Databases & SQL worksheet with answers

What does ROLLBACK do in a transaction?

1. ASaves all changes of the transaction permanently to disk
2. BUndoes the changes made since the transaction began
3. CRepeats the last statement
4. DDeletes the table

What does a CHECK constraint do?

1. AChecks whether a table exists before a query is run on it
2. BRejects rows whose values do not satisfy a condition
3. CCounts the rows in a table
4. DVerifies the user's password

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

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

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

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

Why are indexes created on columns?

1. ATo prevent NULL values
2. BTo speed up searches and joins on those columns
3. CTo save disk space
4. DTo make inserts and updates faster on those columns

What is a database trigger?

1. AA scheduled backup of the database
2. BA type of index that speeds up searches
3. CA query that a user runs by hand
4. DCode that runs automatically when a specified event such as INSERT, UPDATE or DELETE occurs on a table

Table b contains the values 1, 2 and NULL in column y. What does SELECT * FROM a WHERE x NOT IN (SELECT y FROM b) return?

1. AOnly rows of a where x is NULL
2. BEvery row of a
3. CNo rows at all
4. DRows of a where x is not 1 or 2

Given the functional dependencies A → B, B → C and C → D on R(A, B, C, D), what is the closure A⁺?

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