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Understanding the *= Operator in Sybase SQL

If you maintain or work with legacy code on Sybase SQL, you may have come across the *= operator in queries. This operator can be a source of confusion for many, especially because it isn’t widely documented or commonly used in modern SQL practices. In this blog post, we’ll clarify what the *= operator is, its purpose, and how it differs from the more familiar equality operator (=) in SQL.

What is the *= Operator?

The *= operator is used in Sybase SQL to perform outer joins. Specifically, it indicates a left outer join, where one table (known as the outer table) returns all its rows, while the corresponding rows from another table (the inner table) are matched where possible.

Here’s a simple example of how it is used:

SELECT * FROM a, b WHERE a.id *= b.id

How Does This Differ from =?

You might wonder how using *= changes the query’s behavior compared to using the equality operator. Here are the key differences:

  • *= Operator:

    • Indicates a left outer join.
    • Will return all rows from the left table (a), regardless of whether there’s a matching row in the right table (b).
    • If no match is found, results from table b will display as NULL.

  • = Operator:

    • Indicates an inner join.
    • Only returns rows where there is a matching value in both tables (a and b).
    • If there’s no match, those rows will not be included in the result set.

Examples

Consider the following scenarios with two tables A and B:

1. Using the *= Operator:

SELECT * FROM A, B WHERE A.id *= B.id
  • This query will return all records from A, and it will match records from B where the IDs are equal.

2. Using the = Operator:

SELECT * FROM A, B WHERE A.id = B.id
  • In this case, only records from A that have matching records in B will be included.

Conclusion

The *= operator is a relic from earlier SQL syntax and is specific to Sybase, used primarily to perform left outer joins. Understanding this operator is crucial, especially when dealing with legacy codebases that may include it.

By knowing how *= functions in comparison to regular equality operators, you can better navigate and maintain existing code while ensuring that your SQL queries return the desired results. Remember, while it’s important to understand legacy operators, consider transitioning to more standard SQL practices whenever possible to ensure broader compatibility and understanding.

If you have more queries regarding SQL operators or need further clarification, feel free to reach out!