Joins

Module 12 quiz

Practice questions: choose an answer and check it to see the explanation. Nothing here is timed or graded.

1. 1. Orders (1,000 rows) LEFT JOIN customers on customer_key, where 10 keys appear twice in customers. How many rows can the result have?

   Exactly 1,000Fewer than 1,000More than 1,000Exactly 990
   Check
2. 2. Some payments reference accounts missing from the master. What should the clean output do?

   Inner join and drop themReport them as orphans with a reason so totals still reconcileInvent placeholder accountsAssign them to the largest account
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3. 3. pandas merge(..., validate="many_to_one") does what?

   Speeds up the mergeRaises an error if the right-hand keys are not uniqueDrops duplicatesFills missing keys
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4. 4. Why compare row counts and totals before and after a join?

   HabitTo detect fan-out (duplicated rows) and dropped rowsIt is required by SQLTo sort the output
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5. 5. pandas merge on a key column where both sides have NaN keys: do NaN keys match?

   NeverYes, pandas matches NaN to NaN by defaultOnly with how='inner'Only in SQL
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6. 6. A lookup table has the same key twice with different values. What should you do?

   Join anywayFix or deduplicate the lookup by a declared rule, or reject it, before joiningTake the firstAverage them
   Check