是否可以在 Confluent KSQL 查询中使用多个左连接？ 试图加入超过 1 个表的流，如果没有，那么解决方案是什么？

Question

Stream :

describe ammas;

 Field   | Type                        
-------------------------------------  
 ROWTIME | BIGINT           (system)  
 ROWKEY  | VARCHAR(STRING)  (system)   
 ID      | INTEGER                      
 -------------------------------------

For runtime statistics and query details run: DESCRIBE EXTENDED <Stream,Table>;

Table-01 :

ksql> show tables;

Table Name | Kafka Topic | Format    | Windowed 
-------------------------------------------------
ANNAT      | anna        | DELIMITED | false    
APPAT      | appa        | DELIMITED | false    
-------------------------------------------------

Trying to Join stream vs. table-01 is working as expected. Create stream finalstream as select a.id from ammas a left join appat b on a.id = b.id where b.id is null .

But when I tried to join more than one table with stream based on the following query:

ksql> SELECT * FROM ammas cd LEFT JOIN appat ab ON ab.id = cd.id LEFT JOIN annat aa ON aa.id =cd.id;
ServerError:io.confluent.ksql.parser.exception.ParseFailedException
Caused by: null

What is going wrong? Thanks.

Answer 1

KSQL currently (v5.3) only supports a single join operation per statement. If you want to perform multiple joins you have to "daisy-chain" them in multiple statements, eg

CREATE STREAM out1 AS 
  SELECT * FROM ammas cd 
           LEFT JOIN appat ab 
             ON ab.id = cd.id 

CREATE STREAM out_final AS 
  SELECT * FROM out1 o 
           LEFT JOIN annat aa ON o.id =cd.id;

---

Update November 2020: ksqlDB supports multi-way joins were added in ksqlDB v0.9. Confluent Platform 6.0 includes ksqlDB 0.10.2 and therefore also includes this functionality. See the release blog and documentation for details.