DEV Community: Frankline Kibet

SQL JOINS WITH EXAMPLES OF EACH

Frankline Kibet — Wed, 16 Sep 2026 03:07:38 +0000

A join combines rows from two or more tables based on a related column between them. Instead of running separate queries and manually matching up results yourself, a join does that matching for you, in a single query
When working with databases, your data is often stored in more than one table. This is way join are important

*TYPES OF JOINS *

1.** Inner join**
This is a type of join that returns only the rows that have a match in both tables.
example

Left jOIN

Returns all rows from the left table plus matching rows from the right table.

Where there's no match the right table's columns come back as null

3*_.Right join _*
Returns all rows from the right table plus matching rows from the left.

4._ Full outer join _

Return all row from all table.

I used it when when you need a complete view of the table example like in this scenario above I need to check the results table and the subject table to identify areas of improvement.

_self join _

This is a practical
use of self join that really help be in my project. it let me pair each book with it author with ease.

DDL and DML WHAT ARE THEY?

Frankline Kibet — Mon, 14 Sep 2026 12:33:56 +0000

DDL
DDL is an abbreviation for data definition language used to define, modify, and manage the structure of a database.
Common DDL commands
Create - Creates a new table, database, index, or view
Example

Alter
Used to modify the exiting table
Alter can be used to:

Remove column
Add column

Truncate – it empties all rows from a table but keeps its structure

truncate table safari.student;

DML
DML is an abbreviation for data manipulation language,
These are the commands that work with the actual data stored inside tables exmple adding rows, reading rows, updating values, and deleting rows.
Example of commands
Select reads data from one or more tables
Example:

2.Insert
Adds new rows to a table

Update Is used to modifies existing rows. example

Delete is used to removes rows from a table

Diffence
The main difference: DDL defines and changes the structure of a table using commands like CREATE, ALTER, DROP while DML works with the actual data inside that structure commands like SELECT, INSERT, UPDATE, DELETE.

ANALYZING HOTEL BOOKING DATA: A TEMBO HOTEL SQL + POWER BI CASE STUDY

Frankline Kibet — Mon, 14 Sep 2026 04:29:45 +0000

Tembo Hotel had been tracking all of their bookings in Excel, but the data had grown messy over time inconsistent formatting, duplicate entries, and mixed date formats made it impossible to get reliable answers out of it. I was tasked with cleaning the dataset and turning it into something the CEO could actually use to understand the business.

Here's how I approached it, from raw CSV to a working Power BI dashboard.

The cleaning process started by deleting duplicates .

Then standardization values and trimming to remove extra space.

the changing the data format and data type from text to data

after the cleaning process is over I loaded the clean data on table which I will use for analysis.

The CEO wanted to know about his business . Analysis started by answering major business questions
ANALYSIS

Revenue analysis

Guest insighs
staff performance

Cancellation and revenue lost

One of the more important findings: a meaningful share of potential revenue was lost specifically to cancellations, concentrated in certain room types more than others.

I created it as view so that it so that it could be easy to import then on power bi for visulation

Presenting the outcome

Dashboard 2.The revenue analysis
Guest insight

cancelation and revenue lost on it .

REPORT

Data

Frankline Kibet — Mon, 07 Sep 2026 11:58:42 +0000