Hello, and welcome to the SQL Practice Set in PostgreSQL! This package of exercises will help you practice everything you know about basic SQL queries and is perfect for anyone who wants to strengthen their SQL skills.

The difficulty level of the exercises is roughly the same as in the SQL Basics in PostgreSQL course. Topics covered include: SELECTs, aggregate functions (COUNT, SUM, AVG), grouping and ordering results, JOINs, and subqueries. We will start each part with a brief reminder of the syntax needed to solve it and we'll continue with the exercises.

Here's a quick reminder of the basic SELECT syntax:

SELECT name, age
FROM client
WHERE age > 18

You select data by using the SELECT keyword, listing the columns, and then using the FROM keyword where you tell the database to select data from a given table.

To select all columns from a table you can use the asterisk * instead of the column names:

SELECT *
FROM client

To filter the result of your query, you use the WHERE keyword and a condition:

• Use comparison operators (=, !=, <, >, <=, >=) to compare values.
• You can combine conditions with the logical operators: AND, OR, NOT.
• The percentage sign % is used with the LIKE operator to replace any number of unknown characters. For instance, if we want to show only table data for names that start with 'F', we would write:

  WHERE name LIKE 'F%'

• If we want to replace only one unknown character, we use the underscore symbol _. For example:

  WHERE name LIKE '_atherine'

• The NULL represents an unknown or missing value. You use the IS NULL or IS NOT NULL operators to filter NULL values.