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- 2. THE SQL GROUP BY STATEMENT The GROUP BY statement groups rows that have the same values into summary rows, like "find the number of customers in each country". The GROUP BY statement is often used with aggregate functions (COUNT(), MAX(), MIN(), SUM(), AVG()) to group the result-set by one or more columns. GROUP BY Syntax SELECT column_name(s) FROM table_name WHERE condition GROUP BY column_name(s) ORDER BY column_name(s);
- 3. EXAMPLE 1 SELECT COUNT(CustomerID), Country FROM Customers GROUP BY Country;
- 4. EXAMPLE 2 SELECT COUNT(CustomerID), Country FROM Customers GROUP BY Country ORDER BY COUNT(CustomerID) DESC;
- 5. EXAMPLE 3 SELECT Shippers.ShipperName, COUNT(Order s.OrderID) AS NumberOfOrders FROM Orders LEFT JOIN Shippers ON Orders.ShipperID = Shippers.ShipperID GROUP BY ShipperName;
- 6. THE SQL HAVING CLAUSE The HAVING clause was added to SQL because the WHERE keyword cannot be used with aggregate functions. HAVING Syntax SELECT column_name(s) FROM table_name WHERE condition GROUP BY column_name(s) HAVING condition ORDER BY column_name(s);
- 7. SQL HAVING EXAMPLES The following SQL statement lists the number of customers in each country. Only include countries with more than 5 customers: Example SELECT COUNT(CustomerID), Country FROM Customers GROUP BY Country HAVING COUNT(CustomerID) > 5;
- 8. EXAMPLE 1 SELECT Employees.LastName, COUNT(Orders. OrderID) AS NumberOfOrders FROM (Orders INNER JOIN Employees ON Orders.EmployeeID = Employees.EmployeeID) GROUP BY LastName HAVING COUNT(Orders.OrderID) > 10;
- 9. THE SQL EXISTS OPERATOR The EXISTS operator is used to test for the existence of any record in a subquery. The EXISTS operator returns TRUE if the subquery returns one or more records. EXISTS Syntax SELECT column_name(s) FROM table_name WHERE EXISTS (SELECT column_name FROM table_name WHE RE condition);
- 10. EXAMPLE 2 SELECT SupplierName FROM Suppliers WHERE EXISTS (SELECT ProductName FROM Products WHERE Products.SupplierID = Suppliers.supplierID AND Price = 22);
- 11. WHAT IS A STORED PROCEDURE? A stored procedure is a prepared SQL code that you can save, so the code can be reused over and over again. So if you have an SQL query that you write over and over again, save it as a stored procedure, and then just call it to execute it. You can also pass parameters to a stored procedure, so that the stored procedure can act based on the parameter value(s) that is passed
- 12. STORED PROCEDURE SYNTAX CREATE PROCEDURE procedure_name AS sql_statement GO; (Or) Delimiter Create procedure shoppy (IN article int) Begin Select article from shop where article=article; end
- 13. STORED PROCEDURE EXAMPLE The following SQL statement creates a stored procedure named "SelectAllCustomers" that selects all records from the "Customers" table: Example CREATE PROCEDURE SelectAllCustomers AS SELECT * FROM Customers GO;
- 14. EXAMPLE 2 Delimiter Create procedure getproductdesc_with parameters (IN PID INT) Begin Select P.productID,P.ProductName,PD.productDescript ion from product P INNER JOIN productDescription PD on p.productID=PD.ProductID Where P.Products=PID; END
- 15. EXEC Execute the stored procedure above as follows: Example EXEC SelectAllCustomers;
- 16. SQL CREATE INDEX STATEMENT The CREATE INDEX statement is used to create indexes in tables. Indexes are used to retrieve data from the database more quickly than otherwise. The users cannot see the indexes, they are just used to speed up searches/queries.
- 17. CREATE INDEX SYNTAX Creates an index on a table. Duplicate values are allowed: CREATE INDEX index_name ON table_name (column1, column2, ...); CREATE UNIQUE INDEX Syntax Creates a unique index on a table. Duplicate values are not allowed: CREATE UNIQUE INDEX index_name ON table_name (column1, column2, ...);
- 18. CREATE INDEX EXAMPLE The SQL statement below creates an index named "idx_lastname" on the "LastName" column in the "Persons" table: CREATE INDEX idx_lastname ON Persons (LastName); If you want to create an index on a combination of columns, you can list the column names within the parentheses, separated by commas: CREATE INDEX idx_pname ON Persons (LastName, FirstName);
- 19. DROP INDEX STATEMENT The DROP INDEX statement is used to delete an index in a table. MS Access: DROP INDEX index_name ON table_name;
- 20. INTRODUCTION ON TRIGGERS A trigger is a set of actions that are run automatically when a specified change operation (SQL INSERT, UPDATE, or DELETE statement) is performed on a specified table. Triggers are useful for tasks such as enforcing business rules, validating input data, and keeping an audit trail
- 21. USES FOR TRIGGERS: Enforce business rules Validate input data Generate a unique value for a newly-inserted row in a different file. Write to other files for audit trail purposes Query from other files for cross-referencing purposes Access system functions Replicate data to different files to achieve data consistency
- 22. INSERT TRIGGER Example: consider stuff table & stuff_pf table Query: Create trigger After_insert_stuff After insert on stuff for each row Insert into stuff_pf(stuff_id,pf) values (new.id,new.salary*0.10);
- 23. UPDATE TRIGGER Example: consider table 1 as batch and table 2 as rooms Query: Create trigger After_update_batch After update on batch for each row Update rooms Set status=“assigned” Where room_id=new.room_id
- 24. DELETE TRIGGER MySQL BEFORE DELETE Trigger: BEFORE DELETE Trigger in MySQL is invoked automatically whenever a delete operation is fired on the table. In this article, we are going to learn how to create a before delete trigger with its syntax and example. Syntax The following is the syntax to create a BEFORE DELETE trigger in MySQL: CREATE TRIGGER trigger_name BEFORE DELETE ON table_name FOR EACH ROW Trigger_body ;
- 25. DELETE TRIGGER Example: consider table 1 as batch and table 2 as rooms Query: Create trigger After_delete_batch After delete on batch for each row Delete from batch Where room_id=old.room_id