Mandla Mathebula — SQL Playground

Database Schema

Customer

CustomerId (PK)
Firstname
Surname
CustomerStatusId
ChannelId
CreateDateTime
LevelId

Stock

StockId (PK)
Description
Quantity
StockStatusId
StockGroupId

Sale

SaleId (PK)
CustomerId (FK)
CreateDatetime
SaleStatus
CompleteDatetime
SaleChannelId

SaleItem

SaleItemId (PK)
SaleId (FK)
StockId (FK)
Quantity
Price
Tax
Discount

Assessment Questions & Answers

A list of clients with a CustomerStatusId of 1, ordered by Surname then Firstname. Columns: Surname, Firstname, CustomerStatusId, CreateDateTime.
A list of stock items that were sold in January 2018. Stock Description only.
A list of stock items that were not sold in January 2018. Stock Description only.
Top 10 highest selling stock items for January 2018. Fields: Description, Quantity Sold.
Top 10 customers for January 2018 by sale value. Fields: Firstname, Surname, Number of Sales, Value of Sales.

Click a button to load and run the answer:

Explore the raw data (hardcoded for demo purposes):

20 customers, 20 stock items, 20 sales, 35 sale items — all pre-loaded in your browser.

Write SQL

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Results

Click "Run" or select a query above to see results

Mandla's SQL Glossary

Reading Data

SELECT: Choose which columns to show. SELECT Firstname, Surname FROM Customer;
FROM: Which table to pull data from.
WHERE: Filter rows by a condition. WHERE CustomerStatusId = 1
ORDER BY: Sort results. ASC = A→Z, DESC = Z→A.
LIMIT: Return only the first N rows. LIMIT 10
DISTINCT: Remove duplicate rows from results.

Modifying Data

INSERT INTO: Add a new row. INSERT INTO Customer VALUES (...);
UPDATE: Change existing data. UPDATE Customer SET Firstname = 'New' WHERE CustomerId = 1;
DELETE: Remove rows. DELETE FROM Customer WHERE CustomerId = 21;

Combining Tables

INNER JOIN: Combine two tables, keeping only rows that match in both.
ON: The condition that links tables. ON Sale.CustomerId = Customer.CustomerId
NOT IN: Exclude rows where a value appears in a subquery.

Summarising Data

GROUP BY: Group rows together and summarise them (like a pivot table).
SUM(): Add up values. SUM(Quantity * Price)
COUNT(): Count how many rows in each group.
AS: Give a column a display name. SUM(Quantity) AS TotalSold

Key Concepts

Table: A collection of data in rows and columns, like a spreadsheet.
Primary Key (PK): A unique identifier for each row. No duplicates allowed.
Foreign Key (FK): A column that links to another table's Primary Key.
NULL: Means "no value" — different from 0 or empty string.