With as subquery in Oracle

One 、WITH AS brief introduction

    Oracle stay Oracle 9i In the second version of the database, we introduced the SQL; With Clause SQL Allows you to give a subquery block a name （ Also known as subquery refactoring ）, Can be in the Lord SQL References in multiple locations in the query ; The subquery works like Oracle's global temporary tables , Used to improve complexity SQL Query speed .

① This subquery is used to define temporary relationships , So that the output of this temporary relationship can be used , And used by the query associated with the clause .

② Queries related to subqueries can also be written using nested subqueries , But doing so will add more complex reads / debugging SQL Inquire about .

③ This subquery does not support all database systems ;

④ The names assigned to subqueries are like online views or tables ;

⑤Oracle stay Oracle 9i edition 2 In the database, the..., with clauses, is introduced SQL.

⑥with as The temporary table is cleared after the query is completed .

⑦ Added sql The readability of , If multiple subqueries are constructed , The structure will be clearer .

Two 、WITH AS The syntax of the subquery

The syntax of the subquery ：

WITH  Query name 1 AS ( Subquery ) , Query name 2 AS ( Subquery ), Query name 3 AS ( Subquery )  The clause part of the subquery definition 

3、 ... and 、 Use scenarios

3.1、 Find all employees whose salaries exceed the average salary of all employees

① Employee table information

②  Find all employees whose salaries exceed the average salary of all employees WITH AS Subquery statement

WITH tmpTable ( avgValue ) AS ( SELECT AVG( "Salary" ) FROM "EmployeeInfo" ) 
SELECT
 * 
FROM
	"EmployeeInfo",
	tmpTable 
WHERE
	"EmployeeInfo"."Salary" > tmpTable.avgValue

explain ： In this query , Define a temporary relation table using a subquery tmpTable, This temporary table has only 1 Average of attributes avgValue【 namely AVG("Salary") Content 】; Then, we use the query statement to compare the salary and average value of each employee in the basic table （9600） Compare , Only output employee information that is greater than the average salary .

Be careful ： When executing a query with clauses , First evaluate the query mentioned in the clause , And store the output of this assessment in a temporary relationship . After that , The main query that will use the associated clause is finally executed , The query will use the resulting temporary relationship .

③ Finding results  

 3.2、 Get the average salary of employees 、 The highest salary 、 Total salary

① Employee information form

② Get the average salary of employees 、 Maximum salary and total salary

WITH 
tmpAvgAgeTable1 AS (SELECT ROUND(AVG("Salary"), 2) avgSalary FROM "EmployeeInfo"),
tmpMaxAgeTable2 AS (SELECT MAX("Salary") maxSalary FROM "EmployeeInfo" ),
tmpSumAgeTable3 AS (SELECT SUM("Salary") sumSalary FROM "EmployeeInfo")
SELECT * FROM tmpAvgAgeTable1 t1,tmpMaxAgeTable2 t2 ,tmpSumAgeTable3 t3

③ Query results

  Four 、 Reference material

SQL (oracle.com)https://docs.oracle.com/database/121/CNCPT/sqllangu.htm

13.2.15 WITH (Common Table Expressions) (oracle.com)https://docs.oracle.com/cd/E17952_01/mysql-8.0-en/with.html#common-table-expressions