Common Table Expressions, or CTEs, provide a shorthand name to a possibly complex subquery before it is used in a larger query context. This improves readability of the SQL code.
CTEs can be used in combination with SELECT
clauses and INSERT,
DELETE, UPDATE and
UPSERT statements.
Synopsis
Parameters
| Parameter | Description |
|---|---|
table_alias_name |
The name to use to refer to the common table expression from the accompanying query or statement. |
name |
A name for one of the columns in the newly defined common table expression. |
preparable_stmt |
The statement or subquery to use as common table expression. |
Overview
The examples on this page use MovR, a fictional vehicle-sharing application, to demonstrate CockroachDB SQL statements. To follow along, run cockroach demo from the command line to start a temporary, in-memory cluster with the movr dataset preloaded.
For more information about the MovR example application and dataset, see MovR: A Global Vehicle-sharing App.
A query or statement of the form WITH x AS y IN z creates the
temporary table name x for the results of the subquery y, to be
reused in the context of the query z.
For example:
> WITH r AS (SELECT * FROM rides WHERE revenue > 98)
SELECT * FROM users AS u, r WHERE r.rider_id = u.id;
id | city | name | address | credit_card | id | city | vehicle_city | rider_id | vehicle_id | start_address | end_address | start_time | end_time | revenue
---------------------------------------+---------------+------------------+--------------------------------+-------------+--------------------------------------+---------------+---------------+--------------------------------------+--------------------------------------+-----------------------------------+---------------------------+---------------------------+---------------------------+----------
ae147ae1-47ae-4800-8000-000000000022 | amsterdam | Tyler Dalton | 88194 Angela Gardens Suite 94 | 4443538758 | bbe76c8b-4395-4000-8000-00000000016f | amsterdam | amsterdam | ae147ae1-47ae-4800-8000-000000000022 | aaaaaaaa-aaaa-4800-8000-00000000000a | 45295 Brewer View Suite 52 | 62188 Jade Causeway | 2018-12-17 03:04:05+00:00 | 2018-12-17 13:04:05+00:00 | 99.00
c7ae147a-e147-4000-8000-000000000027 | paris | Tina Miller | 97521 Mark Extensions | 8880478663 | d5810624-dd2f-4800-8000-0000000001a1 | paris | paris | c7ae147a-e147-4000-8000-000000000027 | cccccccc-cccc-4000-8000-00000000000c | 47713 Reynolds Mountains Suite 39 | 1417 Stephanie Villages | 2018-12-17 03:04:05+00:00 | 2018-12-18 22:04:05+00:00 | 99.00
75c28f5c-28f5-4400-8000-000000000017 | san francisco | William Wood | 36021 Steven Cove Apt. 89 | 5669281259 | 8ac08312-6e97-4000-8000-00000000010f | san francisco | san francisco | 75c28f5c-28f5-4400-8000-000000000017 | 77777777-7777-4800-8000-000000000007 | 84407 Tony Crest | 55336 Jon Manors | 2018-12-10 03:04:05+00:00 | 2018-12-11 13:04:05+00:00 | 99.00
8a3d70a3-d70a-4000-8000-00000000001b | san francisco | Jessica Martinez | 96676 Jennifer Knolls Suite 91 | 1601930189 | 7d70a3d7-0a3d-4000-8000-0000000000f5 | san francisco | san francisco | 8a3d70a3-d70a-4000-8000-00000000001b | 77777777-7777-4800-8000-000000000007 | 78978 Stevens Ramp Suite 8 | 7340 Alison Field Apt. 44 | 2018-12-19 03:04:05+00:00 | 2018-12-21 10:04:05+00:00 | 99.00
47ae147a-e147-4000-8000-00000000000e | washington dc | Patricia Herrera | 80588 Perez Camp | 6812041796 | 4083126e-978d-4000-8000-00000000007e | washington dc | washington dc | 47ae147a-e147-4000-8000-00000000000e | 44444444-4444-4400-8000-000000000004 | 33055 Julie Dale Suite 93 | 17280 Jill Drives | 2019-01-01 03:04:05+00:00 | 2019-01-01 14:04:05+00:00 | 99.00
(5 rows)
In this example, the WITH clause defines the temporary name r for
the subquery over rides, and that name becomes a valid table name
for use in any table expression of the
subsequent SELECT clause.
This query is equivalent to, but arguably simpler to read than:
> SELECT * FROM users AS u, (SELECT * FROM rides WHERE revenue > 98) AS r
WHERE r.rider_id = u.id;
It is also possible to define multiple common table expressions
simultaneously with a single WITH clause, separated by commas. Later
subqueries can refer to earlier subqueries by name. For example, the
following query is equivalent to the two examples above:
> WITH r AS (SELECT * FROM rides WHERE revenue > 98),
results AS (SELECT * FROM users AS u, r WHERE r.rider_id = u.id)
SELECT * FROM results;
In this example, the second CTE results refers to the first CTE r
by name. The final query refers to the CTE results.
Nested WITH clauses
It is possible to use a WITH clause in a subquery, or even a WITH clause within another WITH clause. For example:
> WITH u AS
(SELECT * FROM
(WITH u_tab AS (SELECT * FROM users) SELECT * FROM u_tab))
SELECT * FROM u;
When analyzing table expressions that mention a CTE name, CockroachDB will choose the CTE definition that is closest to the table expression. For example:
> WITH
u AS (SELECT * FROM users),
v AS (WITH u AS (SELECT * from vehicles) SELECT * FROM u)
SELECT * FROM v;
In this example, the inner subquery SELECT * FROM v will select from
table vehicles (closest WITH clause), not from table users.
New in v20.1: CockroachDB does not support nested WITH clauses containing data-modifying statements. WITH clauses containing data-modifying statements must be at the top level of the query.
Data-modifying statements
It is possible to use a data-modifying statement (INSERT, DELETE,
etc.) as a common table expression, as long as the WITH clause containing the data-modifying statement is at the top level of the query.
For example:
> WITH final_code AS
(INSERT INTO promo_codes(code, description, rules)
VALUES ('half_off', 'Half-price ride!', '{"type": "percent_discount", "value": "50%"}'), ('free_ride', 'Free ride!', '{"type": "percent_discount", "value": "100%"}')
returning rules)
SELECT rules FROM final_code;
rules
+-----------------------------------------------+
{"type": "percent_discount", "value": "50%"}
{"type": "percent_discount", "value": "100%"}
(2 rows)
If the WITH clause containing the data-modifying statement is at a lower level, the statement results in an error:
> SELECT (WITH final_code AS
(INSERT INTO promo_codes(code, description, rules)
VALUES ('half_off', 'Half-price ride!', '{"type": "percent_discount", "value": "50%"}'), ('free_ride', 'Free ride!', '{"type": "percent_discount", "value": "100%"}')
returning rules)
SELECT rules FROM final_code);
ERROR: WITH clause containing a data-modifying statement must be at the top level
SQLSTATE: 0A000
If a common table expression contains
a data-modifying statement (INSERT, DELETE,
etc.), the modifications are performed fully even if only part
of the results are used, e.g., with LIMIT. See Data
Writes in Subqueries for details.
Reusing common table expressions
You can reference a CTE multiple times in a single query, using a WITH operator.
For example:
> WITH
users_ny AS (SELECT name, id FROM users WHERE city='new york'),
vehicles_ny AS (SELECT type, id, owner_id FROM vehicles WHERE city='new york')
SELECT * FROM users_ny JOIN vehicles_ny ON users_ny.id = vehicles_ny.owner_id;
name | id | type | id | owner_id
+------------------+--------------------------------------+------------+--------------------------------------+--------------------------------------+
James Hamilton | 051eb851-eb85-4ec0-8000-000000000001 | skateboard | 00000000-0000-4000-8000-000000000000 | 051eb851-eb85-4ec0-8000-000000000001
Catherine Nelson | 147ae147-ae14-4b00-8000-000000000004 | scooter | 11111111-1111-4100-8000-000000000001 | 147ae147-ae14-4b00-8000-000000000004
(2 rows)
In this single query, you define two CTE's and then reuse them in a table join.
Recursive common table expressions
New in v20.1: CockroachDB supports recursive common table expressions. Recursive common table expressions are common table expressions that contain subqueries that refer to their own output.
Recursive CTE definitions take the following form:
WITH RECURSIVE <cte name> (<columns>) AS (
<initial subquery>
UNION ALL
<recursive subquery>
)
<query>
To write a recursive CTE:
- Add the
RECURSIVEkeyword directly after theWITHoperator in the CTE definition, and before the CTE name. - Define an initial, non-recursive subquery. This subquery defines the initial values of the CTE.
- Add the
UNION ALLkeyword after the initial subquery. - Define a recursive subquery that references its own output. This subquery can also reference the CTE name, unlike the initial subquery.
- Write a parent query that evaluates the results of the CTE.
CockroachDB evaluates recursive CTEs as follows:
- The initial query is evaluated. Its results are stored to rows in the CTE and copied to a temporary, working table. This working table is updated across iterations of the recursive subquery.
- The recursive subquery is evaluated iteratively on the contents of the working table. The results of each iteration replace the contents of the working table. The results are also stored to rows of the CTE. The recursive subquery iterates until no results are returned.
Recursive subqueries must eventually return no results, or the query will run indefinitely.
For example, the following recursive CTE calculates the factorial of the numbers 0 through 9:
WITH RECURSIVE cte (n, factorial) AS (
VALUES (0, 1) -- initial subquery
UNION ALL
SELECT n+1, (n+1)*factorial FROM cte WHERE n < 9 -- recursive subquery
)
SELECT * FROM cte;
n | factorial
+---+-----------+
0 | 1
1 | 1
2 | 2
3 | 6
4 | 24
5 | 120
6 | 720
7 | 5040
8 | 40320
9 | 362880
(10 rows)
The initial subquery (VALUES (0, 1)) initializes the working table with the values 0 for the n column and 1 for the factorial column. The recursive subquery (SELECT n+1, (n+1)*factorial FROM cte WHERE n < 9) evaluates over the initial values of the working table and replaces its contents with the results. It then iterates over the contents of the working table, replacing its contents at each iteration, until n reaches 9, when the WHERE clause evaluates as false.
If no WHERE clause were defined in the example, the recursive subquery would always return results and loop indefinitely, resulting in an error:
WITH RECURSIVE cte (n, factorial) AS (
VALUES (0, 1) -- initial subquery
UNION ALL
SELECT n+1, (n+1)*factorial FROM cte -- recursive subquery with no WHERE clause
)
SELECT * FROM cte;
ERROR: integer out of range
SQLSTATE: 22003
If you are unsure if your recursive subquery will loop indefinitely, you can limit the results of the CTE with the LIMIT keyword.
For example, if we remove the WHERE clause from the factorial example, we can use LIMIT to limit the results and avoid the integer out of range error:
WITH RECURSIVE cte (n, factorial) AS (
VALUES (0, 1) -- initial subquery
UNION ALL
SELECT n+1, (n+1)*factorial FROM cte -- recursive subquery
)
SELECT * FROM cte LIMIT 10;
n | factorial
+---+-----------+
0 | 1
1 | 1
2 | 2
3 | 6
4 | 24
5 | 120
6 | 720
7 | 5040
8 | 40320
9 | 362880
(10 rows)
While this practice works for testing and debugging, we do not recommend it in production.
CockroachDB does not currently support the Postgres recursive CTE variant with the keyword UNION.
Known limitations
Correlated common table expressions
CockroachDB does not support correlated common table expressions. This means that a CTE cannot refer to a variable defined outside the scope of that CTE.
For example, the following query returns an error:
> SELECT * FROM users
WHERE id =
(WITH rides_home AS
(SELECT revenue FROM rides
WHERE end_address = address)
SELECT rider_id FROM rides_home);
ERROR: CTEs may not be correlated
SQLSTATE: 0A000
This query returns an error because the WITH rides_home clause references a column (address) returned by the SELECT statement at the top level of the query, outside the rides_home CTE definition.
For details, see the tracking issue.
See also
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