Databases & SQL worksheet with answers

What does a CHECK constraint do?

1. ARejects rows whose values do not satisfy a condition
2. BChecks whether a table exists before a query is run on it
3. CVerifies the user's password
4. DCounts the rows in a table

Which keyword gives a column or table a temporary alternative name in a query?

1. ANAME
2. BALIAS
3. CRENAME
4. DAS

What does NULL represent in SQL?

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

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

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

Table a has 3 rows and table b has 4 rows. How many rows does SELECT * FROM a CROSS JOIN b return?

1. A12
2. B4
3. C3
4. D7

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

1. ANULL
2. BAn empty string
3. CAn error
4. D'unknown'

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

1. ADENSE_RANK()
2. BROW_NUMBER()
3. CCOUNT()
4. DRANK()

R(A, B, C, D) has the functional dependencies A → B and C → D. What is the candidate key?

1. A{A, C}
2. B{A}
3. C{B, D}
4. D{C}