restrict data already in the join stage, reducing the need for additional filters in the WHERE clause.

2. Result Predictability: Understanding the order of execution allows you to accurately predict the result of the query, which helps ensure that the data retrieved is exactly what you want, without surprises or inconsistencies.

3. Structuring Complex Queries: In more complex queries, understanding the order of execution helps organize the query structure, ensuring that each step is processed at the appropriate time.

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