SQL query beginner's Guide Reading Notes (2) Creating SQL queries, beginner's guide SQL

Source: Internet
Author: User

SQL query beginner's Guide Reading Notes (2) Creating SQL queries, beginner's guide SQL

PARTII: SQL Basics

CHAPTER 4 Creating a Simple Query


This section describes how to create an SQL statement: "Request/Translation/CleanUp/SQL"

 

The SELECT operationin SQL can be broken down into three smaller operations,

Which we will referto as the SELECT statement, the SELECT expression,

And the SELECTquery.

A layer contains a layer and is nested with each other to form a very complex SELECT statement.

 


 

 

Other Keywords of SELECT Statement are familiar. Here we will explain a little bit about group by and HAVING.

Group by-When youuse aggregate functions in the SELECT clause

To produce summaryinformation, you use the group by clause

Divide theinformation into distinct groups. Your database system uses

Any column or listof columns following the group by keywords

Grouping columns. The group by clause is optional, and we'll examine

It further inChapter 13, Grouping Data.

 

HAVING-The HAVINGclause filters the result of aggregate functions

In groupedinformation. It is similar to the WHERE clause in that

HAVING keyword isfollowed by an expression that evaluates to true,

False, orunknown. You can test the expression by using standard comparison

Operators, Booleanoperators, or special operators. HAVING is

Also an optionalclause, and we'll take a closer look at it in Chapter 14,

Filtering GroupedData.


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