JOINS IN SQL

By Rohit Dhanwani

OBJECTIVES

• Define and use different types of joins• INNER JOIN

• EQUI JOIN• NATURAL JOIN

• OUTER JOIN• LEFT OUTER JOIN• RIGHT OUTER JOIN• FULL OUTER JOIN

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PROCESSING MULTIPLE TABLES - JOIN

• Join is a relational operation that causes two or more tables with a common domain to be combined into a single table or view

• Joining columns (i.e., columns linking tables with a common domain) are used to define a join condition

• Generally, the columns in a join condition are the primary key of one table and the foreign key of another table in a 1:M relationship (i.e., joining columns = common columns)

• Occasionally, a join can be established using columns that share a common domain, but not the primary-foreign key relationship (i.e., joining columns ≠ common columns)

• Join condition is specified in WHERE or FROM clause(one join condition for each pair of tables joined)

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RELATIONAL DATA MODEL USED IN THE FOLLOWING MULTIPLE-TABLE QUERIES

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CustomerID CustomerName CustomerAddress CustomerCity

CustomerPostalCodeCustomerState

OrderID OrderDate CustomerID

OrderID ProductID OrderedQuantity

ProductID ProductDescription ProductFinish ProductStandardPrice

ProductLineID

CUSTOMER

ORDER

ORDERLINE

PRODUCT

INNER JOIN

• Inner join - a join in which the join condition contains a comparison operator (e.g., =, >,<) for the values of the joining columns. Returns matching rows (i.e. rows meeting the join condition) [the most popular join]

• Equi-join – a special type of inner join where the equality operator is used in the join condition. That is, the join condition is based on equality between values in the joining columns. Joining columns may appear redundantly in the result table

• Almost every join is an equi-join/inner join, because the join condition is based on the equality of two values — one from each of the tables being joined

• Natural join – same as equi-join except the join is performed over columns withidentical names. Plus, the result table contains only one column for each pair of joining columns

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Creates a Cartesian

product of rows

in involved tables

Finds the matching

rows meeting

the join condition(s)

Identifies

columns to

be presented in

the result table

STEP 1 STEP 2

FROM WHERE SELECT

STEP 3

SupplierID SupplierName

10 Walmart

20 Target

30 Best BuySELECT SupplierName, ShipmentDate

FROM Supplier_T, Shipment_T

WHERE

Supplier_T.SupplierID=Shipment_T.SupplierID;

SU

PP

LIE

R

EQUI JOIN SQL STATEMENT

PROCESSING OF EQUI JOIN - SPECIFYING WITHIN THE WHERE CLAUSE

ShipmentID ShipmentDate SupplierID

1 10-10-2016 20

2 10-12-2016 30

3 11-01-2016SH

IPM

EN

T

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Step 1: FROM Supplier_T, Shipment_T

Step 2: WHERE Supplier_T.SupplierID=Shipment_T.SupplierID

Step 3: SELECT

SupplierName,

ShipmentDate

PROCESSING STEPS OF IMPLICIT EQUI JOIN

SupplierID SupplierName ShipmentID ShipmentDate SupplierID

10 Walmart 1 10-10-2016 20

10 Walmart 2 10-12-2016 30

10 Walmart 3 11-01-2016

20 Target 1 10-10-2016 20

20 Target 2 10-12-2016 30

20 Target 3 11-01-2016

30 Best Buy 1 10-10-2016 20

30 Best Buy 2 10-12-2016 30

30 Best Buy 3 11-01-2016

SupplierID SupplierName ShipmentID ShipmentDate SupplierID

20 Target 1 10-10-2016 20

30 Best Buy 2 10-12-2016 30

SupplierName ShipmentDate

Target 10-10-2016

Best Buy 10-12-2016

• For each customer who placed an order, what is the customer’s name and order number?

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This query produces the

same results as the previous

equi-join exampleMS SQL and ORACLE support the JOIN

…ON syntax without the keyword INNER

EQUI JOIN - SPECIFYING WITHIN THE FROM CLAUSE (INNER JOIN …. ON)

ON clause performs the equality

check for joining columns of the

two tables

Join (explicit) involves multiple tables in the FROM clause

• For each customer who placed an order, what is the customer’s name and order number?

SELECT CustomerID, CustomerName, OrderID

FROM Customer_T INNER JOIN Order_T USING (CustomerID)

ORDER BY OrderID;

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This query produces the

same results as the previous

equi-join example, except

CustomerID column is not

repeated INNER JOIN …USING syntax may not be

supported by all RDBMS

EQUI JOIN - SPECIFYING WITHIN THE FROM CLAUSE (INNER JOIN…..USING)

USING (column name) clause

performs the equality check for

joining columns of the two tables

with identical names

Join (explicit) involves multiple tables in FROM clause

You cannot use a qualifier in the SELECT clause for the joining

column when INNER JOIN…USING syntax is used

• Join Customer and Order tables using Natural join. List all the columns from these tables

SELECT *

FROM Customer_T NATURAL JOIN Order_T

ORDER BY OrderID;

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NATURAL JOIN

NATURAL JOIN performs the

join using CustomerID column.

CustomerID is the attribute that

appears in both tables

This query produces a result table

in which CustomerID column

is not repeated

NATURAL JOIN is not supported by all RDBMS

(Oracle and MySQL support natural join)

You cannot use a qualifier in the SELECT clause for the

joining column when NATURAL JOIN syntax is used

NATURAL JOIN is

very similar to

INNER

JOIN…USING,

except the column

name is not

explicitly mentioned

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NATURAL JOIN (Another Example)

SELECT *

FROM Item_T

NATURAL JOIN

Company_T;

No explicit mention ofjoining columns

Company_ID that existsin both tables is used to join the tables

Company_ID column is not repeated in the result table

OUTER JOIN

• Outer join–a join in which rows that do not have matching values in joining columns are nonetheless included in the result table (as opposed to inner join, in which rows must have matching values in order to appear in the result table)

• Left Outer Join: returns all matching rows plus all rows of the left table that do not have a matching row in the right table

• Right Outer Join: returns all matching rows plus all rows of the right table that do not have a matching row in the left table

• Full Outer Join: returns all matching rows plus all rows of either table that do not have a matching row in the other table

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LEFT OUTER JOIN

• List the customer name, ID number, and order number for all customers. Include customer information even for customers that do not have any order.

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LEFT OUTER JOIN clause causes customer data to appear even if there is no corresponding order data

Unlike INNER join, this will include customer rows with no matching order rows

RIGHT OUTER JOIN is the reverse of LEFT OUTER JOIN

ON

LEFT OUTER JOIN…USING syntax can also be used

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Left Outer Join Results

Result table indicates NULL values for columns with no matching row

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RIGHT OUTER JOIN

• List the customer name, ID number, and order number for all orders. Include order number even for orders with no customer information available.

SELECT Customer_T.CustomerID, CustomerName, OrderID

FROM Customer_T RIGHT OUTER JOIN Order_T ON

Customer_T.CustomerID=Order_T.CustomerID;

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RIGHT OUTER JOIN clause causes order data to appear even if there is no corresponding customer data

Due to the referential integrity constraint, every order will have a valid customer ID. Hence, this query produces the same result table as the previous equi-join example

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Visualization of inner join and outer join with results returned in shaded area

Thank you !

Reference:

Hoffer, Jeffrey A., V. Ramesh, and Heikki Topi "Modern database management" 10th edition