Скачать или смотреть Converting Oracle SQL to SQL Server: Understanding JOINs and Queries

Learn how to convert Oracle SQL queries to SQL Server syntax with ease. Understand the importance of using JOIN statements for clear SQL queries.
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This video is based on the question https://stackoverflow.com/q/62453116/ asked by the user 'Koosh' ( https://stackoverflow.com/u/8834017/ ) and on the answer https://stackoverflow.com/a/62453380/ provided by the user 'Gordon Linoff' ( https://stackoverflow.com/u/1144035/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Converting Oracle SQL to SQL Server: A Clear Path to Mastering JOINs

Migrating from one SQL platform to another can be a daunting task, especially when you want to ensure your SQL queries perform the same function across both systems. If you're looking to convert an Oracle SQL query into a SQL Server query, this guide will guide you through the process, focusing on the use of JOIN statements.

The Problem: Understanding SQL Syntax Differences

You have an Oracle SQL query that looks like this:

[[See Video to Reveal this Text or Code Snippet]]

Your confusion revolves around the use of the FROM clause and whether the commas imply INNER JOIN relationships automatically. Additionally, you wonder if it's acceptable to use the WHERE clause to dictate the join conditions.

Analysing the Original Query Structure

In the original query:

The tables (t1, t2, and t3) are separated by commas in the FROM clause.

Conditions joining the tables are specified in the WHERE clause.

Understanding Comma-Separated Queries

Implicit Joins: The use of commas implies an INNER JOIN. However, this method is considered less clear and can lead to confusion.

WHERE Clause Usage: The WHERE clause in Oracle effectively serves to filter down the results based on conditions, including those that define how tables relate to one another.

The Equivalent SQL Server Query

Your proposed SQL Server query reads:

[[See Video to Reveal this Text or Code Snippet]]

This formulation is clearer for several reasons:

Benefits of Using JOINs

Readability: Using JOIN makes it explicitly clear how tables are related, reducing the cognitive load when reading the query.

Maintenance: In case of changes in table structures or relationships, it’s easier to update specific JOIN conditions without sifting through a WHERE clause filled with complex logic.

Best Practice: The JOIN syntax has been the preferred method for over two decades, leading to better understanding and fewer errors.

Insights on Functionality

Both your original and the rewritten SQL queries should work similarly, returning the same results if set correctly. They are considered functionally equivalent and should, under most circumstances, generate identical execution plans in both Oracle and SQL Server.

Conclusion

In summary, when converting Oracle SQL queries to SQL Server, shifting from a comma-based FROM clause to a structured use of JOIN statements is highly recommended. Not only does it enhance readability and maintainability, but it also aligns with best practices in SQL conventions.

Test your rewritten queries in Oracle to confirm they return the same dataset, ensuring your logic is sound as you transition between databases. Happy querying!