✨ docs: add combining dimensions questions

Bilbottom · Jun 19, 2024 · 06b02a0 · 06b02a0
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diff --git a/docs/challenging-sql-problems/challenging-sql-problems.md b/docs/challenging-sql-problems/challenging-sql-problems.md
@@ -19,9 +19,10 @@ As long as you know your database features and query patterns, these are fairly
 1. [Temperature anomaly detection](problems/bronze/temperature-anomaly-detection.md)
 2. [Personalised customer emails](problems/bronze/personalised-customer-emails.md)
 3. [Suspicious login activity](problems/bronze/suspicious-login-activity.md)
-4. [Customer churn](problems/bronze/customer-churn.md)
-5. [Fibonacci sequence](problems/bronze/fibonacci-sequence.md)
-6. [UK bank holidays](problems/bronze/uk-bank-holidays.md)
+4. [Combining dimensions](problems/bronze/combining-dimensions.md)
+5. [Customer churn](problems/bronze/customer-churn.md)
+6. [Fibonacci sequence](problems/bronze/fibonacci-sequence.md)
+7. [UK bank holidays](problems/bronze/uk-bank-holidays.md)
 
 ### ⚪ Silver Tier
 

diff --git a/docs/challenging-sql-problems/problems/bronze/combining-dimensions--sample-input.sql b/docs/challenging-sql-problems/problems/bronze/combining-dimensions--sample-input.sql
@@ -0,0 +1,23 @@
+```sql
+with
+
+dim_employee_demographics(employee_id, valid_from, valid_until, date_of_birth, gender, ethnicity) as (
+    values
+        (1, '2021-07-12', '9999-12-31', '1995-02-24', 'Female', 'White'),
+        (2, '2023-12-07', '9999-12-31', '1999-12-12', 'Male',   'Asian')
+),
+
+dim_employee_career(employee_id, valid_from, valid_until, job_title, salary) as (
+    values
+        (1, '2021-07-12', '2023-02-18', 'Student',      0.00),
+        (1, '2023-02-19', '9999-12-31', 'Pianist',   2000.00),
+        (2, '2023-12-08', '9999-12-31', 'Paramedic', 4000.00)
+),
+
+dim_employee_contact(employee_id, valid_from, valid_until, email, phone) as (
+    values
+        (1, '2021-07-12', '9999-12-31', '[email protected]',    '123-456-789'),
+        (2, '2023-12-08', '2023-12-31', null,                 '01234 567890'),
+        (2, '2024-01-01', '9999-12-31', '[email protected]', '0300 123 456')
+)
+```
diff --git a/docs/challenging-sql-problems/problems/bronze/combining-dimensions--sample-output.sql b/docs/challenging-sql-problems/problems/bronze/combining-dimensions--sample-output.sql
@@ -0,0 +1,10 @@
+```sql
+solution(employee_id, valid_from, valid_until, date_of_birth, gender, ethnicity, job_title, salary, email, phone) as (
+    values
+        (1, '2021-07-12', '2023-02-18', '1995-02-24', 'Female', 'Black', 'Student',      0.00, '[email protected]',    '123-456-789'),
+        (1, '2023-02-19', '9999-12-31', '1995-02-24', 'Female', 'Black', 'Pianist',   2000.00, '[email protected]',    '123-456-789'),
+        (2, '2023-12-07', '2023-12-07', '1999-12-12', 'Male',   'Asian', null,           null, null,                 null),
+        (2, '2023-12-08', '2023-12-31', '1999-12-12', 'Male',   'Asian', 'Paramedic', 4000.00, null,                 '01234 567890'),
+        (2, '2024-01-01', '9999-12-31', '1999-12-12', 'Male',   'Asian', 'Paramedic', 4000.00, '[email protected]', '0300 123 456')
+)
+```
diff --git a/docs/challenging-sql-problems/problems/bronze/combining-dimensions.md b/docs/challenging-sql-problems/problems/bronze/combining-dimensions.md
@@ -0,0 +1,94 @@
+# Combining dimensions 🔗
+
+> [!SUCCESS] Scenario
+>
+> A human resources department has several dimension tables with information about their employees.
+>
+> For reporting purposes, they need to combine these dimensions into a single dimension table which shows all the information about each employee.
+
+> [!QUESTION]
+>
+> Write a query which combines the three dimension tables below into a single dimension table.
+>
+> The output should have the columns from the three tables combined:
+>
+> - `employee_id`
+> - `valid_from`
+> - `valid_until`
+> - `date_of_birth`
+> - `gender`
+> - `ethnicity`
+> - `job_title`
+> - `salary`
+> - `email`
+> - `phone`
+>
+> The number of rows in the output depends on the number of changes that need to be accounted for, and the `valid_from` and `valid_until` columns should be recalculated as necessary to account for these changes.
+>
+> Order the output by `employee_id` and `valid_from`.
+
+<details>
+<summary>Expand for the DDL</summary>
+--8<-- "docs/challenging-sql-problems/problems/bronze/combining-dimensions.sql"
+</details>
+
+The "dimensions" in this context are [Slowly Changing Dimensions](https://en.wikipedia.org/wiki/Slowly_changing_dimension) of [type 2](https://en.wikipedia.org/wiki/Slowly_changing_dimension#Type_2:_add_new_row) from the [Star Schema modelling framework](https://en.wikipedia.org/wiki/Star_schema).
+
+The solution can be found at:
+
+- [combining-dimensions.md](../../solutions/bronze/combining-dimensions.md)
+
+---
+
+<!-- prettier-ignore -->
+>? INFO: **Sample input**
+>
+> **Employee Demographics Dimension**
+>
+> | employee_id | valid_from | valid_until | date_of_birth | gender | ethnicity |
+> |------------:|:-----------|:------------|:--------------|:-------|:----------|
+> |           1 | 2021-07-12 | 9999-12-31  | 1995-02-24    | Female | Black     |
+> |           2 | 2023-12-07 | 9999-12-31  | 1999-12-12    | Male   | Asian     |
+>
+> **Employee Career Dimension**
+>
+> | employee_id | valid_from | valid_until | job_title |  salary |
+> |------------:|:-----------|:------------|:----------|--------:|
+> |           1 | 2021-07-12 | 2023-02-18  | Student   |    0.00 |
+> |           1 | 2023-02-19 | 9999-12-31  | Pianist   | 2000.00 |
+> |           2 | 2023-12-08 | 9999-12-31  | Paramedic | 4000.00 |
+>
+> **Employee Contact Dimension**
+>
+> | employee_id | valid_from | valid_until | email              | phone        |
+> |------------:|:-----------|:------------|:-------------------|:-------------|
+> |           1 | 2021-07-12 | 9999-12-31  | [email protected]    | 123-456-789  |
+> |           2 | 2023-12-08 | 2023-12-31  | _null_             | 01234 567890 |
+> |           2 | 2024-01-01 | 9999-12-31  | [email protected] | 0300 123 456 |
+>
+--8<-- "docs/challenging-sql-problems/problems/bronze/combining-dimensions--sample-input.sql"
+
+<!-- prettier-ignore -->
+>? INFO: **Sample output**
+>
+> | employee_id | valid_from | valid_until | date_of_birth | gender | ethnicity | job_title |  salary | email              | phone        |
+> |------------:|:-----------|:------------|:--------------|:-------|:----------|:----------|--------:|:-------------------|:-------------|
+> |           1 | 2021-07-12 | 2023-02-18  | 1995-02-24    | Female | Black     | Student   |    0.00 | [email protected]    | 123-456-789  |
+> |           1 | 2023-02-19 | 9999-12-31  | 1995-02-24    | Female | Black     | Pianist   | 2000.00 | [email protected]    | 123-456-789  |
+> |           2 | 2023-12-07 | 2023-12-07  | 1999-12-12    | Male   | Asian     | null      |  _null_ | _null_             | _null_       |
+> |           2 | 2023-12-08 | 2023-12-31  | 1999-12-12    | Male   | Asian     | Paramedic | 4000.00 | _null_             | 01234 567890 |
+> |           2 | 2024-01-01 | 9999-12-31  | 1999-12-12    | Male   | Asian     | Paramedic | 4000.00 | [email protected] | 0300 123 456 |
+>
+--8<-- "docs/challenging-sql-problems/problems/bronze/combining-dimensions--sample-output.sql"
+
+<!-- prettier-ignore -->
+>? TIP: **Hint 1**
+>
+> The `valid_from` values in the output will be each of the distinct `valid_from` values from the three tables for the given employee, so start by constructing an "axis" which is the distinct combination of all `employee_id` and `valid_from` values across the three tables.
+
+<!-- prettier-ignore -->
+>? TIP: **Hint 2**
+>
+> For databases that support it, use [an `ASOF` join](../../../everything-about-joins/syntax/timestamp-joins.md) to add the latest information from each dimension to the "axis" for each `employee_id` and `valid_from` combination.
+>
+> For databases that don't support `ASOF` joins, calculate the `valid_until` values for the `valid_from` values in the "axis" and then join the dimension tables to the axis by where the axis `valid_from`-`valid_until` range in contained within the dimension `valid_from`-`valid_until` range.
diff --git a/docs/challenging-sql-problems/problems/bronze/combining-dimensions.sql b/docs/challenging-sql-problems/problems/bronze/combining-dimensions.sql
@@ -0,0 +1,63 @@
+```sql
+create table dim_employee_demographics (
+    employee_id   int,
+    valid_from    date,
+    valid_until   date,
+    date_of_birth date,
+    gender        varchar,
+    ethnicity     varchar,
+    primary key (employee_id, valid_from)
+);
+insert into dim_employee_demographics
+values
+    (1, '2021-06-13', '2022-06-08', '2004-02-18', 'Female',      'Malaysian'),
+    (2, '2021-10-19', '9999-12-31', '1963-12-12', 'Female',      'Navajo'),
+    (3, '2022-01-29', '9999-12-31', '2000-10-17', 'Genderqueer', 'White'),
+    (4, '2022-04-28', '2024-03-12', '1987-12-13', 'Male',        'Black'),
+    (1, '2022-06-09', '9999-12-31', '2004-02-18', 'Non-binary',  'Malaysian'),
+    (5, '2022-08-31', '9999-12-31', '1999-09-10', 'Female',      'Asian'),
+    (4, '2024-03-13', '9999-12-31', '1987-12-13', 'Female',      'Black')
+;
+
+
+create table dim_employee_career (
+    employee_id int,
+    valid_from  date,
+    valid_until date,
+    job_title   varchar,
+    salary      decimal(10, 2),
+    primary key (employee_id, valid_from)
+);
+insert into dim_employee_career
+values
+    (1, '2021-06-13', '2023-05-27', 'Teacher',             5000.00),
+    (1, '2023-05-28', '9999-12-31', 'Teacher',             6000.00),
+    (2, '2021-10-19', '2023-11-26', 'Data Analyst',        4000.00),
+    (2, '2023-11-27', '2024-03-01', 'Data Analyst',        6500.00),
+    (2, '2024-03-02', '9999-12-31', 'Engineering Manager', 7000.00),
+    (3, '2022-01-29', '2023-04-02', 'Software Engineer',   6000.00),
+    (3, '2023-04-03', '9999-12-31', 'Software Engineer',   8000.00),
+    (4, '2022-06-12', '9999-12-31', 'Founder',                null)
+;
+
+
+create table dim_employee_contact (
+    employee_id int,
+    valid_from  date,
+    valid_until date,
+    email       varchar,
+    phone       varchar,
+    primary key (employee_id, valid_from)
+);
+insert into dim_employee_contact
+values
+    (1, '2021-06-13', '2024-01-29', '[email protected]',        null),
+    (1, '2024-01-30', '9999-12-31', '[email protected]',        '1986474151'),
+    (2, '2021-10-19', '2024-04-04', null,                        null),
+    (2, '2024-04-05', '9999-12-31', '[email protected]',         null),
+    (4, '2022-06-12', '2022-12-01', '[email protected]',   '01246 209863'),
+    (4, '2022-12-02', '2023-11-11', '[email protected]',   '01752 492269'),
+    (4, '2023-11-12', '9999-12-31', '[email protected]', '01270 530950'),
+    (5, '2023-02-17', '9999-12-31', null,                        null)
+;
+```
diff --git a/docs/challenging-sql-problems/solutions/bronze/combining-dimensions--duckdb.sql b/docs/challenging-sql-problems/solutions/bronze/combining-dimensions--duckdb.sql
@@ -0,0 +1,40 @@
+```sql
+with axis as (
+        select employee_id, valid_from
+        from dim_employee_demographics
+    union
+        select employee_id, valid_from
+        from dim_employee_career
+    union
+        select employee_id, valid_from
+        from dim_employee_contact
+)
+
+select
+    employee_id,
+    valid_from,
+    lead(valid_from - 1, 1, '9999-12-31') over (
+        partition by employee_id
+        order by valid_from
+    ) as valid_until,
+
+    dim_employee_demographics.date_of_birth,
+    dim_employee_demographics.gender,
+    dim_employee_demographics.ethnicity,
+
+    dim_employee_career.job_title,
+    dim_employee_career.salary,
+
+    dim_employee_contact.email,
+    dim_employee_contact.phone
+from axis
+    asof left join dim_employee_demographics
+        using (employee_id, valid_from)
+    asof left join dim_employee_career
+        using (employee_id, valid_from)
+    asof left join dim_employee_contact
+        using (employee_id, valid_from)
+order by
+    employee_id,
+    valid_from
+```
diff --git a/docs/challenging-sql-problems/solutions/bronze/combining-dimensions.md b/docs/challenging-sql-problems/solutions/bronze/combining-dimensions.md
@@ -0,0 +1,45 @@
+# Combining dimensions 🔗
+
+> [!TIP]
+>
+> Solution to the following problem:
+>
+> - [combining-dimensions.md](../../problems/bronze/combining-dimensions.md)
+
+## Result Set
+
+Regardless of the database, the result set should look like:
+
+| employee_id | valid_from | valid_until | date_of_birth | gender      | ethnicity | job_title           |  salary | email                     | phone        |
+| ----------: | :--------- | :---------- | :------------ | :---------- | :-------- | :------------------ | ------: | :------------------------ | :----------- |
+|           1 | 2021-06-13 | 2022-06-08  | 2004-02-18    | Female      | Malaysian | Teacher             | 5000.00 | [email protected]        | _null_       |
+|           1 | 2022-06-09 | 2023-05-27  | 2004-02-18    | Non-binary  | Malaysian | Teacher             | 5000.00 | [email protected]        | _null_       |
+|           1 | 2023-05-28 | 2024-01-29  | 2004-02-18    | Non-binary  | Malaysian | Teacher             | 6000.00 | [email protected]        | _null_       |
+|           1 | 2024-01-30 | 9999-12-31  | 2004-02-18    | Non-binary  | Malaysian | Teacher             | 6000.00 | [email protected]        | 1986474151   |
+|           2 | 2021-10-19 | 2023-11-26  | 1963-12-12    | Female      | Navajo    | Data Analyst        | 4000.00 | _null_                    | _null_       |
+|           2 | 2023-11-27 | 2024-03-01  | 1963-12-12    | Female      | Navajo    | Data Analyst        | 6500.00 | _null_                    | _null_       |
+|           2 | 2024-03-02 | 2024-04-04  | 1963-12-12    | Female      | Navajo    | Engineering Manager | 7000.00 | _null_                    | _null_       |
+|           2 | 2024-04-05 | 9999-12-31  | 1963-12-12    | Female      | Navajo    | Engineering Manager | 7000.00 | [email protected]         | _null_       |
+|           3 | 2022-01-29 | 2023-04-02  | 2000-10-17    | Genderqueer | White     | Software Engineer   | 6000.00 | _null_                    | _null_       |
+|           3 | 2023-04-03 | 9999-12-31  | 2000-10-17    | Genderqueer | White     | Software Engineer   | 8000.00 | _null_                    | _null_       |
+|           4 | 2022-04-28 | 2022-06-11  | 1987-12-13    | Male        | Black     | _null_              |  _null_ | _null_                    | _null_       |
+|           4 | 2022-06-12 | 2022-12-01  | 1987-12-13    | Male        | Black     | Founder             |  _null_ | [email protected]   | 01246 209863 |
+|           4 | 2022-12-02 | 2023-11-11  | 1987-12-13    | Male        | Black     | Founder             |  _null_ | [email protected]   | 01752 492269 |
+|           4 | 2023-11-12 | 2024-03-12  | 1987-12-13    | Male        | Black     | Founder             |  _null_ | [email protected] | 01270 530950 |
+|           4 | 2024-03-13 | 9999-12-31  | 1987-12-13    | Female      | Black     | Founder             |  _null_ | [email protected] | 01270 530950 |
+|           5 | 2022-08-31 | 2023-02-16  | 1999-09-10    | Female      | Asian     | _null_              |  _null_ | _null_                    | _null_       |
+|           5 | 2023-02-17 | 9999-12-31  | 1999-09-10    | Female      | Asian     | _null_              |  _null_ | _null_                    | _null_       |
+
+<details>
+<summary>Expand for the DDL</summary>
+--8<-- "docs/challenging-sql-problems/solutions/bronze/combining-dimensions.sql"
+</details>
+
+## Solution
+
+Some SQL solutions per database are provided below.
+
+<!-- prettier-ignore -->
+> SUCCESS: **DuckDB**
+>
+--8<-- "docs/challenging-sql-problems/solutions/bronze/combining-dimensions--duckdb.sql"
diff --git a/docs/challenging-sql-problems/solutions/bronze/combining-dimensions.sql b/docs/challenging-sql-problems/solutions/bronze/combining-dimensions.sql
@@ -0,0 +1,22 @@
+```sql
+solution(employee_id, valid_from, valid_until, date_of_birth, gender, ethnicity, job_title, salary, email, phone) as (
+    values
+        (1, '2021-06-13', '2022-06-08', '2004-02-18', 'Female',      'Malaysian', 'Teacher',             5000.00, '[email protected]',        null),
+        (1, '2022-06-09', '2023-05-27', '2004-02-18', 'Non-binary',  'Malaysian', 'Teacher',             5000.00, '[email protected]',        null),
+        (1, '2023-05-28', '2024-01-29', '2004-02-18', 'Non-binary',  'Malaysian', 'Teacher',             6000.00, '[email protected]',        null),
+        (1, '2024-01-30', '9999-12-31', '2004-02-18', 'Non-binary',  'Malaysian', 'Teacher',             6000.00, '[email protected]',        '1986474151'),
+        (2, '2021-10-19', '2023-11-26', '1963-12-12', 'Female',      'Navajo',    'Data Analyst',        4000.00, null,                        null),
+        (2, '2023-11-27', '2024-03-01', '1963-12-12', 'Female',      'Navajo',    'Data Analyst',        6500.00, null,                        null),
+        (2, '2024-03-02', '2024-04-04', '1963-12-12', 'Female',      'Navajo',    'Engineering Manager', 7000.00, null,                        null),
+        (2, '2024-04-05', '9999-12-31', '1963-12-12', 'Female',      'Navajo',    'Engineering Manager', 7000.00, '[email protected]',         null),
+        (3, '2022-01-29', '2023-04-02', '2000-10-17', 'Genderqueer', 'White',     'Software Engineer',   6000.00, null,                        null),
+        (3, '2023-04-03', '9999-12-31', '2000-10-17', 'Genderqueer', 'White',     'Software Engineer',   8000.00, null,                        null),
+        (4, '2022-04-28', '2022-06-11', '1987-12-13', 'Male',        'Black',     null,                     null, null,                        null),
+        (4, '2022-06-12', '2022-12-01', '1987-12-13', 'Male',        'Black',     'Founder',                null, '[email protected]',   '01246 209863'),
+        (4, '2022-12-02', '2023-11-11', '1987-12-13', 'Male',        'Black',     'Founder',                null, '[email protected]',   '01752 492269'),
+        (4, '2023-11-12', '2024-03-12', '1987-12-13', 'Male',        'Black',     'Founder',                null, '[email protected]', '01270 530950'),
+        (4, '2024-03-13', '9999-12-31', '1987-12-13', 'Female',      'Black',     'Founder',                null, '[email protected]', '01270 530950'),
+        (5, '2022-08-31', '2023-02-16', '1999-09-10', 'Female',      'Asian',     null,                     null, null,                        null),
+        (5, '2023-02-17', '9999-12-31', '1999-09-10', 'Female',      'Asian',     null,                     null, null,                        null)
+)
+```
diff --git a/mkdocs.yml b/mkdocs.yml
@@ -79,6 +79,7 @@ nav:
               - challenging-sql-problems/problems/bronze/temperature-anomaly-detection.md #  window functions
               - challenging-sql-problems/problems/bronze/personalised-customer-emails.md #  string similarity
               - challenging-sql-problems/problems/bronze/suspicious-login-activity.md #  window functions (gaps and islands)
+              - challenging-sql-problems/problems/bronze/combining-dimensions.md #  asof join
               - challenging-sql-problems/problems/bronze/customer-churn.md #  bit-shifting
               - challenging-sql-problems/problems/bronze/fibonacci-sequence.md #  recursive CTE
               - challenging-sql-problems/problems/bronze/uk-bank-holidays.md #  unpivot and unnest
@@ -99,6 +100,7 @@ nav:
               - challenging-sql-problems/solutions/bronze/temperature-anomaly-detection.md
               - challenging-sql-problems/solutions/bronze/personalised-customer-emails.md
               - challenging-sql-problems/solutions/bronze/suspicious-login-activity.md
+              - challenging-sql-problems/solutions/bronze/combining-dimensions.md
               - challenging-sql-problems/solutions/bronze/customer-churn.md
               - challenging-sql-problems/solutions/bronze/fibonacci-sequence.md
               - challenging-sql-problems/solutions/bronze/uk-bank-holidays.md