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Common Table Expressions

Oracle Usage

Common Table Expressions (CTE) provide a way to implement the logic of sequential code or to reuse code. You can define a named sub query and then use it multiple times in different parts of a query statement.

A CTE is implemented using a WITH clause, which is part of the ANSI SQL-99 standard and has existed in Oracle since version 9.2. CTE usage is similar to an inline view or a temporary table. Its main purpose is to reduce query statement repetition and make complex queries simpler to read and understand.

Syntax

WITH <subquery name> AS (<subquery code>)[...]
SELECT <Select list> FROM <subquery name>;

Examples

The following example creates a sub query of the employee count for each department and then use the result set of the CTE in a query.

WITH DEPT_COUNT
(DEPARTMENT_ID, DEPT_COUNT) AS
(SELECT DEPARTMENT_ID, COUNT(*)
FROM EMPLOYEES
GROUP BY DEPARTMENT_ID)
SELECT E.FIRST_NAME ||' '|| E.LAST_NAME AS EMP_NAME,
D.DEPT_COUNT AS EMP_DEPT_COUNT
FROM EMPLOYEES E JOIN DEPT_COUNT D
USING (DEPARTMENT_ID)
ORDER BY 2;

MySQL Usage

Aurora MySQL 5.7 doesn’t support common table expressions (CTE).

Migration Considerations

As a workaround, use views or derived tables in place of non-recursive CTEs. Since non-recursive CTEs are more convenient for readability and code simplification, you can convert the code to use derived tables, which are a subquery in the parent query’s FROM clause. For example, replace the following CTE:

WITH TopCustomerOrders
( SELECT Customer, COUNT(*) AS NumOrders
    FROM Orders
    GROUP BY Customer
)
SELECT TOP 10 * FROM TopCustomerOrders ORDER BY NumOrders DESC;

With the following subquery:

SELECT *
FROM ( SELECT Customer, COUNT(*) AS NumOrders
    FROM Orders
    GROUP BY Customer ) AS TopCustomerOrders
ORDER BY NumOrders DESC
LIMIT 10 OFFSET 0;

When you use derived tables, make sure that the derived table definition is repeated if multiple instances are required for the query.

Converting the code for recursive CTEs is not straight forward, but you can achieve similar functionality using loops.

Examples

Replacing Non-recursive CTEs

Use a derived table to replace non-recursive CTE functionality as follows:

The following example creates and populates an OrderItems table.

CREATE TABLE OrderItems(
    OrderID INT NOT NULL, Item VARCHAR(20) NOT NULL,
    Quantity SMALLINT NOT NULL,
    PRIMARY (OrderID, Item));

INSERT INTO OrderItems (OrderID, Item, Quantity)
VALUES (1, 'M8 Bolt', 100), (2, 'M8 Nut', 100),
(3, 'M8 Washer', 200), (3, 'M6 Washer', 100);

Define a derived table for TotalQty of every order and then join to the OrderItems to obtain the relative quantity for each item.

SELECT O.OrderID, O.Item, O.Quantity, (O.Quantity / AO.TotalQty) * 100 AS PercentOfOrder
FROM OrderItems AS O
    INNER JOIN
    ( SELECT OrderID, SUM(Quantity) AS TotalQty
        FROM OrderItems
        GROUP BY OrderID
    ) AS AO
    ON O.OrderID = AO.OrderID;

For the preceding example, the result looks as shown following.

OrderID  Item       Quantity  PercentOfOrder
1        M8 Bolt    100       100.0000000000
2        M8 Nut     100       100.0000000000
3        M8 Washer  100       33.3333333300
3        M6 Washer  200       66.6666666600

Replacing Recursive CTEs

Use recursive SQL code in stored procedures and SQL loops to replace a recursive CTEs.

The following example creates and populates an Employees table.

CREATE TABLE Employees
( Employee VARCHAR(5) NOT NULL PRIMARY KEY,
    DirectManager VARCHAR(5) NULL);

INSERT INTO Employees (Employee, DirectManager)
VALUES ('John', 'Dave'), ('Jose', 'Dave'),
('Fred', 'John'), ('Dave', NULL);

The following example creates an EmpHierarcy table.

CREATE TABLE EmpHierarchy (LVL INT, Employee VARCHAR(5), Manager VARCHAR(5));

The following example creates a procedure that uses a loop to traverse the employee hierarchy.

CREATE PROCEDURE P()
BEGIN
DECLARE var_lvl INT;
DECLARE var_Employee VARCHAR(5);
SET var_lvl = 0;
SET var_Employee = (SELECT Employee FROM Employees WHERE DirectManager IS NULL);
INSERT INTO EmpHierarchy VALUES (var_lvl, var_Employee, NULL);
WHILE var_lvl <> -1
DO
INSERT INTO EmpHierarchy (LVL, Employee, Manager)
SELECT var_lvl + 1, Employee, DirectManager
FROM Employees
WHERE DirectManager IN (SELECT Employee FROM EmpHierarchy WHERE LVL = var_lvl);
IF NOT EXISTS (SELECT * FROM EmpHierarchy WHERE LVL = var_lvl + 1)
THEN SET var_lvl = -1;
ELSE SET var_lvl = var_lvl + 1;
END IF;
END WHILE;
END;

Run the procedure.

CALL P()

Select all records from the EmpHierarchy table.

SELECT * FROM EmpHierarchy;

Level  Employee  Manager
0      Dave
1      John      Dave
1      Jose      Dave
2      Fred      John

Summary

For more information, see WITH (Common Table Expressions) in the MySQL documentation.