WITH
Syntax
WITH [RECURSIVE] table_reference [(columns_list)] AS ( SELECT ... ) [CYCLE cycle_column_list RESTRICT] SELECT ...
Contents
Description
The WITH
keyword signifies a Common Table Expression (CTE). It allows you to refer to a subquery expression many times in a query, as if having a temporary table that only exists for the duration of a query.
There are two kinds of CTEs:
- Non-Recursive
- Recursive (signified by the
RECURSIVE
keyword, supported since MariaDB 10.2.2)
You can use table_reference
as any normal table in the external SELECT
part. You can also use WITH
in subqueries, as well as with EXPLAIN and SELECT.
Poorly-formed recursive CTEs can in theory cause infinite loops. The max_recursive_iterations system variable limits the number of recursions.
CYCLE ... RESTRICT
MariaDB starting with 10.5.2
The CYCLE clause enables CTE cycle detection, avoiding excessive or infinite loops, MariaDB supports a relaxed, non-standard grammar.
The SQL Standard permits a CYCLE clause, as follows:
WITH RECURSIVE ... ( ... ) CYCLE <cycle column list> SET <cycle mark column> TO <cycle mark value> DEFAULT <non-cycle mark value> USING <path column>
where all clauses are mandatory.
MariaDB does not support this, but from 10.5.2 permits a non-standard relaxed grammar, as follows:
WITH RECURSIVE ... ( ... ) CYCLE <cycle column list> RESTRICT
With the use of CYCLE ... RESTRICT
it makes no difference whether the CTE uses UNION ALL
or UNION DISTINCT
anymore. UNION ALL
means "all rows, but without cycles", which is exactly what the CYCLE
clause enables. And UNION DISTINCT
means all rows should be different, which, again, is what will happen — as uniqueness is enforced over a subset of columns, complete rows will automatically all be different.
Examples
Below is an example with the WITH
at the top level:
WITH t AS (SELECT a FROM t1 WHERE b >= 'c') SELECT * FROM t2, t WHERE t2.c = t.a;
The example below uses WITH
in a subquery:
SELECT t1.a, t1.b FROM t1, t2 WHERE t1.a > t2.c AND t2.c IN(WITH t AS (SELECT * FROM t1 WHERE t1.a < 5) SELECT t2.c FROM t2, t WHERE t2.c = t.a);
Below is an example of a Recursive CTE:
WITH RECURSIVE ancestors AS ( SELECT * FROM folks WHERE name="Alex" UNION SELECT f.* FROM folks AS f, ancestors AS a WHERE f.id = a.father OR f.id = a.mother ) SELECT * FROM ancestors;
Take the following structure, and data,
CREATE TABLE t1 (from_ int, to_ int); INSERT INTO t1 VALUES (1,2), (1,100), (2,3), (3,4), (4,1); SELECT * FROM t1; +-------+------+ | from_ | to_ | +-------+------+ | 1 | 2 | | 1 | 100 | | 2 | 3 | | 3 | 4 | | 4 | 1 | +-------+------+
Given the above, the following query would theoretically result in an infinite loop due to the last record in t1 (note that max_recursive_iterations is set to 10 for the purposes of this example, to avoid the excessive number of cycles):
SET max_recursive_iterations=10; WITH RECURSIVE cte (depth, from_, to_) AS ( SELECT 0,1,1 UNION DISTINCT SELECT depth+1, t1.from_, t1.to_ FROM t1, cte WHERE t1.from_ = cte.to_ ) SELECT * FROM cte; +-------+-------+------+ | depth | from_ | to_ | +-------+-------+------+ | 0 | 1 | 1 | | 1 | 1 | 2 | | 1 | 1 | 100 | | 2 | 2 | 3 | | 3 | 3 | 4 | | 4 | 4 | 1 | | 5 | 1 | 2 | | 5 | 1 | 100 | | 6 | 2 | 3 | | 7 | 3 | 4 | | 8 | 4 | 1 | | 9 | 1 | 2 | | 9 | 1 | 100 | | 10 | 2 | 3 | +-------+-------+------+
However, the CYCLE ... RESTRICT clause (from MariaDB 10.5.2) can overcome this:
WITH RECURSIVE cte (depth, from_, to_) AS ( SELECT 0,1,1 UNION SELECT depth+1, t1.from_, t1.to_ FROM t1, cte WHERE t1.from_ = cte.to_ ) CYCLE from_, to_ RESTRICT SELECT * FROM cte; +-------+-------+------+ | depth | from_ | to_ | +-------+-------+------+ | 0 | 1 | 1 | | 1 | 1 | 2 | | 1 | 1 | 100 | | 2 | 2 | 3 | | 3 | 3 | 4 | | 4 | 4 | 1 | +-------+-------+------+