relational database domain relational calculus language QBE

Qbe:query by Example
? Query Language based on the screen table
? Query Request: Construct a query as a form
? use sample elements (domain variables) to represent possible scenarios for query results
? Query results: Display in tabular format

QBE operating Framework

Constructing several elements of a query

Example element: Field variable, must be underlined

The sample element is a possible value in this field, which does not have to be an element in the query result
Print operator P.: Specifies the attribute columns that are included in the query result
Query criteria: Do not underline
You can use comparison operators >,≥,<,≤,= and ≠ where = can be omitted
Sorting requirements

Conditional query

common set functions:

function name	function
cnt	tuple count
sum	sum
avg	averaging
max	max
min	minimum

inquiry information is the average age of students.

sort the results of a query

Sort Ascending:

To sort the results of a query in ascending order of a property value, simply fill in the corresponding column. AO.?

Sort Descending:

Sort by descending? Do.

Sort multiple columns:

If you sort by multiple columns, use the? AO (i).? Or? Do (i).? Indicates that I is the priority of the sort, the lower the I value, the higher the priority level

Check the names of all the boys, the query results are ordered in ascending order of the department , and the students of the same department are sorted by age.

Modify Operation

two lines representing the pre-and post-change sample elements

Must be operator? U.? Put it on the relationship.

Eg: The age of all students in the computer system is changed to 18 years old

Eg: increase the age of 95001 students by 1 years

Eg: increase the age of all students in the computer system by 1 years

Insert Operation

Eg: the information is 95701 girls, name Zhang San, age 17 years old into the database.

Delete operation

Eg: Delete student 95089

To ensure referential integrity, delete all courses selected by 95089 Students before deleting 95089 students

relational database domain relational calculus language QBE