Multiple With Statements SQL

How can I use multiple CTEs (Common Table Expressions) in a single SQL query?

Multiple WITH statements allow you to define multiple Common Table Expressions (CTEs) within a single SQL query. This enhances readability and modularity by breaking down complex queries into smaller, manageable parts.

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Description

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Example H3

In SQL, Common Table Expressions (CTEs) are temporary, named result sets defined within a single query. They are extremely useful for organizing and structuring complex queries. A single WITH statement can define one CTE. However, you can define multiple CTEs within a single query using multiple WITH statements. This approach is particularly beneficial when dealing with intricate data transformations or when you need to reuse intermediate results in different parts of the query. Each CTE can be referenced multiple times within the main query or other CTEs, promoting code reusability and maintainability. This approach is highly beneficial for queries involving multiple joins or complex calculations. For instance, if you need to calculate the average sales for each product category and then use that average to identify products underperforming, multiple CTEs can streamline the process.

Why Multiple With Statements SQL is important

Multiple WITH statements improve query readability and maintainability by breaking down complex queries into smaller, more manageable parts. This approach promotes code reusability and makes it easier to understand and modify the query logic.

Multiple With Statements SQL Example Usage


WITH
    ProductSales AS (
        SELECT
            product_id,
            SUM(sales_amount) AS total_sales
        FROM
            sales
        GROUP BY
            product_id
    ),
    CategoryAverages AS (
        SELECT
            p.product_id,
            c.category_name,
            p.total_sales,
            (SELECT
                AVG(total_sales)
            FROM
                ProductSales
            ) AS category_average
        FROM
            ProductSales p
        JOIN
            products c ON p.product_id = c.product_id
        WHERE
            c.category_name = 'Electronics'
    )
SELECT
    product_id,
    category_name,
    total_sales,
    category_average
FROM
    CategoryAverages
WHERE
    total_sales < category_average;

Multiple With Statements SQL Syntax

Common Mistakes

Forgetting to alias CTEs, leading to ambiguity in referencing them.
Incorrectly referencing CTEs in the main query, resulting in errors or unexpected results.
Defining CTEs that are not used in the main query, leading to unnecessary computations.
Using incorrect syntax for defining or referencing CTEs.

Frequently Asked Questions (FAQs)

When should I use multiple CTEs instead of nesting sub-queries?

Use multiple CTEs when a query involves several transformation steps—such as calculating averages, joining to dimension tables, then filtering on the derived metrics. By naming each step with its own WITH clause, the logic becomes modular, readable, and reusable across the rest of the statement, unlike deeply nested sub-queries that are hard to debug.

Can CTEs defined in the same statement reference one another?

Yes. After the first CTE is declared, subsequent CTEs in the same query can select from any earlier CTE just as if it were a regular table. This allows you to chain calculations—for example, generate avg_sales in the first CTE and identify under-performing products in the next.

How does Galaxy make writing multi-CTE queries faster?

Galaxy’s context-aware AI copilot autocompletes table names, aliases, and column references across all CTEs, flags unused CTEs, and can even suggest refactoring steps. Combined with the editor’s low-latency UI, you can iterate on complex, multi-WITH queries without getting lost in the syntax.