chapter 04 how to retrieve data from two or more tables mit 22033, database management system by: s....

Chapter 04How to retrieve data from two or

more tablesMIT 22033, Database Management System

By: S. Sabraz Nawaz

MIT 22033 By. S. Sabraz Nawaz Slide 2

Inner Join



The explicit syntax for an inner joinSELECT select_listFROM 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. This syntax for coding an inner join can be referred to as the

explicit syntax. It is also called the SQL-92 syntax because itwas introduced by the SQL-92 standards.


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 othertable.

You can also join tables based on relationships not defined in thedatabase. These are called ad hoc relationships.

If the columns in a join condition have the same name, you have toqualify them with the table names. To code a qualified columnname, type the table name, a period, then the column name.


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

(114 rows)


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 makequalified 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 originaltable name.

Although the AS keyword is optional, it’s a good idea to use itbecause it makes the FROM clause easier to read.

You can use a correlation name for any table in a join withoutusing 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 moreconfusing or difficult to read.


The syntax for an inner join that usescorrelation names

SELECT select_listFROM 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]...


An inner join with correlation names that makethe query more difficult to read

SELECT InvoiceNumber, VendorName, InvoiceDueDate, InvoiceTotal - PaymentTotal - CreditTotal AS BalanceDueFROM Vendors AS v JOIN Invoices AS i ON v.VendorID = i.VendorIDWHERE InvoiceTotal - PaymentTotal - CreditTotal > 0ORDER BY InvoiceDueDate DESC

(40 rows)


An inner join with a correlation name thatsimplifies the query

SELECT InvoiceNumber, InvoiceLineItemAmount, InvoiceLineItemDescriptionFROM Invoices JOIN InvoiceLineItems AS LineItems ON Invoices.InvoiceID = LineItems.InvoiceIDWHERE AccountNo = 540ORDER BY InvoiceDate

(8 rows)


Inner joins that join more than two tables You can think of a multi-table join as a series of two-table 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.


A SELECT statement that joins the Vendors andInvoices tables

SELECT InvoiceNumber, VendorNameFROM Vendors, InvoicesWHERE Vendors.VendorID = Invoices.VendorID

The result set


A statement that joins four tables USE AP SELECT VendorName, InvoiceNumber, InvoiceDate, InvoiceLineItemAmount AS LineItemAmount, AccountDescription FROM Vendors JOIN Invoices ON Vendors.VendorID = Invoices.VendorID JOIN InvoiceLineItems ON Invoices.InvoiceID = InvoiceLineItems.InvoiceID JOIN GLAccounts ON InvoiceLineItems.AccountNo = GLAccounts.AccountNo WHERE InvoiceTotal - PaymentTotal - CreditTotal > 0 ORDER BY VendorName, LineItemAmount DESC

The result set

Outer Join



The explicit syntax for an outer joinSELECT select_listFROM 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. However, you can also use any of the othercomparison operators.

When a row with unmatched columns is retrieved, any columnsfrom the other table that are included in the result set are given nullvalues.


What outer joins do

Joins of this type Keep unmatched rows from

Left outer join The first (left) table

Right outer join The second (right) table

Full outer join Both tables


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

(202 rows)


The Departments table The Employees table


A left outer joinSELECT DeptName, Departments.DeptNo, LastNameFROM Departments LEFT JOIN Employees ON Departments.DeptNo = Employees.DeptNo


A right outer joinSELECT DeptName, Employees.DeptNo, LastNameFROM Departments RIGHT JOIN Employees ON Departments.DeptNo = Employees.DeptNo


A full outer joinSELECT DeptName, Departments.DeptNo, Employees.DeptNo, LastNameFROM Departments FULL JOIN Employees ON Departments.DeptNo = Employees.DeptNo


The Departments table

The Employees table The Projects table


A SELECT statement that joins the three tablesusing left outer joins

SELECT DeptName, LastName, ProjectNoFROM Departments LEFT JOIN Employees ON Departments.DeptNo = Employees.DeptNo LEFT JOIN Projects ON Employees.EmployeeID = Projects.EmployeeIDORDER BY DeptName, LastName, ProjectNo

The result set


A SELECT statement that joins the three tablesusing full outer joins

SELECT DeptName, LastName, ProjectNoFROM Departments FULL JOIN Employees ON Departments.DeptNo = Employees.DeptNo FULL JOIN Projects ON Employees.EmployeeID = Projects.EmployeeIDORDER BY DeptName

The result set

Assignment• What is a Cross Join? Using the tables in the

Accounts Payable database, clearly explain with SQL Script and its corresponding output.


Presentation• What is Normalization?


chapter 04 how to retrieve data from two or more tables mit 22033, database management system by: s....

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