Clickhouse 查詢之 Array JOIN

Array JOIN 子句允許在數據表的內部，與數組或者嵌套的字段進行JOIN操作，從而將一行數據變多行。適用於行轉列操作。

Clickhouse> create table city(province String, city Array(String)) ENGINE=Log;

CREATE TABLE city
(
    `province` String, 
    `city` Array(String)
)
ENGINE = Log

Ok.

0 rows in set. Elapsed: 0.012 sec. 

Clickhouse> insert into city values('hubei',['wuhan','xiangyang']),('guangdong',['guangzhou','shenzhen','zhuhai']);

insert into city values('beijing',[]),('shanghai',[]);


查看原始數據：

Clickhouse> select * from city FORMAT PrettyCompactMonoBlock;

SELECT *
FROM city
FORMAT PrettyCompactMonoBlock

┌─province──┬─city──────────────────────────────┐
│ hubei     │ ['wuhan','xiangyang']             │
│ guangdong │ ['guangzhou','shenzhen','zhuhai'] │
│ beijing   │ []                                │
│ shanghai  │ []                                │
└───────────┴───────────────────────────────────┘

4 rows in set. Elapsed: 0.003 sec. 



1.INNER ARRAY JOIN:

Clickhouse> select province,city from city array join city;

SELECT 
    province, 
    city
FROM city
ARRAY JOIN city

┌─province──┬─city──────┐
│ hubei     │ wuhan     │
│ hubei     │ xiangyang │
│ guangdong │ guangzhou │
│ guangdong │ shenzhen  │
│ guangdong │ zhuhai    │
└───────────┴───────────┘

5 rows in set. Elapsed: 0.003 sec. 

Clickhouse> select province,city original_city,new_city from city array join city as new_city;

SELECT 
    province, 
    city AS original_city, 
    new_city
FROM city
ARRAY JOIN city AS new_city

┌─province──┬─original_city─────────────────────┬─new_city──┐
│ hubei     │ ['wuhan','xiangyang']             │ wuhan     │
│ hubei     │ ['wuhan','xiangyang']             │ xiangyang │
│ guangdong │ ['guangzhou','shenzhen','zhuhai'] │ guangzhou │
│ guangdong │ ['guangzhou','shenzhen','zhuhai'] │ shenzhen  │
│ guangdong │ ['guangzhou','shenzhen','zhuhai'] │ zhuhai    │
└───────────┴───────────────────────────────────┴───────────┘

5 rows in set. Elapsed: 0.006 sec. 

2. LEFT Array JOIN :

Clickhouse> select province,city original_city,new_city from city LEFT array join city as new_city FORMAT PrettyCompactMonoBlock;

SELECT 
    province, 
    city AS original_city, 
    new_city
FROM city
LEFT ARRAY JOIN city AS new_city
FORMAT PrettyCompactMonoBlock

┌─province──┬─original_city─────────────────────┬─new_city──┐
│ hubei     │ ['wuhan','xiangyang']             │ wuhan     │
│ hubei     │ ['wuhan','xiangyang']             │ xiangyang │
│ guangdong │ ['guangzhou','shenzhen','zhuhai'] │ guangzhou │
│ guangdong │ ['guangzhou','shenzhen','zhuhai'] │ shenzhen  │
│ guangdong │ ['guangzhou','shenzhen','zhuhai'] │ zhuhai    │
│ beijing   │ []                                │           │
│ shanghai  │ []                                │           │
└───────────┴───────────────────────────────────┴───────────┘

7 rows in set. Elapsed: 0.003 sec. 

參考：

https://clickhouse.tech/docs/en/sql-reference/statements/select/array-join/