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Chapter 8: Grouping and Aggregate functions of #LearningSQL @OReillyMedia
This thread will be a brief summary and a short one.
Let's dive in
This thread will be a brief summary and a short one.
Let's dive in
1. GROUP BY is one of the most fundamental functions in #MySQL or any kind. It helps us to retrieve counts, distinct values and etc by certain categorical data.
An example:
An example:
2. If you need to filter your tables while using GROUP BY, generally you cannot use WHERE clause, instead use HAVING.
In this example we filtered the data after creating groups. If we'd have used WHERE before GROUP BY, server will raise an error.
In this example we filtered the data after creating groups. If we'd have used WHERE before GROUP BY, server will raise an error.
1⃣Hi everyone, let's start as promised yesterday.
A quick note before starting, check @levikul09 for statistics and python related information.
These tweets are my notes from @OReillyMedia #LearningSQL book chapter 1. All the credit goes to that book.
A quick note before starting, check @levikul09 for statistics and python related information.
These tweets are my notes from @OReillyMedia #LearningSQL book chapter 1. All the credit goes to that book.
2⃣What is a database? Why it is important in data world?
Well this is easy to answer, there are nearly 2.5 quintillion bytes of data created every day. So it is crucial that this data stored in someplace. That place is called a Database, a set of related information.
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Well this is easy to answer, there are nearly 2.5 quintillion bytes of data created every day. So it is crucial that this data stored in someplace. That place is called a Database, a set of related information.
2
3⃣Today we are using Relational Database model most of the time. Before, there were Non-Relational Systems:
☑️Hierarchical Database Systems - same as Decision Trees
☑️Network Database Systems - sets of records and sets of link that defines the relationship
☑️Hierarchical Database Systems - same as Decision Trees
☑️Network Database Systems - sets of records and sets of link that defines the relationship
