Is there any with as mysql?

Yes, WITH AS is a commonly used query statement in MySQL, used to create temporary tables and use them in subsequent queries. Below we introduce the usage and practical application of WITH AS in detail.

1. WITH as syntax

WITH as syntax is mainly divided into the following two parts:

1) WITH: keyword, used to introduce a or Multiple temporary tables.

2) AS: keyword, used to define one or more temporary tables.

The general syntax of the WITH as statement is as follows:

WITH [RECURSIVE] cte_name (column_name1,column_name2,...) AS
(
subquery1
UNION [ALL]
subquery2
UNION [ALL]
subquery3
...
)
SELECT *
FROM cte_name;

where,

cte_name: the name of the temporary table;

column_name1, column_name2,...: column names in the temporary table;

subquery1, subquery2, subquery3...: subquery statements.

2. WITH as practical application

The practical application scenarios of WITH as statement mainly include the following aspects:

1) Recursive query

WITH as statement can perform recursive queries to obtain a result set with a hierarchical structure. The following takes an employee table as an example to illustrate:

WITH RECURSIVE emp AS (
SELECT emp_id, emp_name, emp_manager_id
FROM employees
WHERE emp_manager_id IS NULL
UNION ALL
SELECT e2.emp_id, e2.emp_name, e2.emp_manager_id
FROM employees e2
INNER JOIN emp ON e2.emp_manager_id = emp.emp_id
)
SELECT * FROM emp;

In the above statement, the basic part of the recursive query is to look up the root node in the table (that is, employees without managers). In each recursive iteration, the subquery of the table finds the subordinates of each node at once and adds them to the result. concentrated.

2) Organize the result set

WITH as statement can organize multiple query results together and perform multi-table joint queries. The following takes a product sales and inventory table as an example to illustrate:

WITH sales AS (
SELECT p.product_id, p.product_name, s.sale_id, s.sale_date, s.quantity, s. price
FROM products p
INNER JOIN sales s ON p.product_id = s.product_id
), inventory AS (
SELECT p.product_id, p.product_name, i.inventory_id, i.quantity
FROM products p
INNER JOIN inventory i ON p.product_id = i.product_id
)
SELECT s.sale_id, s.sale_date, s.product_id, s.product_name, s.quantity, s. price, i.inventory_id, i.quantity as inventory_quantity
FROM sales s
INNER JOIN inventory i ON s.product_id = i.product_id;

In the above statement, we first define sales and Inventory two temporary tables, and then perform a joint query on these two tables to output a result set. This result set contains sales information from the sales table and inventory information from the inventory table.

3) Subquery optimization

WITH as statement can also be used to optimize subqueries. Usually, the execution efficiency of subquery statements is very low, and it is easy to cause code confusion and errors. The WITH as statement can improve the readability and maintainability of queries. The following takes an order and order details table as an example to illustrate:

WITH ord AS (
SELECT order_id
FROM orders
WHERE order_date > '2022-01-01'
)
SELECT *
FROM order_details
WHERE order_id IN (SELECT order_id FROM ord);

In the above statement, we first define a temporary table ord to store the requirements order information. Then in subsequent queries, directly use the WHERE order_id IN (SELECT order_id FROM ord) statement to query the order details. This not only improves query efficiency, but also improves code readability and maintainability.

Summary:

This article introduces the usage and practical application scenarios of WITH as statement in MySQL. In actual development, we can flexibly use the WITH as statement according to specific needs to improve query efficiency and code readability and maintainability.

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