Lab 1 · Section 3 of 6

Join and check the tables

Part 3 · TODO 2 and merge checks

First · connect to the lecture

many_to_one is the lecture's key rule in code. Unique student keys on the right prevent registrations from multiplying.

2.8 · Joins: predict the result before running it 2.9 · Lab 1: what you should be able to explain 4.2 · Inventory before editing

Week 1: joins, PDF pp. 42–53; Week 3 inventory is a later connection

Your task

Predict the joined row count. Complete TODO 2, run the merge and inspect both assertions.

Evidence to include

Give before/after row counts, the final row meaning, and what duplicate right-side keys could cause.

Notebook questions and code

Read here; run in your downloaded notebook. Start with its setup cells and run in order. Use your own ID and outputs where requested.

Download Lab 1 notebook

Part 3: Connect the tables

We want one row per course registration, plus the student's programme and study mode.

enrolments stays on the left because it already has one row per registration.
students supplies extra student information.
many_to_one means many enrolment rows can match one student row.

Predict: The merge should have the same number of rows as enrolments. Why?

Python · cell 13 — read here, run in your notebook

# TODO 2 - Complete the merge by replacing the blank with one_student_table.
# Do not change on, how, or validate.

joined = enrolments.merge(
    _____[["student_id", "programme", "study_mode"]],
    on="student_id",
    how="left",
    validate="many_to_one",
)

joined.head()

Python · cell 14 — read here, run in your notebook

print("Rows before merge:", len(enrolments))
print("Rows after merge: ", len(joined))
assert len(joined) == len(enrolments), "Unexpected row change: stop and ask for help."
assert joined["programme"].notna().all(), "A student did not match: stop and ask for help."

print("Check passed: one row still represents one student-course registration.")

Explain after connecting

After the merge, what does one row in joined represent?
Why does the row count stay the same?
What would it mean if the merge created twice as many rows?
Why is many_to_one appropriate here?

Answer each in one short sentence.

What did the lab teach you about the lecture?

Saved only in this browser. This note doesn’t fill in the downloaded notebook; “reviewed” is a personal checklist, not a grade.