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Joining Customer and Order Tables

EasySQL & Data Manipulation00:00
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Your question is Joining Customer and Order Tables. Take a moment with it on the right.

Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).

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Problem

Context

In OpenText analytics work, data is often split across related tables in PostgreSQL, such as customers, orders, or records from OpenText Content Cloud reporting tables. To answer business questions, you need to combine those tables correctly.

Core question

Explain how you would use SQL to join two tables. In your answer, describe how a join matches rows, what role the join condition plays, and when you would choose an INNER JOIN versus a LEFT JOIN. Also explain what happens when a row in one table has no matching row in the other table, and how that affects the result.

You may use a simple example such as joining a customers table to an orders table by customer_id.

Scope guidance

The interviewer is looking for a clear explanation of basic join mechanics in PostgreSQL, not advanced optimization or multi-table query design. A strong answer should include one or two short SQL examples and mention common mistakes such as missing join conditions or filtering away LEFT JOIN results unintentionally.