[Database and SQL study notes] 8. Views in SQL

Tools: SQL Server 2019 Express
OS: Windows 10

Database backup used:teaching.bak

Review the table structure:

t_student (S#, Sname, Sex, Age, Major)
t_teacher (T#, Tname, Age, Title)
t_course (C#, Cname, T#)
t_student_course (S#, C#, Score)

view

In our daily work, when we present database data to users, we need to simplify the user's data point of view.

We can define the data scattered in multiple tables together through the view View, so that the user does not need to enter some complex query statements, but only needs to do a simple query against the view.
This can better adapt to the needs of different users for data; at the same time, it defines the scope of data access for users, which is beneficial to the confidentiality of data.

A view is a virtual table whose contents are defined by a query.Like a real table, a view contains a series of named columns and rows of data.
However, views do not exist in the database as stored sets of data values.Row and column data comes from tables referenced by the query that defines the view, and is dynamically generated when the view is referenced.

create view

Create a view:

create view student_maleas select Sname, Cname, Scorefrom t_student, t_student_course, t_coursewhere t_student.S#=t_student_course.S# and t_student_course.C#=t_course.C#and Sex='male'

After execution, you can open the view under the database in the object explorer, you can find the view we created:

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We can specify the displayed column names:

create view student_male_detail(Name, Course, Score)as select Sname, Cname, Scorefrom t_student, t_student_course, t_coursewhere t_student.S#=t_student_course.S# and t_student_course.C#=t_course.C#and Sex='male'

After creation, we select the view, right-click and select "Select Top 1000 Rows" to see the information.

Alter view

For example, add a new column:

alter view student_male_detailas select t_student.S#, Sname, Cname, Scorefrom t_student, t_student_course, t_coursewhere t_student.S#=t_student_course.S# and t_student_course.C#=t_course.C#and Sex='male'

Effect:

drop view

Delete the student_male_detail view

drop view student_male_detail

Effect:

How to use views

After creating a view, it can be used directly as a table.
The from clause can be used directly.

Example 1: Query the average score of all boys:

select avg(Score)from student_male

Effect:

If we don't use views, the query becomes:

select avg(Score)from t_student, t_student_coursewhere t_student.S#=t_student_course.S# and Sex='male'

So using views can simplify queries.

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Example 2: Query the average test scores of each boy: