SQL Practice and Review

Today's lesson focuses on solidifying your SQL skills through practice and review. You'll work through various exercises to reinforce your understanding of fundamental SQL concepts like selecting data, filtering with WHERE clauses, using operators, and joining tables.

Learning Objectives

  • Write basic SQL SELECT statements to retrieve data from tables.
  • Utilize WHERE clauses with different operators to filter data effectively.
  • Apply aggregate functions to summarize data and derive meaningful insights.
  • Perform JOIN operations to combine data from multiple tables.

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Review: SELECT and WHERE

Let's revisit the basics! The SELECT statement retrieves data, and the WHERE clause filters it based on conditions.

Example: Suppose we have a table named Customers with columns like CustomerID, Name, and City. To retrieve all customers from 'New York', you'd use:

SELECT * FROM Customers WHERE City = 'New York';

The * selects all columns. You can specify particular columns by listing their names (e.g., SELECT Name, City FROM Customers WHERE City = 'New York';).

Remember different operators like =, <>, >, <, >=, <=, and BETWEEN can be used within the WHERE clause.

For example:

SELECT * FROM Orders WHERE OrderDate BETWEEN '2023-01-01' AND '2023-01-31';

Review: Operators and Logical Operators

Operators are crucial for filtering data. Besides comparison operators, logical operators (AND, OR, NOT) are also essential.

Example: To find customers in 'New York' OR 'Los Angeles':

SELECT * FROM Customers WHERE City = 'New York' OR City = 'Los Angeles';

To find customers NOT in 'New York':

SELECT * FROM Customers WHERE NOT City = 'New York';

Review: Aggregate Functions

Aggregate functions summarize data. Common functions include COUNT(), SUM(), AVG(), MIN(), and MAX().

Example: To count the number of orders in an Orders table:

SELECT COUNT(*) FROM Orders;

To find the average order amount:

SELECT AVG(Amount) FROM Orders;

You can use GROUP BY to group results by certain columns. For instance, to calculate the total order amount per customer:

SELECT CustomerID, SUM(Amount) FROM Orders GROUP BY CustomerID;

Review: Joins

Joins combine data from multiple tables based on related columns. The most common types are INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN.

Example: Suppose you have Customers (CustomerID, Name) and Orders (OrderID, CustomerID, Amount).

To get the customer names and their order amounts:

SELECT Customers.Name, Orders.Amount
FROM Customers
INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID;

INNER JOIN only returns rows where there is a match in both tables based on the join condition (Customers.CustomerID = Orders.CustomerID).

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