MIT Database Management Systems

Transcription

1 MIT Database Management Systems Lesson 04: How to retrieve data from two or more tables By S. Sabraz Nawaz Senior Lecturer in MIT FMC, SEUSL

2 How to code an inner join A join is used to combine columns from two or more tables into a result set based on the join conditions you specify. For an inner join, only those rows that satisfy the join condition are included in the result set. A join condition names a column in each of the two tables involved in the join and indicates how the two columns should be compared. In most cases, you ll join two tables based on the relationship between the primary key in one table and a foreign key in the other table. You can also join tables based on relationships not defined in the database. These are called ad hoc relationships. If the columns in a join condition have the same name, you have to qualify them with the table names. To code a qualified column name, type the table name, a period, then the column name. Slide 2

3 Slide 3

4 Inner Join Slide 4

5 The explicit syntax for an inner join SELECT select_list FROM table_1 [INNER] JOIN table_2 ON join_condition_1 [[INNER] JOIN table_3 ON join_condition_2]... Notes The INNER keyword is optional and is seldom used. Slide 5

6 A SELECT statement that joins the Vendors and Invoices tables SELECT InvoiceNumber, VendorName FROM Vendors JOIN Invoices ON Vendors.VendorID = Invoices.VendorID The result set Slide 6

7 When and how to use correlation names Correlation names are temporary table names assigned in the FROM clause. You can use them when long table names make qualified column names long or confusing. A correlation name can also be called a table alias. If you assign a correlation name to a table, you must use that name to refer to the table within your query. You can t use the original table name. Although the AS keyword is optional, it s a good idea to use it because it makes the FROM clause easier to read. You can use a correlation name for any table in a join without using correlation names for all of the other tables. Use correlation names whenever they simplify or clarify the query. Avoid using correlation names when they make a query more confusing or difficult to read. Slide 7

8 The syntax for an inner join that uses correlation names SELECT select_list FROM table_1 [AS] n1 [INNER] JOIN table_2 [AS] n2 ON n1.column_name operator n2.column_name [[INNER] JOIN table_3 [AS] n3 ON n2.column_name operator n3.column_name]... Slide 8

9 An inner join with correlation names SELECT InvoiceNumber, VendorName, InvoiceDueDate FROM Vendors AS v JOIN Invoices AS i ON v.vendorid = i.vendorid ORDER BY InvoiceDueDate DESC Select VendorName, InvoiceNumber, InvoiceDueDate From Vendors AS V Join Invoices I On V.VendorID = i.vendorid Where InvoiceTotal - PaymentTotal - CreditTotal > 0 Order By VendorName Slide 9

10 Inner joins that join more than two tables You can think of a multi-table join as a series of twotable joins proceeding from left to right. The first two tables are joined to produce an interim result set or interim table. Then, the interim table is joined with the next table, and so on. Slide 10

11 Two tables SELECT VendorName, InvoiceNumber FROM Vendors JOIN Invoices ON Vendors.VendorID = Invoices.VendorID ORDER BY VendorName Slide 11

12 Three Tables SELECT VendorName, InvoiceNumber, InvoiceLineItemAmount FROM Vendors JOIN Invoices ON Vendors.VendorID = Invoices.VendorID JOIN InvoiceLineItems ON Invoices.InvoiceID = InvoiceLineItems.InvoiceID ORDER BY VendorName Slide 12

13 Four Tables SELECT VendorName, InvoiceNumber, InvoiceLineItemAmount, AccountDescription FROM Vendors JOIN Invoices ON Vendors.VendorID = Invoices.VendorID JOIN InvoiceLineItems ON Invoices.InvoiceID = InvoiceLineItems.InvoiceID JOIN GLAccounts ON InvoiceLineItems.AccountNo = GLAccounts.AccountNo ORDER BY VendorName Slide 13

14 Outer Join Slide 14

15 The explicit syntax for an outer join SELECT select_list FROM table_1 {LEFT RIGHT FULL} [OUTER] JOIN table_2 ON join_condition_1 [{LEFT RIGHT FULL} [OUTER] JOIN table_3 ON join_condition_2]... How to code an outer join An outer join retrieves all rows that satisfy the join condition, plus unmatched rows in one or both tables. In most cases, you use the equal operator to retrieve rows with matching columns. When a row with unmatched columns is retrieved, any columns from the other table that are included in the result set are given null values. Slide 15

16 What outer joins do Joins of this type Left outer join Right outer join Full outer join Keep unmatched rows from The first (left) table The second (right) table Both tables Slide 16

17 A SELECT statement that uses a left outer join SELECT VendorName, InvoiceNumber, InvoiceTotal FROM Vendors LEFT JOIN Invoices ON Vendors.VendorID = Invoices.VendorID ORDER BY VendorName Slide 17

18 The Departments table The Employees table Slide 18

19 A left outer join SELECT DeptName, Departments.DeptNo, LastName FROM Departments LEFT JOIN Employees ON Departments.DeptNo = Employees.DeptNo Slide 19

20 A right outer join SELECT DeptName, Employees.DeptNo, LastName FROM Departments RIGHT JOIN Employees ON Departments.DeptNo = Employees.DeptNo Slide 20

21 A full outer join SELECT DeptName, Departments.DeptNo, Employees.DeptNo, LastName FROM Departments FULL JOIN Employees ON Departments.DeptNo = Employees.DeptNo Slide 21

22 Reference: Bryan Syverson & Joel Murach. Murach s SQL Server 2012 for Developers. Mike Murach & Associates, Inc. (p ) MIT 22033, Database Management Systems By: S.Sabraz Nawaz 22