Your Data Rarely Comes in One Perfect Table
Published about 1 month ago • 2 min read • DATA FOUNDATIONS
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Why You Need to Understand Joins
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CONNECTING THE DATA
Why You Need to Understand Joins
Real-world information rarely arrives inside one perfect table.
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Real-world data is messy.
Your customers might live in one table. Transactions in another. Products somewhere else.
And the information you actually need? Scattered across all three.
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DATA MODELLING FUNDAMENTALS
What Does a Join Do?
A join connects related datasets using a common field such as:
👤 Customer ID
📦 Product ID
🧾 Order ID
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Once the tables are connected, you can analyse their information together.
That sounds simple—but understanding how those tables interact is one of the most important skills in data analytics and data engineering.
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FOUR JOINS. DIFFERENT RESULTS.
Choose the Wrong One and Your Numbers Can Change
You’ll commonly encounter four types of join:
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đź”— INNER JOIN
Keep only records that have a match in both tables.
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⬅️ LEFT JOIN
Keep everything from the left table and matching records from the right.
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➡️ RIGHT JOIN
Keep everything from the right table and matching records from the left.
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↔️ FULL OUTER JOIN
Keep every record from both tables, whether it has a match or not.
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THE SYNTAX ISN’T THE HARD PART
Understand What Happens After the Join
The real skill is predicting how the join will change your data.
Choose incorrectly and you can:
âś• Lose valid records
âś• Duplicate rows
âś• Inflate totals
âś• Introduce missing values
âś• Change the meaning of your analysis
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The dangerous part is that the final dashboard can still look perfectly reasonable.
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A polished dashboard built on the wrong data model is still wrong.
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THINK BEFORE YOU CONNECT
Ask These Questions First
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1. What does one row represent in each table?
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2. Is the join field unique on either side?
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3. Which unmatched records must be preserved?
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4. Does the final row count make sense?
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LEARN TO THINK BEYOND THE DASHBOARD
Explore Tableau Artisan
Great Tableau developers understand the data underneath the dashboard.
Tableau Artisan helps you move beyond the basics and develop the deeper Tableau skills needed to tackle complex, real-world analytical problems.
Jed
Jellyman Education
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600 1st Ave, Ste 330 PMB 92768, Seattle, WA 98104-2246
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