SA0180 : CASE expression has too many WHEN clauses

Excessive use of WHEN clauses in a CASE expression can complicate query management, especially in complex queries or when working with linked servers.

Description

Using too many WHEN clauses in a CASE expression can lead to several problems in SQL Server, especially when dealing across linked servers. This issue often arises in complex queries where conditions become hard to manage and understand.

For example:

SQL
Copy
1-- Example of problematic query
2SELECT 
3    CASE 
4        WHEN condition1 THEN result1
5        WHEN condition2 THEN result2
6        ... -- Many more WHEN clauses
7        WHEN condition10 THEN result10
8    END
9FROM TableName;

This approach can increase the complexity and reduce the readability of your code. In addition, if such an expression is executed over a linked server, SQL Server may transform it into nested CASE statements, potentially exceeding the allowed nesting level.

  • Complexity and readability: A CASE expression with many WHEN clauses can become difficult to debug and maintain.

  • Runtime Errors: SQL Server limits CASE expression nesting to 10 levels. Queries transformed into nested expressions on linked servers may produce runtime errors.

How to fix

Refactor the CASE expression to improve readability and maintainability, avoiding excessive WHEN clauses.

Follow these steps to address the issue:

  1. Review the complexity of the CASE expression in your query, identifying sections with multiple WHEN clauses.

  2. Where applicable, simplify the CASE expression by breaking it into separate expressions or using alternative logic structures, such as joining additional tables to handle specific conditions.

  3. Consider using a user-defined function or a computed column to encapsulate complex logic outside of the main query flow, thereby reducing complexity within the SELECT statement.

For example:

SQL
Copy
1-- Simplified query with reduced CASE complexity
2SELECT 
3    -- Use a user-defined function to encapsulate complex conditions
4    MyFunction(column1, column2) AS ComputedValue
5FROM 
6    TableName;

Scope

The rule has a Batch scope and is applied only on the SQL script.

Parameters
Name Description Default Value
MaxNumberOfWhenClauses

Maximum number of WHEN clauses.

10
Remarks

The rule does not need Analysis Context or SQL Connection.

Effort To Fix
20 minutes per issue.
Categories

Design Rules, Code Smells

Additional Information

There is no additional info for this rule.

Example Test SQL
SQL
Copy
 1CREATE PROCEDURE tsp_Test_SA0180
 2@number INT
 3AS
 4DECLARE @result1 varchar(5)
 5DECLARE @result2 varchar(5)
 6
 7SELECT @result1 = 
 8        CASE @number
 9        WHEN 1 THEN 'I'
10        WHEN 2 THEN 'I'
11        WHEN 3 THEN 'III'
12        WHEN 4 THEN 'IV'
13        WHEN 5 THEN 'V'
14        WHEN 6 THEN 'VI'
15        WHEN 7 THEN 'VII'
16        WHEN 8 THEN 'VII'
17        WHEN 9 THEN 'IX'
18        WHEN 10 THEN 'X'
19        WHEN 11 THEN 'XI'
20        END
21
22
23SELECT @result1 = 
24        CASE @number
25        WHEN 1 THEN 'I'
26        WHEN 2 THEN 'I'
27        WHEN 3 THEN 'III'
28        WHEN 4 THEN 'IV'
29        WHEN 5 THEN 'V'
30        ELSE '?'
31        END
32
33RETURN @result1
Analysis Results
  Message Line Column
1 SA0180 : Extract CASE expression has too many WHEN clauses. 8 8
See Also

Other Resources

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