selecting on two tables in Cassandra

问题

I use Cassandra for a project, and it's my first project. , and I'm trying to do a simple request on two tables, but that doesn't work...

I want to do something like:

Select * from table1, table2 where table1.test = test and table2.test2 = 123;

Is it possible to request on two tables in Cassandra? And how can I do that?

Thanks

回答1:

I'm trying to do a simple request on two tables

What you're trying to do is known as a "distributed join" and Cassandra is specifically designed to prevent you from doing this.

The way to solve these types of problems, is with a process called denormalization. Let's say you have simple two tables carMake and carModel:

 makeid | make
--------+--------
      1 |  Chevy
      2 |  Dodge
      3 |   Ford

 modelid | makeid | model
---------+--------+---------
      15 |      3 |   Focus
      11 |      3 | Mustang
      32 |      2 | Charger
      82 |      3 |  Fusion

Now, in a traditional RDBMS if I wanted to SELECT all car models with a make of "Ford" I would execute a JOIN query. But with Cassandra, the idea is to solve this problem at the modeling stage, by building a table which supports the ability to query make and model of a car at the same time:

CREATE TABLE carMakeModel (
    carid int,
    make text,
    model text,
    PRIMARY KEY (make,carid));

aploetz@cqlsh:stackoverflow> SELECT * FROM carMakeModel WHERE make='Ford';

 make | carid | model
------+-------+---------
 Ford |     1 | Mustang
 Ford |     2 |   Focus
 Ford |     3 |  Fusion

(3 rows)

Some key points to note here:

make is duplicated as much as is necessary. You'll notice that "Ford" is specified 3 times in the result set. If you had data for 13 models of Fords, you would store the value of "Ford" 13 times.
PRIMARY KEYs in Cassandra are unique. I have carid added as a part of the PRIMARY KEY to ensure uniqueness for each model, otherwise an INSERT for each make would overwrite itself.

来源：https://stackoverflow.com/questions/33981408/selecting-on-two-tables-in-cassandra

标签

cassandra

request

cql