4d. Structured Query Language – JOIN Operation Lingma Acheson Department of Computer and Information Science IUPUI CSCI N207 Data Analysis with Spreadsheets 1

Joining Multiple Tables What if we also want to see the Hours Worked for the previous query? If we need to display columns from different tables, subqueries wont work. Must use a Join Operation. Two tables can be joined together to form a large table based on the matching of the primary key and the foreign key. E.g. Show the names of all the employees and the hours worked. SELECT FirstName, LastName, HoursWorked FROM EMPLOYEE, ASSIGNMENT WHERE EMPLOYEE.EmployeeNumber = ASSIGNMENT.EmployeeNumber; 2

Joining Multiple Tables Join Operation – –Must list all the tables in the FROM clause. –Must indicate which field in what table is matched with which field in what table. –Duplicate field names from different tables must be prefixed by their table names using the dot notation. E.g. SELECT EMPLOYEE.EmployeeNumber, FirstName, LastName, HoursWorked FROM EMPLOYEE, ASSIGNMENT WHERE EMPLOYEE.EmployeeNumber = ASSIGNMENT.EmployeeNumber; 3

Joining Multiple Tables Notice that the records that dont match in two tables wont show in the result. Can apply other WHERE conditions. E.g. Show the names of the employees and their hours worked from the Accounting department. SELECT EMPLOYEE.EmployeeNumber, FirstName, LastName, HoursWorked FROM EMPLOYEE, ASSIGNMENT WHERE EMPLOYEE.EmployeeNumber = ASSIGNMENT.EmployeeNumber AND Department = Accounting; 4

Joining Multiple Tables E.g. Show the names of the employees and their hours worked but only interested in hours worked more than 45. SELECT EMPLOYEE.EmployeeNumber, FirstName, LastName, HoursWorked FROM EMPLOYEE, ASSIGNMENT WHERE EMPLOYEE.EmployeeNumber = ASSIGNMENT.EmployeeNumber AND HoursWorked > 45; 5

Joining Multiple Tables Practice: –Show projects whose maximum hours are smaller than 140. Need to see all the project information, as well as the budget code for departments. –An list of employees, need to see their names, s, department name, and department phone number. Sorted by last name. –How many total hours have been worked on a certain project? Show the project name, and the total hours. (hint: a combination of SUM, WHERE and GROUP BY) 6

Joining Multiple Tables Using table aliases to avoid repeatedly typing long table names – in the FROM clause, use AS XX after a table name with XX as the new shortened name for that table. E.g. Show the names of the employees and their hours worked from the Accounting department. SELECT E.EmployeeNumber, FirstName, LastName, HoursWorked FROM EMPLOYEE AS E, ASSIGNMENT AS A WHERE E.EmployeeNumber = A.EmployeeNumber AND Department = Accounting; 7

Joining Multiple Tables Can join more than two tables. E.g. Show the names of the employees and their hours worked on each project, including the project name. SELECT P.ProjectName, FirstName, LastName, HoursWorked FROM EMPLOYEE AS E, ASSIGNMENT AS A, PROJECT AS P WHERE E.EmployeeNumber = A.EmployeeNumber AND P.ProjectID = A.ProjectID 8

Joining Multiple Tables Using the JOIN … ON key words ACCESS syntax: must use INNER JOIN … ON E.g. Show the names of all the employees and the hours worked. Without using the INNERJOIN key word: SELECT FirstName, LastName, HoursWorked FROM EMPLOYEE AS E, ASSIGNMENT AS A WHERE E.EmployeeNumber = A.EmployeeNumber; Using the INNER JOIN key word: SELECT FirstName, LastName, HoursWorked FROM EMPLOYEE AS E INNER JOIN ASSIGNMENT AS A ON E.EmployeeNumber = A.EmployeeNumber; 9

Joining Multiple Tables Often use the graphic interface to add the tables to be joined. The table will be joined automatically based on the relationships defined. Its handy to create and save a query that shows all the joined data and later create queries based on this grand query, thus no need to join all the time. E.g. Query Saved - ProjectAndAssignment: SELECT * FROM PROJECT AS A INNER JOIN ASSIGNMENT AS A ON P.ProjectID = A.ProjectID; Query - ProjectAndHoursAccounting SELECT ProjectName, HoursWorked FROM ProjectAndAssignment WHERE Department = Accounting; 10