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Invalid Search

Given the following table, what is the percentage of invalid search result, and the total number of searches for each country?

Schema

mysql> describe SearchCategory;
+-----------------+---------------+------+-----+---------+-------+
| Field           | Type          | Null | Key | Default | Extra |
+-----------------+---------------+------+-----+---------+-------+
| country         | varchar(10)   | NO   | PRI | NULL    |       |
| search_cat      | varchar(10)   | NO   | PRI | NULL    |       |
| num_search      | int(10)       | YES  |     | NULL    |       |
| zero_result_pct | decimal(10,0) | YES  |     | NULL    |       |
+-----------------+---------------+------+-----+---------+-------+
4 rows in set (0.01 sec)

Sample data

mysql> select * from SearchCategory LIMIT 3;
+---------+------------+------------+-----------------+
| country | search_cat | num_search | zero_result_pct |
+---------+------------+------------+-----------------+
| CN      | dog        |    9700000 |            NULL |
| CN      | home       |    1200000 |              13 |
| CN      | tax        |       1200 |              99 |
+---------+------------+------------+-----------------+
3 rows in set (0.00 sec)

Result table | country | num_search | zeo_result_pct |

Load Data

Load the database file db.sql to localhost MySQL. A Search database will be created.

mysql < db.sql -uroot -p

Observation

This question tests one single skill: given an aggregated table, the goal is to aggregated one level higher. The tricky point is to handle numerous NULL cases. In the naive way, we may do the following:

SELECT
  country
  ,SUM(num_search) AS num_search
  ,SUM(num_search * zero_result_pct) AS sum_zero_result
FROM SearchCategory
GROUP BY country;
+---------+------------+-----------------+
| country | num_search | sum_zero_result |
+---------+------------+-----------------+
| CN      |   11881200 |        26498800 |
| UAE     |       NULL |            NULL |
| UK      |     198000 |          398000 |
| US      |     211500 |          218500 |
+---------+------------+-----------------+
4 rows in set (0.00 sec)

Then divide the two columns to get net percentage of zero result searches.

SELECT
  country, 
  num_search,
  ROUND(sum_zero_result / num_search, 2) AS zero_result_pct
FROM (
  SELECT
    country
    ,SUM(num_search) AS num_search
    ,SUM(num_search * zero_result_pct) AS sum_zero_result
  FROM SearchCategory
  GROUP BY country) AS a;
+---------+------------+-----------------+
| country | num_search | zero_result_pct |
+---------+------------+-----------------+
| CN      |   11881200 |            2.23 |
| UAE     |       NULL |            NULL |
| UK      |     198000 |            2.01 |
| US      |     211500 |            1.03 |
+---------+------------+-----------------+
4 rows in set (0.00 sec)

Trouble-shooting NULL

Here are some cases where NULL will not cause trouble.

  • If search_cat is NULL, it does not change the result, because it is neither aggregated over, nor grouped by.
  • If both num_search and zero_result_pct are NULL, their product is NULL. This row contributes to neither the numerator nor the denominator.
  • If country is NULL but the other columns are not, we get a row in the result table for the NULL group. In this exercise, we do not deal with such pathological case. The table structure implies that country-num_search is a composite primary key, which cannot contain NULL.
  • If the summed num_search is null for every category in a country, the denominator of outer query is 0. Fortunately, division by 0 yields 0, which is expected.

The Hidden Error If zero_result_pct is NULL, the multiplying it with num_search results in NULL. Summing over it with other numeric values does not change the sum, but the denominator includes the num_search columns. We are effectively treating this row as having zero invalid result!

Take China for example, the dog category has the highest volume of searches, and yet we are treating it as having zero invalid result. The resulting net percentage is grossly underestimated.

SELECT * FROM SearchCategory WHERE country = 'CN';
+---------+------------+------------+-----------------+
| country | search_cat | num_search | zero_result_pct |
+---------+------------+------------+-----------------+
| CN      | dog        |    9700000 |            NULL |
| CN      | home       |    1200000 |              13 |
| CN      | tax        |       1200 |              99 |
| CN      | travel     |     980000 |              11 |
+---------+------------+------------+-----------------+
4 rows in set (0.00 sec)

To fix the denominator, we add a case statement. See that the num_search column drops lower for China and US.

SELECT
  country
  ,SUM(CASE WHEN zero_result_pct IS NOT NULL THEN num_search ELSE NULL END) AS num_search
  ,SUM(num_search * zero_result_pct) AS sum_zero_result
FROM SearchCategory
GROUP BY country;
+---------+------------+-----------------+
| country | num_search | sum_zero_result |
+---------+------------+-----------------+
| CN      |    2181200 |        26498800 |
| UAE     |       NULL |            NULL |
| UK      |     198000 |          398000 |
| US      |     199500 |          218500 |
+---------+------------+-----------------+

Finally, we can get the proper estimate of invalid search. See how much we had underestimated the zero_result_pct earlier!

SELECT
  country, 
  num_search,
  ROUND(sum_zero_result / num_search, 2) AS zero_result_pct
FROM (
  SELECT
    country
    ,SUM(CASE WHEN zero_result_pct IS NOT NULL THEN num_search ELSE NULL END) AS num_search
    ,SUM(num_search * zero_result_pct) AS sum_zero_result
  FROM SearchCategory
  GROUP BY country) AS a;
+---------+------------+-----------------+
| country | num_search | zero_result_pct |
+---------+------------+-----------------+
| CN      |    2181200 |           12.15 |
| UAE     |       NULL |            NULL |
| UK      |     198000 |            2.01 |
| US      |     199500 |            1.10 |
+---------+------------+-----------------+
4 rows in set (0.00 sec)

Optional: MySQL8

WITH tmp AS (
SELECT
    country
    ,SUM(CASE WHEN zero_result_pct IS NOT NULL THEN num_search ELSE NULL END) AS num_search
    ,SUM(num_search * zero_result_pct) AS sum_zero_result
  FROM SearchCategory
  GROUP BY country
)
SELECT
  country, 
  num_search,
  ROUND(sum_zero_result / num_search, 2) AS zero_result_pct
FROM tmp;
+---------+------------+-----------------+
| country | num_search | zero_result_pct |
+---------+------------+-----------------+
| CN      |    2181200 |           12.15 |
| UAE     |       NULL |            NULL |
| UK      |     198000 |            2.01 |
| US      |     199500 |            1.10 |
+---------+------------+-----------------+
4 rows in set (0.00 sec)

Parting Thought

Though this notebook is meant to be exhaustive, you should communicate with interviewers on which columns can be NULL and which cannot. Some columns cannot be NULL by design. Also, ask for how to present the result where value is NULL: whether to leave it as NULL or replace it with more informative text.

See solution here.