2. aggregate function

Question 1

When we want to limit the results of a query based on values of the individual rows, use

Answer 1

WHERE

Question 2

When we want to limit the results of a query based on an aggregate property, use

Answer 2

HAVING

Question 3

HAVING statement always comes ___ GROUP BY, but ___ ORDER BY and LIMIT.

Answer 3

HAVING statement always comes after GROUP BY, but before ORDER BY and LIMIT.

Question 4

What does LEFT JOIN do?

Answer 4

A left join will keep all rows from the first table, regardless of whether there is a matching row in the second table.

Question 5

What are the three requirements for primary keys?

Answer 5

• None of the values can be NULL
• Each value must be unique (i.e., you can’t have two customers with the same customer_id in the customers table)
• A table can not have more than one primary key column

Question 6

What is primary key?

Answer 6

a column that uniquely identifies each row of that table

Question 7

what is foreign key?

Answer 7

When the primary key for one table appears in a different table, it is called a foreign key.

Question 8

Which one is often more descriptive? Primary key or foreign key?

Answer 8

Foreign key

Question 9

Why is this important to identify primary key and foreign key?

Answer 9

The most common types of joins will be joining a foreign key from one table with the primary key from another table. For instance, when we join orders and customers, we join on customer_id, which is a foreign key in orders and the primary key in customers.