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,