Can CASE Statements Be Used Directly in JOIN Conditions in SQL Server?

SQL Server JOIN Conditions and CASE Statements: A Closer Look

When working with Microsoft SQL Server 2008 R2's system views, like sys.partitions and sys.allocation_units, the relationship between them often depends on the sys.allocation_units.type value. A common approach to joining these tables might involve a CASE statement within the JOIN condition.

Let's examine this scenario: Connecting sys.indexes, sys.partitions, and sys.allocation_units requires a conditional join based on the allocation_units.type. An initial attempt might look like this:

<code class="language-sql">SELECT *
FROM sys.indexes i
JOIN sys.partitions p ON i.index_id = p.index_id
JOIN sys.allocation_units a ON CASE
    WHEN a.type IN (1, 3) THEN a.container_id = p.hobt_id
    WHEN a.type IN (2) THEN a.container_id = p.partition_id
END</code>

This, however, results in a syntax error. The issue stems from the fact that CASE statements in SQL Server produce scalar values, not executable statements. They can't directly control the equality comparison within the JOIN clause.

The Solution: Boolean Evaluation with CASE

To resolve this, we need to restructure the CASE statement to return a Boolean value (1 for true, 0 for false) that can then be compared:

<code class="language-sql">SELECT *
FROM sys.indexes i
JOIN sys.partitions p ON i.index_id = p.index_id
JOIN sys.allocation_units a ON 
    CASE
        WHEN a.type IN (1, 3) AND a.container_id = p.hobt_id THEN 1
        WHEN a.type IN (2) AND a.container_id = p.partition_id THEN 1
        ELSE 0
    END = 1</code>

By using AND within the WHEN clauses and comparing the resulting CASE expression to 1, we effectively create a conditional join. This demonstrates that while CASE statements are powerful, their use within JOIN conditions requires careful consideration of their return type and the need for explicit Boolean evaluation. Direct assignment or equality testing within the CASE statement itself within a JOIN is not supported.

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