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

SQL Server JOINs and CASE Statements: A Common Pitfall

In SQL Server 2008 R2 (and later versions), attempting to use a CASE statement directly within a JOIN condition to define the join criteria can lead to syntax errors. Let's examine why and how to solve this.

Consider the relationship between sys.partitions and sys.allocation_units, where the join depends on the sys.allocation_units.type value. A naive approach might look like this:

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

This results in the error "Incorrect syntax near '='". The problem is that the CASE expression itself doesn't produce a Boolean (TRUE/FALSE) result suitable for a join condition. Instead, it returns either a.container_id = p.hobt_id or a.container_id = p.partition_id, neither of which is a valid join condition on its own.

The Correct Approach: Boolean Evaluation

To fix this, we need the CASE expression to yield a Boolean value. We can achieve this by structuring the CASE statement to return 1 for true and 0 for false, and then comparing the result to 1:

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

Now, the CASE statement evaluates to 1 only when the appropriate conditions (based on a.type) and the equality check (a.container_id = p.hobt_id or a.container_id = p.partition_id) are both true. The = 1 comparison then provides the necessary Boolean result for the JOIN condition. This revised query correctly joins the tables. This method ensures that the JOIN condition evaluates to a true or false value, enabling the join to function as intended.

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