DEV Community: Wickliff Odoyo

Mastering SQL Joins and Window Functions (With Clear Examples)

Wickliff Odoyo — Mon, 02 Mar 2026 18:24:53 +0000

SQL Jpins and Windows functions

1.SQL JOINS

-Joins allow you to retrieve data from multiple tables based on a relationship between them.
-For example when you have motre than two tables to use,e.g project,employee and department table.

a.Inner join

-inner join is a join that return only matching records from both tables.

b.Left join

Returns all records from the left table and matching records from the right table.

c.Right join

Returns all records from the right table and matching from the left.

d.Full outer join

Returns everything from both tables.

e.cross join

-returns a combination of rows from both tables .

f.Self join

-joins table to itself

g.Natural join

-used to join both tables which returns all records from the tables.

2.WINDOW FUNCTIONS

-Window functions perform calculations across a set of rows without collapsing them like GROUP BY does.

1️⃣ ROW_NUMBER()

-Assigns a unique number to each row.

2️⃣ RANK()

Similar to ROW_NUMBER but handles ties.

3️⃣ SUM() OVER()

Calculates running totals.

4️⃣ Running Total

How Analysts Translate Messy Data,DAX,and Dashboards into Action Using Power BI.

Wickliff Odoyo — Sun, 08 Feb 2026 17:48:03 +0000

Power BI

-Power BI helps analysts turn messy data into clear insights that support real business decisions.This involves keys like:

.Cleaning data
.Using Dax for analysis
.Building dashboards
-Power BI has many views which helps the analyst to analyze data:Table view,Report view ,Model view etc.

Messy Data to meaningful Data

-Real world data is far from perfect.Files may come from different sources,columns may be missing,and values may not follow a format.For example ,a sales dataset might list countries with different spellings or have empty values for revenue.

-Using power BI and power query,analysts clean and organize data.They remove :Duplicates,fix errors,and standardize formats.
-Poor Quality data can lead to incorrect insights,which in turn can result in bad business decisions.Clean data creates a trustworthy foundation for analylis.
.For example is my data from messy to cleaned and DAX calculated columns and power query which is used in the power BI to transform Data.

DAX functions to answer Questions

-The DAX function is used after the data has been prepared.The DAX data analysis expression is used by the analysts to create calculations that answer real questions.(Using an impression called DAX measure).

-DAX measures include:
1.Totals-Which gives the total count or the total sum of something.
.This calculations one may run them when on table view or in report view using DAX Measure.

2.Average-which which gives the mean of the product or sales calculated for.

3.We have many more functions in DAX Measure:
-Sumif
-Averageif
-Mean
-Median
-If
-Date
.This calculated functions of data are being viewed in the report view using visuals.

.DAX functions help us to tract problems,identify trends and spot potential problems in business early.

Dahboards

-Dashboards are insights that are well designed to show well represented data that has been analysed which tells the story easily.It uses Power BI Designed charts,tables and KPLs to allow easy quick decision masking by business owners.
1.KPLs

2.Charts

3.Tables

.Power BI helps bridge the gap between raw data and real world action.This due to cleaning messy data using DAX to create meaningful calculations and building intuitive dashboards,analysts enable organizations to make smarter decisions.

Data Modelling And Schemas in Power Bi,Relationships and clarification:Beginner.

Wickliff Odoyo — Sun, 01 Feb 2026 19:18:44 +0000

Whsat is Data Modelling:

-Data modelling is the process of organizing tables and defining relationships between them so power Bi can understand how data connects.
-It work like a map of connections;
.Tables=locations where we assume data is being entered
.Relationships=Roads connecting them as a connecter
.Dax calculations=Directions that rely on the roads being correct

A good model ensures the following in the data:

-Accurate calculations
-Better performance
-Easier report building
-Scalability for large datasets

What is a Schema in power Bi

-A schema describes the structure of your data model-how tables are arranged and connected.

Power Bi typically follows data warehouse schemas,mainly:

1.Star schema
2.Snowflake Schema
3.Flat table(single Table)

1.Star Schema

-The star schema is the most recommended structure in Power Bi.

Structure

.it has:
1.Fact Table(center)
2.Dimension Tables(Around it)

Fact Table:

1.Sales:
-OrderID
-ProductId
-CustomerID
-DateID
-SalesAmount

Dimension Tables

-Products (ProductId,ProductName,Category)
-Customers(CustomerID,CustomerName,Region)
-Date(DateID,Yeah,Month,Day)

Its importance:

.Simple relationships
.Faster performance
.Easier DAX performance
.Clear visuals

Snowflake Schema

-The snowflake schema is a more normalized version of the star schema.

Structure

.Dimensions are split into multiple related tables.e.g
product-categories-subcategories
.This reduces data duplications
.Useful for complex hierarchies

Cardinality

-its how rows match
-ome -to -many(1:*)
one customer -many sales
-one-to-oe
rare
-many-to-many(:)
avoid if possible

Filter Direction(Important)

Single Direction
.Filter flow from dimension-fact
.Prevents confusion

Microsoft Excel Beginner friendly Overview.

Wickliff Odoyo — Sat, 24 Jan 2026 10:29:18 +0000

What is a microsoft Excel?

Microsoft excel is a spreadsheet program used to store,organize,calculate and analyze data.

Basic layout of a Microsft Excel

1.Workbook

.It is an excel file called workbook.

Workshhet

-Inside a workbook are worksheets.

Cells

-cells are made up of columns and rows where they meet.e.g A1,B2

Columns and Rows

-columns are vertical while rows are horizontal.
-Columns(A,B,C)
-Rows(1,2,3)

Data Validation

-Data validation controls only what a user might want.
e.g-only allows lists,allows numbers,allows dates,prevent text where numbers are expected,it also removes duplicates.
-You select a cell go to thr data on the text bar:Data validation,then you choose your direction of vsalidation.

Data sorting

-It is the arranging data in a specific order.
-TYPES:A-Z,smallest to largest,oldest to newest.
-To do this you click anywhere in the cells,go to data sort,choose a colum and sort.

Formatting

-It makes data easy to read
COMMON FORMATTING OPTIONS
.Font style,size,color
.Bold/italics
.Borders
.Cell fill color
.Number formats

NUMBER FORMATTING
.Currency
.Percentage
.Date
.Time
.Comma style

Conditional Formatting

-Automatically formats cells based on rules.
HOW TO USE
a)Select data
b)Home :conditional formatting
c)Choose a rule:greater than,color scales,data bars,icon sets.

Filtering

-Shows only what you need.
-YOU filter by selecting your header row,go to data then filter and click down row.

What is placed in a cell

-Texts(names,labels)
-Numbers(prices,scores,values)
-Formulas(calculations)

Various formulas in e.g

Basic functions

-All formulas start with (=) e.g
.Sum(A1:A5)which adds all the values in that range(=SuM(A1:A5))

.Average(A1:A5)which averages the values in that range{=AVERAGE(A1:A5)

.Count(which count cells with numbers)

-We also have other functions as MAX AND MIN.

Text functios

-Concat
-Left
-Mid
-Right

Logical

-If
-And
-Or
-Ifs

Lookup & Reference

-Vlookup
-Index
-Match
-Hlookup

Pivot Tables and Dashboards

-Pivot table is used to summarize and analyze raw data quickly.
e.g
-It groups data by category,by date and region.
-Calculates totals,average ,counts and percemtages.
-Pivot is created by clicking any cell then inserting.

DASHBOARD
-A dashboards is a visual summary of key metrics often built using pivot tables.e.g
.chats(bars,lines,pie)
.filters,slicers to interact with data.

BEGINER FRIENDLY

Wickliff Odoyo — Sat, 17 Jan 2026 07:17:55 +0000

BEGINNER'S GUIDE TO GIT: PUSH, PULL AND TRACKING CHANGES

Wickliff Odoyo — Sat, 17 Jan 2026 07:08:01 +0000

Git

It is a free and open source distributed version control system designed to handle everything from small to very large projects with speed and efficiency.It is installed in the browswer by writting git and clicking the the sighn im account.

GitHub

A web based platform for developers to store,manage,and share code.

Gitbash

It is a command-line interface (CLI)tool for windows that provides an emulation layer for a unix-like environment.

Version Control(git)

Solves a mistake by keeping one single file but recording every single change you ever made.

1. Tracking Changes

In git, you don't just hit Save(ctrl + s). Tracking changes happens in three steps:

a).Modify:

You change your code.
In my case am going to create a (New text document file) to track my changes in the repository and local host,while al write a statement to confirm the changes.Then track the changes also in the github resository created.(which in my case the reporitory will be learning).WHERE WE INITIALISE GIT FIRST ( USING COMMIT INIT).Then as for me I created a repository (Learning) in the github hence I can add my new folder and track changes.

b).Stage (git add):

You "mark" which changes you want to save.
We stage our commandline by using the gitbash to add the file to the github by using command (git add).So that we can add it in the reporitory in the github and track the changes if made in the local disc or github repository.

c).Commit (git commit):

You officially save the changes with a message describing what you did (e.g.,"fixed the login button")
We use the command line (git commit -m"first commit") to make changes in the repository in github.

2.What is "Pulling"?

It is like downloading updates.
if your working on a team ,your teammates might have added new code to the shared project. To make sure you have the latest version on your computer, you pull the code.

-Command:

git pull

-Analogy

Checking your email to see if anyone sent you new documents.where you use (git fetch and git merge to pull changes from repository)

3.What is "pushing"?

It is the opposite of pulling. It is basically uploading your work to a git repository. After you have committed changes locally on your computer, no one else can see them yet. To share your work with the team (or save it to a site like GitHub),you push your commits.

-Command:

git push

-Anatomy:

Sending an email with your updated document so others can see it.

The Workflow in 4 steps

to master git as a beginner, just remember this cycle:

1. Pull:

get the latest updates from your team.

2. Work:

Write your amazing code.

3.Commit:

Record your changes locally with a descriptive note.

DEV Community: Wickliff Odoyo

Mastering SQL Joins and Window Functions (With Clear Examples)

SQL Jpins and Windows functions

1.SQL JOINS

a.Inner join

b.Left join

c.Right join

d.Full outer join

e.cross join

f.Self join

g.Natural join

2.WINDOW FUNCTIONS

How Analysts Translate Messy Data,DAX,and Dashboards into Action Using Power BI.

Power BI

Messy Data to meaningful Data

DAX functions to answer Questions

Dahboards

Data Modelling And Schemas in Power Bi,Relationships and clarification:Beginner.

Whsat is Data Modelling:

A good model ensures the following in the data:

What is a Schema in power Bi

Power Bi typically follows data warehouse schemas,mainly:

1.Star Schema

Structure

Fact Table:

Dimension Tables

Its importance:

Snowflake Schema

Structure

Cardinality

Filter Direction(Important)

Microsoft Excel Beginner friendly Overview.

What is a microsoft Excel?

Basic layout of a Microsft Excel

1.Workbook

Workshhet

Cells

Columns and Rows

Data Validation

Data sorting

Formatting

Conditional Formatting

Filtering

What is placed in a cell

Various formulas in e.g

Basic functions

Text functios

Logical

Lookup & Reference

Pivot Tables and Dashboards

BEGINER FRIENDLY

BEGINNER'S GUIDE TO GIT: PUSH, PULL AND TRACKING CHANGES

Git

GitHub

Gitbash

Version Control(git)

1. Tracking Changes

a).Modify:

b).Stage (git add):

c).Commit (git commit):

2.What is "Pulling"?

-Command:

-Analogy

3.What is "pushing"?

-Command:

-Anatomy:

The Workflow in 4 steps

1. Pull:

2. Work:

3.Commit:

4.Push:Send your changes up to the cloud for everyone else.