Databases & SQL worksheet with answers

In a LIKE pattern, what does the _ (underscore) wildcard stand for?

1. AAny letter but no digits
2. BZero or more characters
3. CExactly one character
4. DA space character

In PostgreSQL and MySQL, which clause returns only the first 10 rows of a result?

1. ATOP ROWS 10
2. BMAX 10
3. CFIRST 10 ONLY
4. DLIMIT 10

Which aggregate function counts rows?

1. AAVG
2. BMAX
3. CSUM
4. DCOUNT

In relational algebra, the projection operator (π) corresponds to:

1. ASorting the result
2. BCombining two tables on a key
3. CChoosing which columns appear in the result
4. DChoosing which rows satisfy a given condition

Which statement about HAVING is correct?

1. AIt sorts the results
2. BIt cannot be used with GROUP BY
3. CIt can use aggregate functions to filter groups
4. DIt must come before WHERE in the SELECT statement

A column holds the values 10, 20 and NULL. What does AVG(col) return?

1. A10
2. B30
3. C15
4. DNULL

What is the result of SELECT COUNT(col) FROM t if all values in col are NULL and the table has 10 rows?

1. ANULL
2. B0
3. CAn error
4. D10

Why do many DBMSs use strict two-phase locking, holding exclusive locks until commit?

1. AIt makes deadlocks impossible
2. BIt prevents cascading aborts, because nobody can read uncommitted writes
3. CIt allows transactions to read dirty data safely
4. DIt removes the need for a transaction log