How to Implement Conditional WHERE Clauses in SQL Server?

Conditional WHERE clause in SQL Server

In SQL Server, the conditional WHERE clause allows you to specify criteria for filtering data based on specific conditions. In some cases, you may need a conditional WHERE clause to evaluate the condition and adjust the filter accordingly. Here's how to effectively construct and use conditional WHERE clauses.

Grammar

The basic conditional WHERE clause follows the following syntax:

<code class="language-sql">WHERE 逻辑表达式 AND (CASE WHEN 条件 THEN 表达式1 ELSE 表达式2 END)</code>

Example

Let’s consider the example you provided:

<code class="language-sql">SELECT
    DateAppr,
    TimeAppr,
    ...
FROM
    [test].[dbo].[MM]
WHERE
    DateDropped = 0
    AND (CASE
            WHEN @JobsOnHold = 1 THEN DateAppr >= 0
            ELSE DateAppr != 0
        END)</code>

This query attempts to filter data for which DateDropped is 0 and DateAppr meets a specific condition based on the @JobsOnHold value. However, the syntax is incorrect.

Correct grammar

To fix syntax errors, enclose the conditional expression in parentheses:

<code class="language-sql">SELECT
    DateAppr,
    TimeAppr,
    ...
FROM
    [test].[dbo].[MM]
WHERE
    DateDropped = 0
    AND (
        (ISNULL(@JobsOnHold, 0) = 1 AND DateAppr >= 0)
        OR
        (ISNULL(@JobsOnHold, 0) != 1 AND DateAppr != 0)
    )</code>

Explanation

• ISNULL(@JobsOnHold, 0): This ensures that if @JobsOnHold is NULL or missing, it is treated as 0.
The
• OR operator ensures that the condition works correctly in both cases.

When @JobsOnHold is 1, DateAppr must be greater than or equal to 0. Otherwise, if @JobsOnHold is not 1, then DateAppr must not be 0.

Other notes

• Use parentheses to group and prioritize conditions for greater clarity.
• Make sure the expression types match.
• Avoid using too many nested conditions as they can be difficult to read and maintain.

By following these guidelines, you can effectively use conditional WHERE clauses in SQL Server to filter data based on specific criteria.

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