DA SQL and Database Querying 3

Question 1

What is a primary key in a relational database?

Accepted Answer

A column that uniquely identifies each row

Answer

A primary key uniquely identifies each record in a table and cannot contain NULL or duplicate values.

Question 2

Which SQL clause limits the number of rows returned by a query?

Accepted Answer

All of the above (syntax varies by database)

Answer

Different databases use different syntax: LIMIT (MySQL/PostgreSQL), TOP (SQL Server), or ROWNUM (Oracle) to restrict the number of returned rows.

Question 3

What does the COALESCE function do in SQL?

Accepted Answer

Returns the first non-NULL value in a list

Answer

COALESCE evaluates its arguments in order and returns the first non-NULL value, making it useful for handling missing data.

Question 4

What is a CTE (Common Table Expression) in SQL?

Accepted Answer

A temporary named result set defined within a WITH clause

Answer

A CTE is a temporary, named result set created with the WITH keyword that can be referenced within the same query.

Question 5

Which SQL aggregate function returns the highest value in a column?

Accepted Answer

MAX()

Answer

MAX() returns the largest value in the specified column, and works with numeric, date, and string data types.

DA - Data Analyst Practice Test