First and Last Call On the Same Day

First and last call of the day with the same person

Calls(caller_id, recipient_id, call_time). Report users whose first and last call on a day were with the same other person, counting a user whether they were the caller or the recipient. The earlier write-up contradicted itself (its Step-3 set listed user 3 as qualifying, but the final output silently dropped it). The clean approach: unpivot each call into two "(user, partner, time)" rows, then compare each user's first-of-day partner with their last-of-day partner.

WITH bidir AS (                       -- each call seen from both participants' side
    SELECT caller_id    AS user_id, recipient_id AS partner, call_time FROM Calls
    UNION ALL
    SELECT recipient_id AS user_id, caller_id    AS partner, call_time FROM Calls
),
ends AS (
    SELECT DISTINCT user_id,
           FIRST_VALUE(partner) OVER (PARTITION BY user_id, DATE(call_time) ORDER BY call_time)            AS first_partner,
           FIRST_VALUE(partner) OVER (PARTITION BY user_id, DATE(call_time) ORDER BY call_time DESC)        AS last_partner
    FROM bidir
)
SELECT DISTINCT user_id
FROM ends
WHERE first_partner = last_partner;

Why this is correct

• Unpivot with UNION ALL so a user is considered on every call they took part in, as caller or recipient — exactly what "regardless of being the caller or the recipient" requires.
• FIRST_VALUE(... ORDER BY call_time) gives the day's first partner; the same with DESC gives the last. Equal → the day's bookend calls were with the same person.
• A day with a single call trivially qualifies (first = last), which is the intended semantics.

Pitfalls

• If two calls share the exact call_time, the "first/last partner" tie is resolved arbitrarily — add a deterministic tiebreaker (e.g. , partner) if your data allows identical timestamps.
• Don't join first/last calls back by matching on call_time equality (the old approach) — it breaks on duplicate timestamps and silently loses rows.

Takeaways

• Unpivot caller/recipient with UNION ALL to treat a user symmetrically.
• FIRST_VALUE ascending vs descending over (user, day) gives first/last partner in one pass.

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Re-authored for correctness for this guide (the prior worked example contradicted its own output). See also: Window Functions, UNION, Date Functions.

🎯 STRICT STANDOUT: Why / worked / when-not / failure / drills — First and Last Call On the Same Day

Why this concept exists (judgment chain)

Symmetric “user as caller or recipient” requires unpivoting edges with UNION ALL. FIRST_VALUE asc vs desc over (user, day) compares bookend partners without fragile time-equality joins.

Worked example with numbers or traced steps

Calls: A↔B 09:00, A↔C 10:00, A↔B 18:00 same day
bidir: A sees partners B,C,B; first=B last=B → A qualifies.
B sees A,A (if only those); first=last=A → B may qualify.
Single call day: first=last trivially true.
Tie same call_time: add ORDER BY call_time, partner for stability.

When NOT to use / named alternative

Do not join first/last by matching equal timestamps only — duplicate times lose rows. Do not analyze only caller_id (misses recipient-side bookends). Prefer MIN/MAX(call_time) + join back to partner if windows unavailable — more joins, same idea.

Failure / ops fingerprint

Fingerprint: user 3 in intermediate set but missing from output (old bug); timezone DATE(call_time) shifts bookends. Ops: store UTC; deterministic ORDER BY; test single-call and multi-partner days.

Hostile-panel drills (defend the decision)

Q1. Why UNION ALL unpivot?
Model answer: Each call contributes two (user, partner, time) rows so both parties are evaluated symmetrically.

Q2. How do FIRST_VALUE asc and desc help?
Model answer: Asc → first partner of day; desc → last partner; equality means bookends match.

Q3. Single-call day?
Model answer: First partner equals last partner — qualifies under intended semantics.

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