What Does '*=' Mean in Microsoft SQL Server Joins?

Outer Join Syntax in Microsoft SQL Server: Demystifying "*="

In the vast landscape of SQL queries, we may encounter unfamiliar conventions that can puzzle us. One such convention is the mysterious "*=" operator that appears in joins. Let's delve into what this symbol truly signifies.

Question:

Navigating the intricacies of a Microsoft SQL Server query, an unfamiliar join syntax caught our attention. Specifically, we encountered "=" being used in a join expression. What elucidations can be provided to help unravel the enigmatic meaning behind "="?

Answer:

Steeped in the annals of TSQL history, this syntax is a vestige of the pre-SQL Server 2005 era. It represents outer join syntax, a departure from the ANSI JOIN standards that have become prevalent in contemporary database management systems.

Key Points:

• The "*" operator in this context indicates that the join is of the outer variety.
• Outer joins retrieve rows from the participating tables based on equality in the specified columns, even if one or both of the rows have NULL values in those columns.
• The syntax "

  WHERE t.column =* s.column

  " establishes a relationship between the columns named "column" in the tables "t" and "s," allowing rows from both tables to be retrieved based on their matching values.
• It is prudent to note that this syntax is no longer the preferred approach for performing outer joins in modern SQL Server implementations.

Historical Note:

The "" syntax serves as a relic from an earlier era of TSQL, preceding the introduction of ANSI JOINs in SQL Server 2005. With the advent of the revised ANSI syntax, the "" convention has largely faded into obsolescence.

Conclusion:

The "*=" operator unveils a glimpse into TSQL's rich history, showcasing syntax that has been supplanted by more modern standards. Comprehending its meaning empowers us to navigate legacy systems with ease and appreciate the evolution of database technology through time.

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