Examples of Using the WITH AS Statement in MySQL

1. What is the WITH AS statement?

WITH Clauses, also known as Common Table Expressions (CTEs), are a way to create temporary result sets in a SQL query that exist within the scope of a single statement so that they can be referenced multiple times in the query. It makes SQL queries more modular and readable.

2. How it works

WITH The syntax of the clause is as follows:

WITH cte_name (column_name1, column_name2, ...) AS (
    -- CTE query definition
    SELECT column1, column2, ...
    FROM table_name
    WHERE condition
)
-- Main query that references the CTE
SELECT column_name1, column_name2, ...
FROM cte_name
WHERE condition;

Among them:

• cte_name is the name of the CTE.
• (column_name1, column_name2, …) is optional and is used to define column names for the CTE; providing optional column names improves readability.
• SELECT column1, column2, … FROM table_name WHERE condition is the definition part of the CTE query.

Here is a simple example demonstrating how to use theWITH Clause:

WITH DepartmentCTE AS (
    SELECT
        DepartmentID,
        COUNT(*) AS EmployeeCount
    FROM Employees
    GROUP BY DepartmentID
)
SELECT
    ,
    
FROM Departments D
LEFT JOIN DepartmentCTE E ON  = ;

In this example, DepartmentCTE is a CTE that counts the number of employees in each department. The main query then retrieves relevant information from both the Departments table and the CTE, enabling a modular query that retrieves information from both tables. This makes the query more readable and easier to maintain.

CTE can be seen as a more concise and readable way of querying, especially for situations where the same subquery results need to be referenced multiple times.

I can also use it this way if there are multiple result sets:

WITH
    CTE1 AS (
        -- CTE1 definition
        SELECT * FROM Table1
    ),
    CTE2 AS (
        -- CTE2 definition
        SELECT * FROM Table2
    )
-- Main query that references the CTEs
SELECT * FROM CTE1, CTE2
WHERE  = ;

In this example, there are two CTEs: CTE1 and CTE2. in the main query, you can refer to these two CTEs and join them with a WHERE clause to retrieve results based on a common condition.

Note that each CTE definition should contain a valid query. The association between these CTEs can be accomplished in the main query by a JOIN or other join condition.

Note that if multiple result sets are involved, it may be necessary to use different CTEs or combine them as appropriate.WITH The main purpose of the clause is to improve the readability and modularity of the query, allowing the query logic to be expressed more clearly in the main query.

3. Official Documents

For detailed instructions, please refer toMySQL Official Documentation。

Relevant summary from WITH (excerpts)

1. The use of the with clause allows subqueries to reuse the same with query block, called by select (with clause can only be referenced by the select query block), generally in the with query used more than one case. In the reference to the select statement before the definition, the same level can only define the with keyword can only be used once, more than one with a comma split.

2. The last with clause and the following query can not have a comma between, only through the right bracket split, with clause of the query must be enclosed in brackets

3. If the with clause is defined and not used in the query, then it will report ora-32035 error: the name of the query defined in the with clause is not referenced. (at least one with query name is not referenced, the solution is to remove the unquoted with query), note: as long as there is a reference to the back of the can be, do not have to be referenced in the main query, such as the back of the with query is also referenced, it is also possible.

4. The query defined in the previous with clause can be used in the subsequent with clause. However, you cannot nest with clauses within the same with clause.

5. When a query block name is the same as a table name or other object, the parser searches from the inside out, giving preference to subquery block names.

The result column of the query has an alias and must be referenced with an alias or *.

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