Common Table Expressions

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New in v2.0: 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

Overview

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 o AS (SELECT * FROM orders WHERE id IN (33, 542, 112))
  SELECT *
    FROM customers AS c, o
   WHERE o.customer_id = c.id;

In this example, the WITH clause defines the temporary name o for the subquery over orders, 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 customers AS c, (SELECT * FROM orders WHERE id IN (33, 542, 112)) AS o
   WHERE o.customer_id = c.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 o       AS (SELECT * FROM orders WHERE id IN (33, 542, 112)),
       results AS (SELECT * FROM customers AS c, o WHERE o.customer_id = c.id)
  SELECT * FROM results;

In this example, the second CTE results refers to the first CTE o 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 a AS (SELECT * FROM (WITH b AS (SELECT * FROM c)
                            SELECT * FROM b))
  SELECT * FROM a;

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 a AS (TABLE x),
       b AS (WITH a AS (TABLE y)
             SELECT * FROM a)
  SELECT * FROM b;

In this example, the inner subquery SELECT * FROM a will select from table y (closest WITH clause), not from table x.

Data Modifying Statements

It is possible to use a data-modifying statement (INSERT, DELETE, etc.) as a common table expression.

For example:

> WITH v AS (INSERT INTO t(x) VALUES (1), (2), (3) RETURNING x)
  SELECT x+1 FROM v

However, the following restriction applies: only WITH sub-clauses at the top level of a SQL statement can contain data-modifying statements. The example above is valid, but the following is not:

> SELECT x+1 FROM
    (WITH v AS (INSERT INTO t(x) VALUES (1), (2), (3) RETURNING x)
     SELECT * FROM v);

This is not valid because the WITH clause that defines an INSERT common table expression is not at the top level of the query.

Known Limitations

Referring to a CTE by name more than once

It is currently not possible to refer to a common table expression by name more than once.

For example, the following query is invalid because the CTE a is referred to twice:

> WITH a AS (VALUES (1), (2), (3))
  SELECT * FROM a, a;

Using CTEs with data-modifying statements

If a common table expression containing data-modifying statement is not referred to by the top level query, either directly or indirectly, the data-modifying statement will not be executed at all.

For example, the following query does not insert any row, because the CTE a is not used:

> WITH a AS (INSERT INTO t(x) VALUES (1), (2), (3))
  SELECT * FROM b;

Also, the following query does not insert any row, even though the CTE a is used, because the other CTE that uses a are themselves not used:

> WITH a AS (INSERT INTO t(x) VALUES (1), (2), (3)),
       b AS (SELECT * FROM a)
  SELECT * FROM c;

To determine whether a modification will effectively take place, use EXPLAIN and check whether the desired data modification is part of the final plan for the overall query.

Using CTEs with views

It is not yet possible to use a common table expression inside the selection query used to define a view.

Using CTEs with VALUES clauses

It is not yet possible to use a common table expression define outside of a VALUES clause in a subquery inside the VALUES clause, for example:

> WITH a AS (...) VALUES ((SELECT * FROM a));

Using CTEs with Set Operations

It is not yet possible to use a common table expression defined outside of a set expression in the right operand of a set operator, for example:

> WITH a AS (SELECT 1)
  SELECT * FROM users UNION SELECT * FROM a; -- "a" used on the right, not yet supported.

For UNION, you can work around this limitation by swapping the operands. For the other set operators, you can inline the definition of the CTE inside the right operand.

See also

• Subqueries
• Selection Queries
• Table Expressions
• EXPLAIN

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