Common Table Expressions
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SQL Server Usage
Common Table Expressions (CTE) are part of the ANSI standard since SQL:1999, simplify queries and make them more readable by defining a temporary view, or derived table, that a subsequent query can reference. You can use SQL Server CTEs as the target of DML modification statements. They have similar restrictions as updateable views.
SQL Server CTEs provide recursive functionality in accordance with the ANSI 99 standard. Recursive CTEs can reference themselves and re-run queries until the data set is exhausted, or the maximum number of iterations is exceeded.
CTE Syntax
WITH <CTE NAME> AS ( SELECT .... ) SELECT ... FROM CTE
Recursive CTE Syntax
WITH <CTE NAME> AS ( <Anchor SELECT query> UNION ALL <Recursive SELECT query with reference to <CTE NAME>> ) SELECT ... FROM <CTE NAME>...
Examples
Create and populate an OrderItems table.
CREATE TABLE OrderItems ( OrderID INT NOT NULL, Item VARCHAR(20) NOT NULL, Quantity SMALLINT NOT NULL, PRIMARY KEY(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 CTE to calculate the total quantity in every order and then join to the OrderItems table to obtain the relative quantity for each item.
WITH AggregatedOrders AS ( SELECT OrderID, SUM(Quantity) AS TotalQty FROM OrderItems GROUP BY OrderID ) SELECT O.OrderID, O.Item, O.Quantity, (O.Quantity / AO.TotalQty) * 100 AS PercentOfOrder FROM OrderItems AS O INNER JOIN AggregatedOrders AS AO ON O.OrderID = AO.OrderID;
The preceding example produces the following results.
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
Using a recursive CTE, create and populate the Employees table with the DirectManager for each employee.
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);
Use a recursive CTE to display the employee-management hierarchy.
WITH EmpHierarchyCTE AS
(
-- Anchor query retrieves the top manager
SELECT 0 AS LVL,
Employee,
DirectManager
FROM Employees AS E
WHERE DirectManager IS NULL
UNION ALL
-- Recursive query gets all Employees managed by the previous level
SELECT LVL + 1 AS LVL,
E.Employee,
E.DirectManager
FROM EmpHierarchyCTE AS EH
INNER JOIN
Employees AS E
ON E.DirectManager = EH.Employee
)
SELECT *
FROM EmpHierarchyCTE;
The preceding example produces the following results.
LVL Employee DirectManager 0 Dave NULL 1 John Dave 1 Jose Dave 2 Fred John
For more information, see Recursive Queries Using Common Table Expressions
PostgreSQL Usage
PostgreSQL conforms to the ANSI SQL-99 standard and implementing CTEs in PostgreSQL is similar to SQL Server.
CTE is also known as WITH query. This type of query helps you to simplify long queries, it is similar to defining temporary tables that exist only for the running of the query. The statement in a WITH clause can be a SELECT, INSERT, UPDATE, or DELETE, and the WITH clause itself is attached to a primary statement that can also be a SELECT, INSERT, UPDATE, or DELETE.
CTE Syntax
WITH <CTE NAME> AS ( SELECT OR DML ) SELECT OR DML Recursive CTE
Recursive CTE Syntax
WITH RECURSIVE <CTE NAME> AS ( <Anchor SELECT query> UNION ALL <Recursive SELECT query with reference to <CTE NAME>> ) SELECT OR DML
Examples
Create and populate an OrderItems table.
CREATE TABLE OrderItems ( OrderID INT NOT NULL, Item VARCHAR(20) NOT NULL, Quantity SMALLINT NOT NULL, PRIMARY KEY(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);
Create a CTE.
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;
PostgreSQL provides an additional feature when using a CTE as a recursive modifier. The following example uses a recursive WITH clause to access its own result set.
WITH RECURSIVE t(n) AS ( VALUES (0) UNION ALL SELECT n+1 FROM t WHERE n < 5) SELECT * FROM t; WITH RECURSIVE t(n) AS ( VALUES (0) UNION ALL SELECT n+1 FROM t WHERE n < 5) SELECT * FROM t; n 0 ... 5
Note that using the SQL Server example will get undesired results.
Define a CTE to calculate the total quantity in every order and then join to the OrderItems table to obtain the relative quantity for each item.
WITH AggregatedOrders
AS
( SELECT OrderID, SUM(Quantity) AS TotalQty
FROM OrderItems
GROUP BY OrderID
)
SELECT O.OrderID, O.Item,
O.Quantity,
(O.Quantity / AO.TotalQty) * 100 AS PercentOfOrder
FROM OrderItems AS O
INNER JOIN
AggregatedOrders AS AO
ON O.OrderID = AO.OrderID;
The preceding example produces the following results.
OrderID Item Quantity PercentOfOrder 1 M8 Bolt 100 100 2 M8 Nut 100 100 3 M8 Washer 100 0 3 M6 Washer 200 0
This is because when you divide INT by INT, you get a round result. If you use another data type such as DECIMAL, there will be no problem. To fix the current issue, cast the columns using ::decimal.
AS ( SELECT OrderID, SUM(Quantity) AS TotalQty FROM OrderItems GROUP BY OrderID ) SELECT O.OrderID, O.Item, O.Quantity, trunc((O.Quantity::decimal / AO.TotalQty::decimal)*100,2) AS PercentOfOrder FROM OrderItems AS O INNER JOIN AggregatedOrders AS AO ON O.OrderID = AO.OrderID;
The preceding example produces the following results.
OrderID Item Quantity PercentOfOrder 1 M8 Bolt 100 100 2 M8 Nut 100 100 3 M8 Washer 100 33.33 3 M6 Washer 200 66.66
Unlike in SQL Server, for RECURSIVE WITH query, use the RECURSIVE keyword in PostgreSQL.
Use a recursive CTE to display the employee-management hierarchy.
WITH RECURSIVE EmpHierarchyCTE AS ( -- Anchor query retrieves the top manager SELECT 0 AS LVL, Employee, DirectManager FROM Employees AS E WHERE DirectManager IS NULL UNION ALL -- Recursive query gets all Employees managed by the previous level SELECT LVL + 1 AS LVL, E.Employee, E.DirectManager FROM EmpHierarchyCTE AS EH INNER JOIN Employees AS E ON E.DirectManager = EH.Employee ) SELECT * FROM EmpHierarchyCTE;
The preceding example produces the following results.
LVL Employee DirectManager 0 Dave 1 John Dave 1 Jose Dave 2 Fred John
For more information, see WITH Queries (Common Table Expressions)