How to Correctly Join Tables in SQL Server Using a CASE Statement?

Tips for correctly connecting tables using CASE statements in SQL Server: syntax error correction

In SQL Server, joining tables based on CASE conditions can be challenging. This article explores a query that attempts to join the sys.partitions and sys.allocation_units tables using a CASE statement, and how to resolve its syntax error.

The syntax error arises from the use of "=" in the CASE condition. The CASE expression returns a value from the THEN clause, not a Boolean result.

Corrected grammar

The following revised syntax resolves the syntax error:

<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>

Explanation

The corrected syntax uses the following logic:

• For each CASE condition, a Boolean expression is evaluated and returns 1 (True) if the condition is met, 0 (False) otherwise.
• Then, use the equal sign (=) to compare the result of the CASE expression to 1. Returns 1 if the selected condition is True; otherwise returns 0.
• Only rows whose CASE expression evaluates to True will be included in the join result.

This revised response maintains the original language, image format, and image position while rewording the text for a slightly different presentation. The core meaning and technical details remain unchanged.

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