Advantages of Using Common Table Expressions (CTEs) in SQL.

The main advantages of CTEs in SQL queries include improving readability, supporting recursive queries, avoiding duplicate subqueries, and enhancing modular and debugging capabilities. 1. Improve readability: Make the structure clearer by splitting complex queries into multiple independent logical blocks; 2. Support recursive queries: The logic is simpler when processing hierarchical data, suitable for deep traversal; 3. Avoid duplicate subqueries: define multiple references at a time, reduce redundancy and improve efficiency; 4. Better modularization and debugging capabilities: Each CTE block can be run and verified separately, making it easier to troubleshoot problems.

Anyone who has used Common Table Expressions (CTEs) knows that it does have many benefits in SQL queries. Especially when you are faced with complex queries and multi-layer nesting, CTEs can help you write the code more clearly and easily maintain.

1. Improve readability: Make complex query structure clearer

The most intuitive advantage of CTEs is to improve the readability of SQL. Compared to large segments of nested subqueries, CTE splits the query into multiple logical blocks, each part is independently defined, and then references it in the main query. This not only makes it easier for you to understand, but it is also easier for others to take over and modify.

For example:

 WITH top_customers AS (
    SELECT customer_id, SUM(order_amount) AS total_spent
    FROM orders
    GROUP BY customer_id
    HAVING SUM(order_amount) > 1000
)
SELECT * FROM top_customers
WHERE total_spent > 5000;

In the above code, a CTE called top_customers is defined first, and then high-consumption users are selected. The entire process is written in steps, which is much clearer than writing it all in FROM or WHERE.

2. Support recursive query: it is more convenient to process hierarchical data

CTEs also has a very powerful feature that supports recursive queries (Recursive CTE), which is very useful for processing tree structures or hierarchical data, such as organizational structures, directory structures, comments and replies chains, etc.

For example, find the superior and subordinate relationship of an employee:

 WITH RECURSIVE employee_hierarchy AS (
    SELECT employee_id, manager_id, name, 1 AS level
    FROM employees
    WHERE manager_id IS NULL

    UNION ALL

    SELECT e.employee_id, e.manager_id, e.name, eh.level 1
    FROM employees e
    INNER JOIN employee_hierarchy eh ON e.manager_id = eh.employee_id
)
SELECT * FROM employee_hierarchy;

This writing method can be found layer by layer, without repeatedly writing JOIN, and the logic is clear, especially suitable for data analysis of deep traversal classes.

3. Avoid duplicate subqueries: reduce redundancy and improve efficiency

Sometimes a subquery will be used multiple times, for example, you count a certain indicator multiple times in a report. If you write a subquery every time, it will not only be troublesome, but also prone to errors. CTEs allow you to extract this part separately and reference it multiple times in the main query.

for example:

 WITH monthly_sales AS (
    SELECT EXTRACT(MONTH FROM order_date) AS month, SUM(amount) AS total
    FROM orders
    GROUP BY EXTRACT(MONTH FROM order_date)
)
SELECT month, total FROM monthly_sales WHERE total > 10000
UNION ALL
SELECT month, total FROM monthly_sales WHERE total <= 10000;

monthly_sales is used twice here, but only needs to be written once. This can reduce the burden of database parsing and executing duplicate statements, and is also helpful to performance.

4. Better modularization and debugging capabilities

CTEs allows you to encapsulate every layer of logic like writing functions. This not only helps with post-maintenance, but also facilitates debugging. You can run each CTE block separately to check whether the intermediate results are correct, without having to run a full large query at once.

Debugging tips:

• Run the CTE section separately to see if the output is as expected
• If the main query error occurs, first rule out the problem with the CTE data
• When multiple CTEs are combined, step by step verification is superimposed in order.

CTEs are not all-purpose, but in most cases it makes SQL easier to read, more flexible and more modular. Especially when your query begins to become "nested layer by layer" and "uncomprehensible", try to use CTE to take apart, and there may be unexpected results.

Basically all this is not complicated but practical.

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