Pavel Tisner

AI_ASSIST.md

@@ -6,16 +6,74 @@ Document one session where you used an LLM to help with a query or a design deci
 
 ## The problem
 
-TODO: What were you trying to solve? Paste the relevant SQL or schema fragment.
+I was creating the `vw_dim_zones` view for the star schema. The starter comment suggested selecting `location_id`, `zone`, and `borough`, but when I tried to run the view with this column order, PostgreSQL returned an error.
+
+Relevant SQL:
+
+```sql
+CREATE OR REPLACE VIEW vw_dim_zones AS
+SELECT
+    location_id,
+    zone,
+    borough
+FROM nyc_taxi.raw_zones;
+```
+
+The error was:
+
+```text
+ERROR: cannot change name of view column "borough" to "zone"
+Hint: Use ALTER VIEW ... RENAME COLUMN ... to change name of view column instead.
+```
 
 ## The prompt
 
-TODO: What did you ask the AI? Include the context you provided.
+I asked the LLM why this query failed and whether the issue was related to the `zone` column being highlighted in DBeaver.
+
+I provided the context that this was a PostgreSQL view for the NYC Taxi assignment, based on `nyc_taxi.raw_zones`, and that the existing view seemed to work when I used this order:
+
+```sql
+CREATE OR REPLACE VIEW vw_dim_zones AS
+SELECT
+    location_id,
+    borough,
+    zone
+FROM nyc_taxi.raw_zones;
+```
 
 ## The response
 
-TODO: What did it suggest? Did it work first try?
+The LLM explained that the problem was not the DBeaver highlighting of `zone`. The issue was that `CREATE OR REPLACE VIEW` in PostgreSQL does not allow changing the names/order of existing view columns in a way that would rename an existing column.
+
+The view had already been created with this column order:
+
+```text
+location_id, borough, zone
+```
+
+So trying to replace it with this order:
+
+```text
+location_id, zone, borough
+```
+
+made PostgreSQL interpret the second column as being renamed from `borough` to `zone`.
+
+The LLM suggested either dropping and recreating the view, or keeping the existing column order. I kept the existing column order:
+
+```sql
+CREATE OR REPLACE VIEW vw_dim_zones AS
+SELECT
+    location_id,
+    borough,
+    zone
+FROM nyc_taxi.raw_zones;
+```
+
+This worked.
 
 ## Reflection
 
-TODO: Did you understand *why* the suggestion worked, or did you accept it blindly?
+I understood why the suggestion worked. The problem was not with the raw data or with the `zone` column name itself. The problem was how PostgreSQL handles `CREATE OR REPLACE VIEW`: it can replace the query behind a view, but it does not freely allow changing the existing view column names by reordering columns.
+
+Keeping the same column order solved the issue because PostgreSQL no longer interpreted the replacement as a column rename. In this case, the view still contains all required dimension columns: `location_id`, `borough`, and `zone`, so the star schema remains valid.

data_dictionary.md

@@ -4,14 +4,14 @@ Document both views. State the grain in one sentence, identify the keys, and lis
 
 ## vw_fact_trips
 
-- **Grain:** TODO (one sentence, e.g. "One row per ...")
-- **Primary key:** TODO
-- **Foreign keys:** TODO
-- **Measures:** TODO (columns you would SUM or AVG)
+- **Grain:** One row per taxi trip from `nyc_taxi.raw_trips`, excluding rows where `fare_amount` is less than 0
+- **Primary key:** No declared primary key. The source table does not provide a unique trip ID
+- **Foreign keys:** `pickup_location_id` and `dropoff_location_id` reference `vw_dim_zones.location_id`
+- **Measures:** `passenger_count`, `trip_distance`, `fare_amount`, `extra`, `mta_tax`, `tip_amount`, `tolls_amount`, `improvement_surcharge`, `total_amount`, `congestion_surcharge`
 
 ## vw_dim_zones
 
-- **Grain:** TODO
-- **Primary key:** TODO
-- **Foreign keys:** TODO (or "none")
-- **Measures:** TODO (or "none, descriptive attributes only")
+- **Grain:** One row per taxi zone location
+- **Primary key:** `location_id`
+- **Foreign keys:** None
+- **Measures:** None, descriptive attributes only

schema_setup.sql

@@ -5,7 +5,9 @@
 -- TODO: complete the SELECT (location_id, zone, borough).
 CREATE OR REPLACE VIEW vw_dim_zones AS
 SELECT
-    -- TODO
+    location_id,
+    borough,
+    zone
 FROM nyc_taxi.raw_zones;
 
 -- Fact: one row per taxi trip.
@@ -15,10 +17,27 @@ FROM nyc_taxi.raw_zones;
 -- TODO: complete the SELECT and the WHERE.
 CREATE OR REPLACE VIEW vw_fact_trips AS
 SELECT
-    -- TODO
+    vendor_id,
+    CAST(pickup_datetime AS TIMESTAMP) AS pickup_datetime,
+    dropoff_datetime,
+    store_and_fwd_flag,
+    rate_type_id,
+    pickup_location_id,
+    dropoff_location_id,
+    passenger_count,
+    trip_distance,
+    fare_amount,
+    extra,
+    mta_tax,
+    tip_amount,
+    tolls_amount,
+    improvement_surcharge,
+    total_amount,
+    payment_type,
+    congestion_surcharge,
+    trip_type
 FROM nyc_taxi.raw_trips
--- TODO: WHERE fare_amount >= 0
-;
+WHERE fare_amount >= 0;
 
 -- Join-readiness test (run after creating the views; it must run without error
 -- and return a count close to the vw_fact_trips row count):

validation_queries.sql

@@ -5,16 +5,42 @@
 
 -- 1. Duplicate check: are there rows with the same vendor_id, pickup_datetime, dropoff_datetime?
 -- TODO: GROUP BY the three columns and keep only groups with HAVING COUNT(*) > 1.
-
+SELECT
+    vendor_id,
+    pickup_datetime,
+    dropoff_datetime,
+    COUNT(*) AS duplicate_count
+FROM nyc_taxi.raw_trips
+GROUP BY
+    vendor_id,
+    pickup_datetime,
+    dropoff_datetime
+HAVING COUNT(*) > 1;
 
 -- 2. Null integrity: how many rows have a NULL pickup_location_id or dropoff_location_id?
 -- TODO: count the NULLs (COUNT(*) FILTER (WHERE ... IS NULL) is handy for several columns at once).
-
+SELECT
+    COUNT(*) FILTER (WHERE pickup_location_id IS NULL) AS null_pickup_location_id_count,
+    COUNT(*) FILTER (WHERE dropoff_location_id IS NULL) AS null_dropoff_location_id_count
+FROM nyc_taxi.raw_trips;
 
 -- 3. Range validation: what are the min and max fare_amount? Are there negative values?
 -- TODO: SELECT MIN(fare_amount), MAX(fare_amount), and a count of rows where fare_amount < 0.
-
+SELECT
+    MIN(fare_amount) AS min_fare_amount,
+    MAX(fare_amount) AS max_fare_amount,
+    COUNT(*) FILTER (WHERE fare_amount < 0) AS negative_fares
+FROM nyc_taxi.raw_trips;
 
 -- 4. Relationship check: which pickup_location_id values in nyc_taxi.raw_trips do NOT exist in nyc_taxi.raw_zones?
 -- TODO: LEFT JOIN nyc_taxi.raw_zones ... WHERE z.location_id IS NULL  (or NOT EXISTS).
 -- Do NOT use NOT IN: a single NULL in the subquery hides every orphan.
+SELECT
+    t.pickup_location_id,
+    COUNT(*) AS row_count
+FROM nyc_taxi.raw_trips AS t
+LEFT JOIN nyc_taxi.raw_zones AS z
+    ON t.pickup_location_id = z.location_id
+WHERE t.pickup_location_id IS NOT NULL
+  AND z.location_id IS NULL
+GROUP BY t.pickup_location_id

verification_results.sql

@@ -4,19 +4,101 @@
 
 -- 1. Volume: how many total rows in vw_fact_trips? How many rows per borough?
 --    What is the most common pickup/dropoff location combination?
--- TODO
+SELECT
+    COUNT(*) AS total_rows
+FROM vw_fact_trips;
+
+SELECT
+    z.borough,
+    COUNT(*) AS row_count
+FROM vw_fact_trips AS t
+JOIN vw_dim_zones AS z
+    ON t.pickup_location_id = z.location_id
+GROUP BY
+    z.borough
+ORDER BY
+    row_count DESC;
+
+SELECT
+    pickup_zones.zone AS pickup_zone,
+    dropoff_zones.zone AS dropoff_zone,
+    COUNT(*) AS row_count
+FROM vw_fact_trips AS t
+JOIN vw_dim_zones AS pickup_zones
+    ON t.pickup_location_id = pickup_zones.location_id
+JOIN vw_dim_zones AS dropoff_zones
+    ON t.dropoff_location_id = dropoff_zones.location_id
+GROUP BY
+    pickup_zones.zone,
+    dropoff_zones.zone
+ORDER BY
+    row_count DESC
+LIMIT 1
+
 -- (Take a screenshot of the per-borough counts and save it as assets/borough_count.png.)
 
 
 -- 2. Revenue: which pickup zone (name, not ID) generated the highest total fare_amount?
 --    Which pickup zone collected the highest total fare_amount on any single day?
--- TODO
+SELECT
+    z.zone AS pickup_zone,
+    SUM(t.fare_amount) AS total_fare_amount
+FROM vw_fact_trips AS t
+JOIN vw_dim_zones AS z
+    ON t.pickup_location_id = z.location_id
+GROUP BY
+    z.zone
+ORDER BY
+    total_fare_amount DESC
+LIMIT 1;
+
+SELECT
+    CAST(t.pickup_datetime AS DATE) AS pickup_date,
+    z.zone AS pickup_zone,
+    SUM(t.fare_amount) AS total_fare_amount
+FROM vw_fact_trips AS t
+JOIN vw_dim_zones AS z
+    ON t.pickup_location_id = z.location_id
+GROUP BY
+    CAST(t.pickup_datetime AS DATE),
+    z.zone
+ORDER BY
+    total_fare_amount DESC
+LIMIT 1;
 
 
 -- 3. Geospatial: total number of trips and average trip_distance for each borough.
--- TODO
+SELECT
+    z.borough,
+    COUNT(*) AS total_trips,
+    AVG(t.trip_distance) AS avg_trip_distance
+FROM vw_fact_trips AS t
+JOIN vw_dim_zones AS z
+    ON t.pickup_location_id = z.location_id
+GROUP BY
+    z.borough
+ORDER BY
+    total_trips DESC;
 
 
 -- 4. Time patterns: which day of the week had the highest total tip_amount?
 --    What hour of the day has the highest average tip?
--- TODO
+SELECT
+    TO_CHAR(pickup_datetime, 'Day') AS day_of_week,
+    SUM(tip_amount) AS total_tip_amount
+FROM vw_fact_trips
+GROUP BY
+    TO_CHAR(pickup_datetime, 'Day')
+ORDER BY
+    total_tip_amount DESC
+LIMIT 1;
+
+SELECT
+    EXTRACT(HOUR FROM pickup_datetime) AS pickup_hour,
+    AVG(tip_amount) AS avg_tip_amount
+FROM vw_fact_trips
+GROUP BY
+    EXTRACT(HOUR FROM pickup_datetime)
+ORDER BY
+    avg_tip_amount DESC
+LIMIT 1;