In software engineering, it is common practice to group instructions as small and easily comprehensible units—namely functions or methods. This makes the code reusable and improves readability.
Even though SQL has functions and procedures as well, they are not the right tools for building easily understandable and reusable units. In SQL, neither functions nor procedures are first-class citizens in the same way that subqueries are.0 The building block of SQL are queries1—not instructions.
To make queries reusable, SQL-92 introduced views. Once created, a view has a name in the database schema so that other queries can use it like a table.
SQL:1999 added the
with clause to define “statement scoped views”. They are not stored in the database schema: instead, they are only valid in the query they belong to. This makes it possible to improve the structure of a statement without polluting the global namespace.
with clause is an optional prefix for
WITH query_name (column_name1, ...) AS (SELECT ...) SELECT ...
The syntax after the keyword
with is the same as it is for
create view: it starts with the query name, and optionally3 and in parenthesis the name of the columns it returns. The keyword
as finally introduces the definition itself (the query)—again in parentheses.
With is not a stand alone command like
create view is: it must be followed by
select. This query (and subqueries it contains) can refer to the just defined query name in their
with clause can introduce multiple query names by separating them with a comma (the
with keyword is not repeated). Each of these queries can refer to the query names previously defined within the same
with clause4 (an often neglected rule—see Compatibility):
WITH query_name1 AS ( SELECT ... ) , query_name2 AS ( SELECT ... FROM query_name1 ... ) SELECT ...
Query names defined using
with mask existing tables or views with the same name.5
Most databases process
with-queries in the same way that they process views: they replace the reference to the query by its definition and optimize the overall query.
The PostgreSQL database is different in this regard: it optimizes each
with query and the main statement independent of each other.
with query is referred to multiple times, some databases cache (i.e. “materialize”) its result to prevent double execution.
Read more about this in “
with Clause: Performance Impacts”.
with clause was introduced with SQL:1999 as a set of optional features. Leaving the
recursive variant aside, the features are T121 for top-level
with clauses and T122 for
with clauses in subqueries.
The basic functionality of
with is well-supported. The single area where products behave differently is name resolution. It is especially noteworthy that
with is often treated like
with recursive.6 The more subtle incompatibilities are related to qualified table names (
schema.table cannot refer to a
with query)7 and views used in the scope of
with queries (the query inside the view does not “see” the outer
Views can cover some of the use cases. However, this can easily lead to an unreasonable number of views (“namespace pollution”). In those cases, subqueries are often the better option.
with as DML prefix (PostgreSQL, SQL Server, SQLite)
SQL Server can also use a
with query as a target for DML statements (basically building an updatable view).
The Oracle Database supports function and procedure declarations within the
with clause since version 12cR1 (documentation).
Starting with 9.1, the PostgreSQL database supports DML statements (
delete) within the body of
with queries (documentation). When using the (also proprietary)
returning clause of the DML statement, the
with query actually returns data (e.g. the rows just inserted).