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# Common Table Expressions (CTE)
## 1. Overview
CTE (Common Table Expressions) supports defining one or more temporary result sets (called common tables) using the `WITH` clause. These result sets can be referenced multiple times in subsequent parts of the same query. CTE provides a clean way to construct complex queries, making SQL code more readable and maintainable.
> Note: This feature is available since version 2.0.10.
## 2. Syntax Definition
The simplified SQL syntax for CTE is as follows:
```sql
with_clause:
WITH cte_name [(col_name [, col_name] ...)] AS (subquery)
[, cte_name [(col_name [, col_name] ...)] AS (subquery)] ...
```
- Supports simple and nested CTEs: One or more CTEs can be defined in a `WITH` clause, and CTEs can reference each other in a nested way (forward references are **not** allowed, meaning a CTE cannot reference another CTE that has not yet been defined).
- Name conflict between CTE and source table: If a CTE has the same name as a source table, only the CTE is visible in the outer scope, and the source table is shadowed.
- Multiple references to CTE: The same CTE can be referenced multiple times in the outer query.
- EXPLAIN / EXPLAIN ANALYZE support: `EXPLAIN` or `EXPLAIN ANALYZE` can be used on the entire query, but **not** on the `subquery` inside a CTE definition.
- Column count constraint: The number of column names specified in a CTE definition must match the number of output columns from the `subquery`, otherwise an error will be thrown.
- Unused CTE: A query can still execute normally if a defined CTE is not referenced in the main query body.
## 3. Examples
Using tables `table1` and `table2` from the [Sample Data](../Reference/Sample-Data.md) as source tables:
### 3.1 Simple CTE
```sql
WITH cte1 AS (SELECT device_id, temperature FROM table1 WHERE temperature IS NOT NULL),
cte2 AS (SELECT device_id, humidity FROM table2 WHERE humidity IS NOT NULL)
SELECT * FROM cte1 JOIN cte2 ON cte1.device_id = cte2.device_id LIMIT 10;
```
Result
```
+---------+-----------+---------+--------+
|device_id|temperature|device_id|humidity|
+---------+-----------+---------+--------+
| 100| 90.0| 100| 45.1|
| 100| 90.0| 100| 35.2|
| 100| 90.0| 100| 35.1|
| 100| 85.0| 100| 45.1|
| 100| 85.0| 100| 35.2|
| 100| 85.0| 100| 35.1|
| 100| 85.0| 100| 45.1|
| 100| 85.0| 100| 35.2|
| 100| 85.0| 100| 35.1|
| 100| 88.0| 100| 45.1|
+---------+-----------+---------+--------+
Total line number = 10
It costs 0.075s
```
### 3.2 CTE with the Same Name as Source Table
```sql
WITH table1 AS (SELECT time, device_id, temperature FROM table1 WHERE temperature IS NOT NULL)
SELECT * FROM table1 LIMIT 5;
```
Result
```
+-----------------------------+---------+-----------+
| time|device_id|temperature|
+-----------------------------+---------+-----------+
|2024-11-30T09:30:00.000+08:00| 101| 90.0|
|2024-11-30T14:30:00.000+08:00| 101| 90.0|
|2024-11-29T10:00:00.000+08:00| 101| 85.0|
|2024-11-27T16:39:00.000+08:00| 101| 85.0|
|2024-11-27T16:40:00.000+08:00| 101| 85.0|
+-----------------------------+---------+-----------+
Total line number = 5
It costs 0.103s
```
### 3.3 Nested CTE
```sql
WITH
table1 AS (SELECT device_id, temperature FROM table1 WHERE temperature IS NOT NULL),
cte1 AS (SELECT device_id, temperature FROM table2 WHERE temperature IS NOT NULL),
table2 AS (SELECT temperature FROM table1),
cte2 AS (SELECT temperature FROM table1)
SELECT * FROM table2;
```
Result
```
+-----------+
|temperature|
+-----------+
| 90.0|
| 90.0|
| 85.0|
| 85.0|
| 85.0|
| 85.0|
| 90.0|
| 85.0|
| 85.0|
| 88.0|
| 90.0|
| 90.0|
+-----------+
Total line number = 12
It costs 0.050s
```
- Forward references are **not** supported
```sql
WITH
cte2 AS (SELECT temperature FROM cte1),
cte1 AS (SELECT device_id, temperature FROM table1)
SELECT * FROM cte2;
```
Error message
```
Msg: org.apache.iotdb.jdbc.IoTDBSQLException: 550: Table 'database1.cte1' does not exist.
```
### 3.4 Multiple References to CTE
```sql
WITH cte AS (SELECT device_id, temperature FROM table1 WHERE temperature IS NOT NULL)
SELECT * FROM cte WHERE temperature > (SELECT avg(temperature) FROM cte);
```
Result
```
+---------+-----------+
|device_id|temperature|
+---------+-----------+
| 101| 90.0|
| 101| 90.0|
| 100| 90.0|
| 100| 88.0|
| 100| 90.0|
| 100| 90.0|
+---------+-----------+
Total line number = 6
It costs 0.203s
```
### 3.5 EXPLAIN Support
- Supported on the entire query
```sql
EXPLAIN WITH cte AS (SELECT * FROM table1) SELECT * FROM cte;
```
Result
```
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| distribution plan|
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐ |
| │OutputNode-7 │ |
| │OutputColumns-[time, region, plant_id, device_id, model_id, maintenance, temperature, humidity, status, arrival_time] │ |
| │OutputSymbols: [time, region, plant_id, device_id, model_id, maintenance, temperature, humidity, status, arrival_time]│ |
| └──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘ |
| │ |
| │ |
| ┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐ |
| │Collect-42 │ |
| │OutputSymbols: [time, region, plant_id, device_id, model_id, maintenance, temperature, humidity, status, arrival_time]│ |
| └──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘ |
| ┌───────────────────────────────────────────────────────────┴────────────────────────────────────────────────────────────┐ |
| │ │ |
| ┌───────────┐ ┌───────────┐ |
| │Exchange-49│ │Exchange-50│ |
| └───────────┘ └───────────┘ |
| │ │ |
| │ │ |
|┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐ ┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐|
|│DeviceTableScanNode-41 │ │DeviceTableScanNode-40 │|
|│QualifiedTableName: database1.table1 │ │QualifiedTableName: database1.table1 │|
|│OutputSymbols: [time, region, plant_id, device_id, model_id, maintenance, temperature, humidity, status, arrival_time]│ │OutputSymbols: [time, region, plant_id, device_id, model_id, maintenance, temperature, humidity, status, arrival_time]│|
|│DeviceNumber: 3 │ │DeviceNumber: 3 │|
|│ScanOrder: ASC │ │ScanOrder: ASC │|
|│PushDownOffset: 0 │ │PushDownOffset: 0 │|
|│PushDownLimit: 0 │ │PushDownLimit: 0 │|
|│PushDownLimitToEachDevice: false │ │PushDownLimitToEachDevice: false │|
|│RegionId: 2 │ │RegionId: 1 │|
|└──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘ └──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘|
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
Total line number = 29
It costs 0.065s
```
- Not supported for internal queries of CTE
```sql
WITH cte AS (EXPLAIN SELECT * FROM table1) SELECT * FROM cte;
```
Error message
```
Msg: org.apache.iotdb.jdbc.IoTDBSQLException: 700: line 1:14: mismatched input 'EXPLAIN'. Expecting: <query>
```