Table Joins
  • 17 Aug 2021
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Table Joins

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Overview

When writing query joins in SQL lab, there are several restrictions:

  1. Joined tables must be from the same database.
  2. Joined tables can be from different schemas within a database.
  3. Joined tables can be from the same schema within a database.

Examples

Example for Restriction 1

There exists a table called public.example_one in a Postgresql database called post and another table called public.example_two in an Athena database called ath.

A SQL query such as:

SELECT * FROM post.public.example_one 
JOIN ath.public.example_two ON example_one.id = example_two.id

...will be invalid. Even if the two databases were both Postgresql databases, you will not be able to join. Tables must originate from the same database.

Example for Restriction 2

There exists a table called schema_one.example_one and another table called schema_two.example_two in the same database.

A SQL query such as:

SELECT * FROM schema_one.example_one
JOIN schema_two.example_two ON example_one.id = example_two.id

...will be valid.

Example for Restriction 3

There exists a table called public.example_one and another table called public.example_two in the same database.

A SQL query such as:

SELECT * FROM public.example_one
JOIN public.example_two ON example_one.id = example_two.id

...will be valid.

Specify the database you are using in SQL Lab

From the SQL Editor, in the Database field, ensure that you specify the database that contains your tables/schemas.

Screen Shot 2021-08-15 at 10.48.37 PM.png


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