OCA /OCP Oracle Database 11g A ll-in-One Exam Guide- P56 pdf

Consider the following queries: Query 1: select * from jobs cross join job_history; Query 2: select * from jobs j cross join job_history jh where j.job_id='AD_PRES'; Query 1 takes the 19

SELECT table1.column, table2.column

FROM table1

CROSS JOIN table2;

It is important to observe that no join condition is specified using the ON or USING keyword A Cartesian product associates the rows from table1 with every row in table2

Conditions that limit the results are permitted in the form of WHERE clause

restrictions If table1 and table2 contain x and y number of rows, respectively, the Cartesian product will contain x times y number of rows The results from a cross join

may be used to identify orphan rows or generate a large dataset for use in application testing Consider the following queries:

Query 1: select * from jobs cross join job_history;

Query 2: select * from jobs j cross join job_history jh

where j.job_id='AD_PRES';

Query 1 takes the 19 rows and 4 columns from the JOBS table and the 10 rows and 5 columns from the JOB_HISTORY table and generates one large set of 190 records with 9 columns SQL*Plus shows all instances of identically named columns

as separate headings labeled with the column name (unless they are aliased) SQL Developer appends an underscore and number to each shared column name and uses it as the heading The JOB_ID column is common to both the JOBS and JOB_ HISTORY tables The headings in SQL Developer are labeled JOB_ID and JOB_ID_1, respectively Query 2 generates the same Cartesian product as the first, but the 190 rows are constrained by the WHERE clause condition and only 10 rows are returned

Figure 12-13 shows a cross join between the REGIONS and COUNTRIES tables

There are 4 rows in REGIONS and 25 rows in COUNTRIES Since the WHERE clause limits the REGIONS table to 2 of 4 rows, the Cartesian product produces 50 (25 × 2) records The results are sorted alphabetically, first on the REGION_NAME and then on the COUNTRY_NAME The first record has the pair of values, Asia and Argentina When the REGION_NAME changes, the first record has the pair of values, Africa and Argentina Notice that the COUNTRY_NAME values are repeated for every REGION_NAME

EXAM TIP When using the cross join syntax, a Cartesian product is

intentionally generated Inadvertent Cartesian products are created when there are insufficient join conditions in a statement Joins that specify fewer

than N – 1 join conditions when joining N tables or that specify invalid join

conditions may inadvertently create Cartesian products A pure natural join

between two tables sharing no identically named columns results in a Cartesian join, since two tables are joined but less than one condition

is available

Exercise 12-4: Work with Joins Using SQL Developer or SQL*Plus, connect

to the WEBSTORE schema and produce a report of customers who purchased the 11G

All-in-One Guide (PRODUCT_ID=2) The report must contain the customer’s name,

the product description, and the quantity ordered There are several approaches to

solving this question Your approach may differ from this solution

1 Start SQL*Plus and connect to the WEBSTORE schema

Figure 12-13 The cross join

Two-Minute Drill

Write SELECT Statements to Access Data from More Than One Table Using Equijoins and Nonequijoins

• Equijoining occurs when one query fetches column values from multiple tables in which the rows fulfill an equality-based join condition

• A pure natural join is performed using the NATURAL JOIN syntax when the source and target tables are implicitly equijoined using all identically named columns

• The JOIN USING syntax allows a natural join to be formed on specific columns with shared names

• Dot notation refers to qualifying a column by prefixing it with its table name

and a dot or period symbol This designates the table a column originates

from and differentiates it from identically named columns from other tables

• The JOIN ON clause allows the explicit specification of join columns

regardless of their column names This provides a flexible joining format

• The ON, USING, and NATURAL keywords are mutually exclusive and

therefore cannot appear together in a join clause

• A nonequijoin is performed when the values in the join columns fulfill the

join condition based on an inequality operator

Join a Table to Itself Using a Self-Join

• A self-join is required when the join columns originate from the same table

Conceptually, the source table is duplicated and a target table is created The

self-join then works as a regular join between two discrete tables

• Storing hierarchical data in a relational table requires a minimum of two

columns per row One column stores an identifier of the row’s parent record,

and the second stores the row’s identifier

View Data That Does Not Meet a Join Condition

Using Outer Joins

• When equijoins and nonequijoins are performed, rows from the source and

target tables are matched These are referred to as inner joins

• An outer join is performed when rows that are not retrieved by an inner join

are included for retrieval

• A left outer join between the source and target tables returns the results of an

inner join and the missing rows it excluded from the source table

• A right outer join between the source and target tables returns the results of an

inner join and the missing rows it excluded from the target table

• A full outer join returns the combined results of a left outer join and a right

outer join

Generate a Cartesian Product of Two or More Tables

• A Cartesian product is sometimes called a cross join and refers to the set of

data created by merging the rows from two or more tables with each other

• The count of the rows returned from a Cartesian product is equal to the

number of rows in the source table multiplied by the number of rows in the

target table

• Joins that specify fewer than N – 1 join conditions when joining N tables, or

that specify invalid join conditions, inadvertently create Cartesian products

A SELECT * FROM EMPLOYEES NATURAL JOIN DEPARTMENTS;

B SELECT * FROM EMPLOYEES E NATURAL JOIN DEPARTMENTS D ON E.DEPARTMENT_ID=D.DEPARTMENT_ID;

C SELECT * FROM EMPLOYEES NATURAL JOIN DEPARTMENTS USING (DEPARTMENT_ID);

D None of the above

2 The EMPLOYEES and DEPARTMENTS tables have two identically named columns: DEPARTMENT_ID and MANAGER_ID Which statements join

these tables based on both column values? (Choose all that apply.)

A SELECT * FROM EMPLOYEES NATURAL JOIN DEPARTMENTS;

B SELECT * FROM EMPLOYEES JOIN DEPARTMENTS USING (DEPARTMENT_ID,MANAGER_ID);

C SELECT * FROM EMPLOYEES E JOIN DEPARTMENTS D ON E.DEPARTMENT_ID=D.DEPARTMENT_ID AND E.MANAGER_ID=D MANAGER_ID;

D None of the above

3 Which join is performed by the following query?

SELECT E.JOB_ID ,J.JOB_ID FROM EMPLOYEES E JOIN JOBS J ON (E.SALARY < J.MAX_SALARY);

(Choose the best answer.)

A Equijoin

B Nonequijoin

C Cross join

D Outer join

4 Which of the following statements are syntactically correct? (Choose all that apply.)

A SELECT * FROM EMPLOYEES E JOIN DEPARTMENTS D USING (DEPARTMENT_ID);

B SELECT * FROM EMPLOYEES JOIN DEPARTMENTS D USING (D.DEPARTMENT_ID);

C SELECT D.DEPARTMENT_ID FROM EMPLOYEES JOIN DEPARTMENTS D

USING (DEPARTMENT_ID);

D None of the above

5 Which of the following statements are syntactically correct? (Choose all that

apply.)

A SELECT E.EMPLOYEE_ID, J.JOB_ID PREVIOUS_JOB, E.JOB_ID

CURRENT_JOB FROM JOB_HISTORY J CROSS JOIN EMPLOYEES E ON

(J.START_DATE=E.HIRE_DATE);

B SELECT E.EMPLOYEE_ID, J.JOB_ID PREVIOUS_JOB, E.JOB_ID

CURRENT_JOB FROM JOB_HISTORY J JOIN EMPLOYEES E ON

(J.START_DATE=E.HIRE_DATE);

C SELECT E.EMPLOYEE_ID, J.JOB_ID PREVIOUS_JOB, E.JOB_ID

CURRENT_JOB FROM JOB_HISTORY J OUTER JOIN EMPLOYEES E ON

(J.START_DATE=E.HIRE_DATE);

D None of the above

6 Choose one correct statement regarding the following query:

SELECT * FROM EMPLOYEES E

JOIN DEPARTMENTS D ON (D.DEPARTMENT_ID=E.DEPARTMENT_ID)

JOIN LOCATIONS L ON (L.LOCATION_ID =D.LOCATION_ID);

A Joining three tables is not permitted

B A Cartesian product is generated

C The JOIN ON clause may be used for joins between multiple tables

D None of the above

7 How many rows are returned after executing the following statement?

SELECT * FROM REGIONS R1 JOIN REGIONS R2 ON (R1.REGION_

ID=LENGTH(R2.REGION_NAME)/2);

The REGIONS table contains the following row data (Choose the best answer.)

1 Europe

2 Americas

3 Asia

4 Middle East and Africa

A 2

B 3

C 4

D None of the above

B The rows returned represent those locations that have no COUNTRY_ID.

C The rows returned represent the COUNTRY_ID values for all the rows in the LOCATIONS table

D None of the above

9 Which of the following statements are syntactically correct? (Choose all that apply.)

A SELECT JH.JOB_ID FROM JOB_HISTORY JH RIGHT OUTER JOIN JOBS J

ON JH.JOB_ID=J.JOB_ID

B SELECT JOB_ID FROM JOB_HISTORY JH RIGHT OUTER JOIN JOBS J ON (JH.JOB_ID=J.JOB_ID)

C SELECT JOB_HISTORY.JOB_ID FROM JOB_HISTORY OUTER JOIN JOBS

ON JOB_HISTORY.JOB_ID=JOBS.JOB_ID

D None of the above

10 If the REGIONS table, which contains 4 rows, is cross joined to the COUNTRIES

table, which contains 25 rows, how many rows appear in the final results set? (Choose the best answer.)

A 100 rows

B 4 rows

C 25 rows

D None of the above

Self Test Answers

1 þ D The queries in B and C incorrectly contain the NATURAL keyword If

this is removed, they will join the DEPARTMENTS and EMPLOYEES tables based on the DEPARTMENT_ID column

ý A, B, and C A performs a pure natural join that implicitly joins the

two tables on all columns with identical names, which, in this case, are DEPARTMENT_ID and MANAGER_ID

2 þ A, B, and C These clauses demonstrate different techniques to join the

tables on both the DEPARTMENT_ID and MANAGER_ID columns

ý D.

3 þ B The join condition is an expression based on the less than inequality

operator Therefore, this join is a nonequijoin

ý A, C, and D A would be correct if the operator in the join condition

expression was an equality operator The CROSS JOIN keywords or the

absence of a join condition would result in C being true D would be

true if one of the OUTER JOIN clauses was used instead of the JOIN

ON clause

4 þ A This statement demonstrates the correct usage of the JOIN USING

clause

ý B, C, and D B is incorrect because only nonqualified column names are

allowed in the brackets after the USING keyword C is incorrect because the

column in brackets after the USING keyword cannot be referenced with a

qualifier in the SELECT clause

5 þ B demonstrates the correct usage of the JOIN ON clause.

ý A, C, and D A is incorrect since the CROSS JOIN clause cannot contain

the ON keyword C is incorrect since the OUTER JOIN keywords must be

preceded by the LEFT, RIGHT, or FULL keyword

6 þ C The JOIN ON clause and the other join clauses may all be used for

joins between multiple tables The JOIN ON and JOIN USING clauses

are better suited for N-way table joins

ý A, B, and D A is false, since you may join as many tables as you wish

A Cartesian product is not created, since there are two join conditions and

three tables

7 þ B Three rows are returned For the row with a REGION_ID value of 2,

the REGION_NAME is Asia and half the length of the REGION_NAME is

also 2 Therefore this row is returned The same logic results in the rows with

REGION_ID values of 3 and 4 and REGION_NAME values of Europe and

Americas being returned

ý A, C, and D.

8 þ A The right outer join fetches the COUNTRY.COUNTRY_ID values that

do not exist in the LOCATIONS table in addition to performing an inner

join between the tables The WHERE clause then eliminates the inner join

results The rows remaining represent those countries for which there are no

locations

ý B, C, and D.

10 þ A The cross join associates every four rows from the REGIONS table

25 times with the rows from the COUNTRIES table, yielding a result set that contains 100 rows

ý B, C, and D.

CHAPTER 13

Subqueries and Set Operators

Exam Objectives

In this chapter you will learn to

• 051.7.1 Define Subqueries

• 051.7.2 Describe the Types of Problems That the Subqueries Can Solve

• 051.7.3 List the Types of Subqueries

• 051.7.4 Write Single-Row and Multiple-Row Subqueries

• 051.8.1 Describe Set Operators

• 051.8.2 Use a Set Operator to Combine Multiple Queries into a Single Query

• 051.8.3 Control the Order of Rows Returned

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