A hierarchical query is a type of SQL query that handles hierarchical model data. They are special case of more general recursive fixpoint queries, which compute transitive closures.
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In standard SQL:1999 hierarchical queries are implemented by way of recursive common table expressions (CTEs). Unlike the Oracle extension described below, the recursive CTEs were designed with fixpoint semantics from the beginning. The recursive CTEs from the standard were relatively close to the existing implementation in IBM DB2 version 2. Recursive CTEs are also supported by Microsoft SQL Server (since SQL Server 2008 R2), Firebird 2.1, PostgreSQL 8.4+, SQLite 3.8.3+, Oracle 11g Release 2, IBM Informix version 11.50+ and CUBRID. Tableau and TIBCO Spotfire do not support CTEs.
An alternative syntax is the non-standard CONNECT BY construct; it was introduced by Oracle in the 1980s. Prior to Oracle 10g, the construct was only useful for traversing acyclic graphs because it returned an error on detecting any cycles; in version 10g Oracle introduced the NOCYCLE feature (and keyword), making the traversal work in the presence of cycles as well.
Without Common-table-expressions or a connected-by clause it is possible to achieve hierarchical queries with user-defined recursive functions.
CONNECT BY
CONNECT BY is supported by EnterpriseDB, Oracle database, CUBRID, IBM Informix and DB2 although only if it is enabled as a compatibility mode. The syntax is as follows:
The output from the above query would look like:
level | employee | empno | manager -------+-------------+-------+--------- 1 | KING | 7839 | 2 | JONES | 7566 | 7839 3 | SCOTT | 7788 | 7566 4 | ADAMS | 7876 | 7788 3 | FORD | 7902 | 7566 4 | SMITH | 7369 | 7902 2 | BLAKE | 7698 | 7839 3 | ALLEN | 7499 | 7698 3 | WARD | 7521 | 7698 3 | MARTIN | 7654 | 7698 3 | TURNER | 7844 | 7698 3 | JAMES | 7900 | 7698 2 | CLARK | 7782 | 7839 3 | MILLER | 7934 | 7782 (14 rows)Pseudo-columns
Unary operators
The following example returns the last name of each employee in department 10, each manager above that employee in the hierarchy, the number of levels between manager and employee, and the path between the two:
Functions
SYS_CONNECT_BY_PATHCommon table expression
A Common Table Expression, or CTE, (in SQL) is a temporary named result set, derived from a simple query and defined within the execution scope of a SELECT, INSERT, UPDATE, or DELETE statement.
CTEs can be thought of as alternatives to derived tables (subquery), views, and inline user-defined functions.
Common table expressions are supported by Teradata, DB2, Firebird, Microsoft SQL Server, Oracle (with recursion since 11g release 2), PostgreSQL (since 8.4), MariaDB (since 10.2), SQLite (since 3.8.3), HyperSQL and H2 (experimental). Oracle calls CTEs "subquery factoring". Though MySQL does not support common table expressions there exists a demonstration of WITH RECURSIVE emulation using stored procedures.
The syntax for a Recursive CTE is as follows:
where with_query‘s syntax is:
Recursive CTEs (or "recursive subquery factoring" in Oracle jargon) can be used to traverse relations (as graphs or trees) although the syntax is much more involved because there are no automatic pseudo-columns created (like LEVEL above); if these are desired, they have to be created in the code. See MSDN documentation or IBM documentation for tutorial examples.
The RECURSIVE keyword is not usually needed after WITH in systems other than PostgreSQL.
In SQL:1999 a recursive (CTE) query may appear anywhere a query is allowed. It's possible, for example, to name the result using CREATE [RECURSIVE] VIEW. Using a CTE inside an INSERT INTO, one can populate a table with data generated from a recursive query; random data generation is possible using this technique without using any procedural statements.
An example of a recursive query computing the factorial of numbers from 0 to 9 is the following: