Databases & SQL worksheet with answers

What does UNION do with the results of two SELECT queries?

1. ACombines them into one result and removes duplicate rows
2. BKeeps only rows that appear in the first query but not the second
3. CJoins them side by side on a key
4. DKeeps only rows that appear in both

Which statement adds a new column to an existing table?

1. AUPDATE TABLE
2. BALTER TABLE
3. CINSERT COLUMN
4. DCREATE COLUMN

What is a candidate key?

1. AAny column that contains NULL values
2. BA column that is waiting to become a foreign key
3. CA set of columns shared between two tables to link their rows
4. DA minimal set of columns that uniquely identifies each row

What does SELECT SUM(amount) FROM orders return when the WHERE clause matches no rows?

1. ANULL
2. BAn error
3. CAn empty string
4. D0

When is WHERE EXISTS (subquery) true for a row?

1. AWhen the subquery returns at least one row
2. BWhen the subquery returns no NULL values
3. CWhen the subquery returns exactly one row
4. DWhen the subquery returns a number greater than zero

What does the "A" in ACID stand for?

1. AAccuracy
2. BAggregation
3. CAvailability
4. DAtomicity

After customers LEFT JOIN orders, why use COUNT(o.id) rather than COUNT(*) to count orders per customer?

1. ACOUNT(*) ignores rows with NULL values
2. BCOUNT(o.id) counts only distinct customers
3. CCOUNT(*) counts the NULL-filled row as 1 for customers without orders
4. DCOUNT(*) is not allowed after a join

When is a schedule of concurrent transactions called serializable?

1. AWhen no transaction ever reads data
2. BWhen its effect is equivalent to some serial execution of the same transactions
3. CWhen the transactions run on one CPU only
4. DWhen every transaction is given a serial number