Sql Common Table Expressions

Sql Common Table Expressions
Nikhil
Published on 2021-08-15 06:00:20

generating values

Most databases do not have a native way of generating a series of numbers for ad-hoc use; however, common table expressions can be used with recursion to emulate that type of function.

The following example generates a common table expression called Numbers with a column i which has a row fo numbers 1-5:

--Give a table name `Numbers" and a column `i` to hold the numbers
WITH Numbers(i) AS (
--Starting number/index
SELECT 1
--Top-level UNION ALL operator required for recursion
UNION ALL
--Iteration expression:
SELECT i + 1
--Table expression we first declared used as source for recursion
FROM Numbers
--Clause to define the end of the recursion
WHERE i < 5
)
--Use the generated table expression like a regular table
SELECT i FROM Numbers;

i
1
2
3
4
5

This method can be used with any number interval, as well as other types of data.

recursively enumerating a subtree

WITH RECURSIVE ManagedByJames(Level, ID, FName, LName) AS (
-- start with this row
SELECT 1, ID, FName, LName
FROM Employees
WHERE ID = 1
UNION ALL
-- get employees that have any of the previously selected rows as manager
SELECT ManagedByJames.Level + 1,
Employees.ID,
Employees.FName,
Employees.LName
FROM Employees
JOIN ManagedByJames
ON Employees.ManagerID = ManagedByJames.ID
ORDER BY 1 DESC -- depth-first search
)
SELECT * FROM ManagedByJames;

Level ID FName LName
1 1 James Smith
2 2 John Johnson
3 4 Johnathon Smith
2 3 Michael Williams

Temporary query

These behave in the same manner as nested subqueries but with a different syntax.

WITH ReadyCars AS (
SELECT *
FROM Cars
WHERE Status = 'READY'
)
SELECT ID, Model, TotalCost
FROM ReadyCars
ORDER BY TotalCost;

ID Model TotalCost
1 Ford F-150 200
2 Ford F-150 230

Equivalent subquery syntax

SELECT ID, Model, TotalCost
FROM (
SELECT *
FROM Cars
WHERE Status = 'READY'
) AS ReadyCars
ORDER BY TotalCost

recursively going up in a tree

WITH RECURSIVE ManagersOfJonathon AS (
-- start with this row
SELECT *
FROM Employees
WHERE ID = 4
UNION ALL
-- get manager(s) of all previously selected rows
SELECT Employees.*
FROM Employees
JOIN ManagersOfJonathon
ON Employees.ID = ManagersOfJonathon.ManagerID
)
SELECT * FROM ManagersOfJonathon;

Id FName LName PhoneNumber ManagerId DepartmentId
4 Johnathon Smith 1212121212 2 1
2 John Johnson 2468101214 1 1
1 James Smith 1234567890 NULL 1

Recursively generate dates, extended to include team rostering as example

DECLARE @DateFrom DATETIME = '2016-06-01 06:00'
DECLARE @DateTo DATETIME = '2016-07-01 06:00'
DECLARE @IntervalDays INT = 7
-- Transition Sequence = Rest & Relax into Day Shift into Night Shift
-- RR (Rest & Relax) = 1
-- DS (Day Shift) = 2
-- NS (Night Shift) = 3
;WITH roster AS
(
SELECT @DateFrom AS RosterStart, 1 AS TeamA, 2 AS TeamB, 3 AS TeamC
UNION ALL
SELECT DATEADD(d, @IntervalDays, RosterStart),
CASE TeamA WHEN 1 THEN 2 WHEN 2 THEN 3 WHEN 3 THEN 1 END AS TeamA,
CASE TeamB WHEN 1 THEN 2 WHEN 2 THEN 3 WHEN 3 THEN 1 END AS TeamB,
CASE TeamC WHEN 1 THEN 2 WHEN 2 THEN 3 WHEN 3 THEN 1 END AS TeamC
FROM roster WHERE RosterStart < DATEADD(d, [email protected], @DateTo)
)
SELECT RosterStart,
ISNULL(LEAD(RosterStart) OVER (ORDER BY RosterStart), RosterStart + @IntervalDays) AS
RosterEnd,
CASE TeamA WHEN 1 THEN 'RR' WHEN 2 THEN 'DS' WHEN 3 THEN 'NS' END AS TeamA,
CASE TeamB WHEN 1 THEN 'RR' WHEN 2 THEN 'DS' WHEN 3 THEN 'NS' END AS TeamB,
CASE TeamC WHEN 1 THEN 'RR' WHEN 2 THEN 'DS' WHEN 3 THEN 'NS' END AS TeamC
FROM roster

Result

I.e. For Week 1 TeamA is on R&R, TeamB is on Day Shift and TeamC is on Night Shift.

sql table

Oracle CONNECT BY functionality with recursive CTEs

Oracle's CONNECT BY functionality provides many useful and nontrivial features that are not built-in when using SQL standard recursive CTEs. This example replicates these features (with a few additions for sake of completeness), using SQL Server syntax. It is most useful for Oracle developers finding many features missing in their hierarchical queries on other databases, but it also serves to showcase what can be done with a hierarchical query in general.

WITH tbl AS (
SELECT id, name, parent_id
FROM mytable)
, tbl_hierarchy AS (
/* Anchor */
SELECT 1 AS "LEVEL"
--, 1 AS CONNECT_BY_ISROOT
--, 0 AS CONNECT_BY_ISBRANCH
, CASE WHEN t.id IN (SELECT parent_id FROM tbl) THEN 0 ELSE 1 END AS CONNECT_BY_ISLEAF
, 0 AS CONNECT_BY_ISCYCLE
, '/' + CAST(t.id AS VARCHAR(MAX)) + '/' AS SYS_CONNECT_BY_PATH_id
, '/' + CAST(t.name AS VARCHAR(MAX)) + '/' AS SYS_CONNECT_BY_PATH_name
, t.id AS root_id
, t.*
FROM tbl t
WHERE t.parent_id IS NULL -- START WITH parent_id IS NULL
UNION ALL
/* Recursive */
SELECT th."LEVEL" + 1 AS "LEVEL"
--, 0 AS CONNECT_BY_ISROOT
--, CASE WHEN t.id IN (SELECT parent_id FROM tbl) THEN 1 ELSE 0 END AS
CONNECT_BY_ISBRANCH
, CASE WHEN t.id IN (SELECT parent_id FROM tbl) THEN 0 ELSE 1 END AS CONNECT_BY_ISLEAF
, CASE WHEN th.SYS_CONNECT_BY_PATH_id LIKE '%/' + CAST(t.id AS VARCHAR(MAX)) + '/%'
THEN 1 ELSE 0 END AS CONNECT_BY_ISCYCLE
, th.SYS_CONNECT_BY_PATH_id + CAST(t.id AS VARCHAR(MAX)) + '/' AS
SYS_CONNECT_BY_PATH_id
, th.SYS_CONNECT_BY_PATH_name + CAST(t.name AS VARCHAR(MAX)) + '/' AS
SYS_CONNECT_BY_PATH_name
, th.root_id
, t.*
FROM tbl t
JOIN tbl_hierarchy th ON (th.id = t.parent_id) -- CONNECT BY PRIOR id = parent_id
WHERE th.CONNECT_BY_ISCYCLE = 0) -- NOCYCLE
SELECT th.*
--, REPLICATE(' ', (th."LEVEL" - 1) * 3) + th.name AS tbl_hierarchy
FROM tbl_hierarchy th
JOIN tbl CONNECT_BY_ROOT ON (CONNECT_BY_ROOT.id = th.root_id)
ORDER BY th.SYS_CONNECT_BY_PATH_name; -- ORDER SIBLINGS BY name

CONNECT BY features demonstrated above, with explanations:

Clauses

  • CONNECT BY: Specifies the relationship that defines the hierarchy.
  • START WITH: Specifies the root nodes.
  • ORDER SIBLINGS BY: Orders results properly.

Parameters

  • NOCYCLE: Stops processing a branch when a loop is detected. Valid hierarchies are Directed Acyclic
  • Graphs, and circular references violate this construct.

Operators

  • PRIOR: Obtains data from the node's parent.
  • CONNECT_BY_ROOT: Obtains data from the node's root.

Pseudocolumns

  • LEVEL: Indicates the node's distance from its root.
  • CONNECT_BY_ISLEAF: Indicates a node without children.
  • CONNECT_BY_ISCYCLE: Indicates a node with a circular reference.

Functions

  • SYS_CONNECT_BY_PATH: Returns a flattened/concatenated representation of the path to the node from its root.

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