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Data fundamentals
Count the right thing when combining tables.
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Data fundamentals
Count the right thing when combining tables.
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Source-based teaching example · Fictional data · Reviewed 2026-10-03. Microsoft Mark · locally synthesized. On a small screen, use fullscreen for diagram details or read the transcript.
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0:00 · What you will learn
Count the right thing when combining tables.
What appears on screen
Count the right thing when combining tables.
0:05 · The idea
Decide what each row represents before counting. Combining tables can create multiple rows for one person. Keep missing values different from zero.
What appears on screen
Decide what each row represents before counting. Combining tables can create multiple rows for one person. Keep missing values different from zero.
0:18 · One useful term
Join. Combining tables using a shared field, such as a person ID.
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Combining tables using a shared field, such as a person ID.
0:26 · A worked example · 1
Person A has two application rows. Person B has none. A joined table can contain two rows for A.
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Person A has two application rows. Person B has none. A joined table can contain two rows for A.
0:36 · A worked example · 2
Counting those rows as people would count A twice.
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Counting those rows as people would count A twice.
0:42 · Your turn to think
Are two application rows always two different people? Pause and decide before continuing.
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Are two application rows always two different people? Pause the video if you need more time.
0:59 · Why this answer works
No. Count the intended unit, such as unique people or applications. Keep the original tables and record the transformation.
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No. Count the intended unit, such as unique people or applications. Keep the original tables and record the transformation.
1:10 · Check 1
Name what one row represents.
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Name what one row represents.
1:15 · Check 2
Check repeated IDs after a join.
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Check repeated IDs after a join.
1:21 · Check 3
Preserve missing values and source records.
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Preserve missing values and source records.
1:26 · 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.
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Open the lesson Try the new case Check and revise The full task and its requirements are in the linked lesson.
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