Grokking the Data Engineering Interview
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Data Contracts and Schema Changes

An online marketplace keeps a dashboard of the value of new listings per day, which is the price times the quantity on offer. One morning it showed these numbers:

SELECT day_no, COUNT(*) AS listings, SUM(price_cents * qty) AS listed_cents
FROM listings
GROUP BY day_no
ORDER BY day_no;

Output:

+--------+----------+--------------+
| day_no | listings | listed_cents |
+--------+----------+--------------+
|     11 |        2 |        21500 |
|     12 |        2 |        17800 |
|     13 |        2 |            0 |
|     14 |        2 |          154 |

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