Data fundamentals
Count the right thing when combining tables.
Reviewed: 2026-10-03
Source-based teaching example · Fictional data
Narration: Microsoft Mark · locally synthesized

00:00:00.000 — What you will learn

Count the right thing when combining tables.

Visual description:
Count the right thing when combining tables.

00:00:05.500 — The idea

Decide what each row represents before counting. Combining tables can create multiple rows for one person. Keep missing values different from zero.

Visual description:
Decide what each row represents before counting. Combining tables can create multiple rows for one person. Keep missing values different from zero.

00:00:18.042 — One useful term

Join. Combining tables using a shared field, such as a person ID.

Visual description:
Combining tables using a shared field, such as a person ID.

00:00:26.042 — A worked example · 1

Person A has two application rows. Person B has none. A joined table can contain two rows for A.

Visual description:
Person A has two application rows. Person B has none. A joined table can contain two rows for A.

00:00:36.333 — A worked example · 2

Counting those rows as people would count A twice.

Visual description:
Counting those rows as people would count A twice.

00:00:42.292 — Your turn to think

Are two application rows always two different people? Pause and decide before continuing.

Visual description:
Are two application rows always two different people?
Pause the video if you need more time.

00:00:59.000 — Why this answer works

No. Count the intended unit, such as unique people or applications. Keep the original tables and record the transformation.

Visual description:
No. Count the intended unit, such as unique people or applications. Keep the original tables and record the transformation.

00:01:10.792 — Check 1

Name what one row represents.

Visual description:
Name what one row represents.

00:01:15.792 — Check 2

Check repeated IDs after a join.

Visual description:
Check repeated IDs after a join.

00:01:21.000 — Check 3

Preserve missing values and source records.

Visual description:
Preserve missing values and source records.

00:01:26.750 — Now try a different case

Open the lesson and try its different practice case. Use every check in the full task. Watching alone does not mark the lesson complete.

Visual description:
Open the lesson
Try the new case
Check and revise
The full task and its requirements are in the linked lesson.