Databases & SQL worksheet with answers

What does NULL represent in SQL?

1. AA missing or unknown value
2. BA false Boolean
3. CThe number zero in numeric columns
4. DAn empty string

What does COMMIT do in a transaction?

1. ALocks the whole database
2. BStarts a new table
3. CMakes all changes of the transaction permanent
4. DUndoes all changes made since the transaction began

What is a composite key?

1. AA key that allows duplicate values
2. BA key made up of two or more columns
3. CA key stored in a separate table
4. DA key that is generated by the database

What does EXCEPT (called MINUS in Oracle) return?

1. ARows that appear in both queries
2. BRows of the first query that do not appear in the second
3. CAll rows from both queries without duplicates
4. DRows of the second query that do not appear in the first one

What does SELECT COALESCE(phone, 'unknown') FROM customers return for a customer whose phone is NULL?

1. A'unknown'
2. BAn empty string
3. CAn error
4. DNULL

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

1. ACombining two tables on a key
2. BChoosing which columns appear in the result
3. CSorting the result
4. DChoosing which rows satisfy a given condition

Which window function assigns 1, 2, 3, ... to rows within each partition without ties?

1. ACOUNT()
2. BROW_NUMBER()
3. CRANK()
4. DDENSE_RANK()

When is a schedule of concurrent transactions called serializable?

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