What Does the '=' Operator Modified by '*' Mean in a SQL Join Condition?

Comprehending the Enigmatic "=" Operator

In the realm of SQL programming, the use of the "=" operator is ubiquitous. However, when encountering the expression "=*" within a join statement, seasoned SQL developers may pause for a moment, puzzled by its unfamiliar syntax. This article aims to demystify this enigmatic operator, shedding light on its significance and usage.

Now, let's delve into the query that has sparked this curiosity:

WHERE table1.yr =* table2.yr -1

This snippet represents a join condition in Microsoft SQL Server, where the equality operator "=" is adjoined by an asterisk. This syntax dates back to an era predating SQL Server 2005 and finds no place in the ANSI JOIN specification.

To unravel its meaning, it's crucial to understand its historical context. This syntax facilitates an outer join, a type of join that retains all rows from the specified tables, regardless of whether they possess matching values in the join condition.

In the given example, the "*" indicates that the join should select all rows from both table1 and table2. The expression "table2.yr -1" subtracts 1 from the value of the "yr" column in table2 before performing the comparison.

In essence, the query retrieves all rows from table1 that match the values in table2's "yr" column shifted one year back. It's a specific and useful syntax that arguably has its place in certain scenarios.

While this syntax has fallen into disuse in favor of more standardized approaches, it serves as a testament to the evolutionary nature of SQL, reflecting the continual refinement and adaptation of programming languages to meet the evolving needs of developers.

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