Lesson-5-Complex-Queries-Aggregate-Function-Nested-Queries-Triggers.pptx

Complex Queries
(Aggregate Function,
Nested Queries,
Triggers)

AGGREGATES FUNCTION IN SQL
• An aggregate function in SQL performs a calculation on
multiple values and returns a single value.
• Aggregate functions often use with the GROUP BY and
HAVING clauses of the SELECT statement.
• Various types of SQL aggregate functions are: Count, Sum,
Avg, Min, Max
• NULL values discarded when aggregate functions are applied
to a particular column.

This function returns the number of rows in a database table.
Syntax: COUNT(columnname)
Examples:
SELECT COUNT(product_id)
FROM Products;
SELECT COUNT(product_id)
FROM Products
WHERE unit_price > 4;
COUNT() FUNCTION

This function returns the total sum of a numeric column.
Syntax: SUM(columnname)
Examples:
SELECT SUM(unit_price)
FROM Products;
SELECT COUNT(customer_id), city
FROM Customers
GROUP BY city
HAVING SUM(points) > 3000;
SUM() FUNCTION

This function calculates the average of a set of values.
Syntax: AVG(columnname)
Examples:
SELECT AVG(quantity_in_stock)
FROM Products;
SELECT department_id, AVG(salary)
FROM Employees
GROUP BY department_id;
AVG() FUNCTION

This function returns the lowest value (minimum) in a set of non-NULL
values.
Syntax: MIN(columnname)
Example:
SELECT MIN(quantity_in_stock)
FROM Products;
MIN() FUNCTION

This function returns the highest value (maximum) in a set of non-NULL
values.
Syntax: MAX(columnname)
Example:
SELECT MAX(quantity_in_stock)
FROM Products;
MAX() FUNCTION

CORRELATED NESTED QUERIES
• Evaluated once for each row in the outer query.
• Correlated nested queries are used for row-by-row
processing.
• Each nested sub-query is executed once for every row of
the outer query.
• A correlated nested query is one way of reading every row
in a table and comparing values in each row against related
data.

Example:
SELECT first_name, last_name
FROM employee a
WHERE TargetSale >
(SELECT Sales
FROM sale
WHERE emp_id = a.emp_id);
Correlated Nested Queries

TRIGGERS IN SQL
Triggers in SQL serve as a mechanism for automatically
executing a set of SQL statements in response to certain
events occurring in the database. These events can include
data manipulation operations (e.g., INSERT, UPDATE,
DELETE) on specific tables.

TRIGGERS IN SQL
Scenario: In a university database, there's a table named
Grades that stores students' grades for various courses. We
want to implement a trigger that automatically updates a
student's overall GPA (Grade Point Average) whenever a new
grade is inserted or updated in the Grades table.
Objective: We'll create a trigger that calculates the overall
GPA of a student whenever a new grade is inserted or
updated in the Grades table.

TRIGGERS IN SQL
Tables:
Grades: Contains student grades for different courses.
Students: Contains student information including their overall GPA.
Steps:
Create a Trigger: We'll create a trigger that fires after each insert or
update operation on the Grades table.
Calculate GPA: Inside the trigger, we'll calculate the overall GPA of the
student based on their grades in different courses.
Update Student's GPA: Finally, we'll update the student's overall GPA in
the Students table.

CREATE TRIGGER UpdateGPA
AFTER INSERT ON Grades
FOR EACH ROW
BEGIN
DECLARE total_credits INT;
DECLARE total_grade_points DECIMAL(5, 2);
DECLARE new_gpa DECIMAL(5, 2);
-- Calculate total credits and grade points for the student
SELECT SUM(course_credits), SUM(grade * course_credits)
INTO total_credits, total_grade_points
FROM Grades
WHERE student_id = NEW.student_id;
-- Calculate GPA
IF total_credits > 0 THEN
SET new_gpa = total_grade_points / total_credits;
ELSE
SET new_gpa = 0; -- To handle the case when there are no grades yet
END IF;
-- Update student's GPA
UPDATE Students
SET gpa = new_gpa
WHERE student_id = NEW.student_id;
END;
CREATE A TRIGGER
CALCULATE GPA
UPDATE GPA
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Lesson-5-Complex-Queries-Aggregate-Function-Nested-Queries-Triggers.pptx

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