Common table expressions|V4.3.5| docs|Distributed Database

Common table expressions

Last Updated：2025-12-05 01:38:29 Updated

Common table expressions (CTEs) are named temporary result sets that are valid only for the current statement and are not stored as objects. They are used only during query execution. Unlike derived tables, CTEs can be self-referencing and can be referenced multiple times within the same query.

Scenarios:

CTEs can be used to reuse the same subquery in multiple places, avoiding the need to write the same logic repeatedly.
They can simplify recursive queries, such as those used to find hierarchical data.
CTEs can break down complex queries into smaller parts, making the query logic clearer and easier to understand.

OceanBase Database supports both nonrecursive and recursive CTEs.

CTE syntax

Common table expressions (CTEs) are optional parts of DML statement syntax and are defined using the WITH clause. If the WITH clause contains multiple clauses, they can be separated by commas. Each clause provides a subquery to generate a result set and associates the subquery with a name. The syntax is as follows:

 WITH [RECURSIVE]
      cte_name [(column_name [, column_name] ...)] AS (subquery)
      [, cte_name [(column_name [, column_name] ...)] AS (subquery)] ...

Parameters

Parameter	Description
`[RECURSIVE]`	An optional keyword used to specify whether to create a recursive CTE. Recursive CTE: Specify the `RECURSIVE` keyword. A recursive CTE refers to itself within the CTE and is used to process data with a recursive structure, such as trees or graphs. A recursive CTE includes a base query (starting condition) in its definition and performs recursive operations on the results of this base query until a stopping condition is met. Non-recursive CTE: Do not specify the `RECURSIVE` keyword. A non-recursive CTE does not refer to itself and is used to build a temporary result set that does not involve recursive operations.
`cte_name`	The name of the CTE, which can be referenced by a table containing the `WITH` clause.
`column_name`	The list of selected column names used to specify aliases for columns in the CTE. This allows you to use more readable column names in the main query.
`AS(subquery)`	The CTE subquery used to generate the CTE result set. The `AS` keyword must be followed by parentheses.

If a list of names in parentheses follows the CTE name, these names are column names and must match the number of columns in the SELECT statement of the CTE. If no column names are specified, the column names will be derived from the AS(subquery) part, specifically from the first SELECT list.

Applicable scenarios for the WITH clause

The WITH clause can be used in the following scenarios:

At the beginning of a SELECT statement.
```
WITH ... SELECT ...
```

At the beginning of a subquery (including derived table subqueries).

SELECT ... WHERE id IN (WITH ... SELECT ...) ...
SELECT * FROM (WITH ... SELECT ...) AS dt ...

Immediately before a statement that contains a SELECT statement.

INSERT ... WITH ... SELECT ...
REPLACE ... WITH ... SELECT ...
CREATE TABLE ... WITH ... SELECT ...
CREATE VIEW ... WITH ... SELECT ...

Only one WITH clause is allowed at the same level. If the WITH clause contains multiple clauses, they can be separated by commas.

WITH cte1 AS (...), cte2 AS (...) SELECT ...

The WITH clause can define one or more CTEs, but each CTE name must be unique within the clause. The following example is invalid:

WITH cte1 AS (...), cte1 AS (...) SELECT ...

Structure of a recursive CTE

A recursive CTE has the following structure:

If a CTE in the WITH clause refers to itself, the WITH clause must start with WITH RECURSIVE. Otherwise, the RECURSIVE keyword is not required.
A recursive CTE subquery consists of two parts separated by UNION [ALL | DISTINCT]:
Note
- UNION or UNION DISTINCT: Merges the result sets of two or more SELECT statements into a single set and removes duplicate rows.
- UNION ALL: Merges the result sets of two or more SELECT statements into a single set without removing duplicate rows.
SELECT ... -- Returns the initial row set UNION ALL SELECT ... -- Returns additional rows The first SELECT generates one or more initial rows for the CTE and does not reference the CTE name. The second SELECT generates additional rows and performs a recursive operation by referencing the CTE name in its FROM clause. The recursion stops when this part no longer generates new rows. Therefore, a recursive CTE consists of a non-recursive SELECT part and a recursive SELECT part. Each SELECT part can be a union of multiple SELECT statements.

 The data types of the result columns of the CTE are inferred only from the columns in the non-recursive SELECT part, and all columns can be nullable. The types from the recursive SELECT part are ignored.
 Each iteration of the recursive part operates only on the rows generated in the previous iteration. If the recursive part contains multiple query blocks, each query block's iterations are scheduled in an unspecified order, and each query block operates on the rows generated in its previous iteration or by other query blocks since the last iteration.

 Here is an example:
 WITH RECURSIVE cte1 (n) AS
(
  SELECT 1                /* Non-recursive part that retrieves a single row to generate the initial row set */
  UNION ALL
  SELECT n + 2 FROM cte1 WHERE n < 10     /* Recursive part that generates a new value greater than the previous n value by 2, until n is not less than 10 */
)
SELECT * FROM cte1;
 Limitations
 The following syntax constraints apply to recursive CTE subqueries:
    
     The recursive SELECT part cannot contain the following structures:
      
       Aggregate functions, such as SUM()
       Window functions
       GROUP BY
       ORDER BY
       DISTINCT
      
     The recursive SELECT part must reference the subquery in its FROM clause only once. It can reference tables other than the CTE and join with the CTE. If such a join is used, the CTE cannot be on the right side of a LEFT JOIN.
    
 For recursive CTEs, the cost estimates displayed by EXPLAIN represent the cost of each iteration, which may differ significantly from the total cost. The optimizer cannot predict the number of iterations because it cannot determine when the WHERE clause will become false.
    
     The recursive SELECT and non-recursive SELECT parts cannot contain the LIMIT structure.
    
 Examples
 The following example creates a student table with columns for student ID, name, and mentor ID, to demonstrate the difference between recursive and non-recursive CTEs.
 First, we create the student table and insert some data:
 obclient> CREATE TABLE student (
    student_id INT PRIMARY KEY,
    name VARCHAR(100),
    mentor_id INT,
    FOREIGN KEY (mentor_id) REFERENCES student(student_id)
);
Query OK, 0 rows affected

obclient> INSERT INTO student (student_id, name, mentor_id) VALUES
(1, 'Alice', NULL),
(2, 'Bob', 1),
(3, 'Charlie', 1),
(4, 'David', 2),
(5, 'Eve', 3);
Query OK, 5 rows affected
 In this data model, Alice is the top-level student with no mentor. Bob and Charlie are Alice's students, David is Bob's student, and Eve is Charlie's student.
 Non-recursive CTE example
 A non-recursive CTE does not reference itself. For example, if we want to select all of Alice's direct students (the first-level students), we can use a non-recursive CTE like this:
 WITH Alice_Students AS (
    SELECT * FROM student WHERE mentor_id = 1
)
SELECT * FROM Alice_Students;
 The result will be:
 +------------+---------+-----------+
| student_id | name    | mentor_id |
+------------+---------+-----------+
|          2 | Bob     |         1 |
|          3 | Charlie |         1 |
+------------+---------+-----------+
2 rows in set
 Recursive CTE example
 Now, if we want to find all of Alice's direct and indirect students (all levels of students), we need to use a recursive CTE. A recursive CTE references itself to query down each level.
 WITH RECURSIVE Student_Hierarchy AS (
    -- Anchor member: Select Alice as the starting point
    SELECT student_id, name, mentor_id FROM student WHERE mentor_id IS NULL
    UNION ALL
    -- Recursive member: Select direct students based on the ID of the previous level's student
    SELECT s.student_id, s.name, s.mentor_id
    FROM student s
    INNER JOIN Student_Hierarchy sh ON s.mentor_id = sh.student_id
)
SELECT * FROM Student_Hierarchy;
 This recursive CTE first selects Alice as the starting point and then recursively selects each student's direct students until there are no more students.
 The result will be:
 +------------+---------+-----------+
| student_id | name    | mentor_id |
+------------+---------+-----------+
|          1 | Alice   |      NULL |
|          3 | Charlie |         1 |
|          2 | Bob     |         1 |
|          5 | Eve     |         3 |
|          4 | David   |         2 |
+------------+---------+-----------+
5 rows in set
 In this example, the recursive CTE allows us to traverse the student hierarchy, while the non-recursive CTE only selects students at a fixed level.

Common table expressions

CTE syntax

Parameters

Applicable scenarios for the WITH clause

Structure of a recursive CTE

Note

Limitations

Examples

Non-recursive CTE example

Recursive CTE example