Multiple Table Queries
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Objectives
 Retrieve data from more than one table by joining
tables
 Using IN and EXISTS to query multiple tables
 Nested Subqueries
 Using aliases
 Joining a table to itself
 UNION, INTERSECT and MINUS
 ALL and ANY operators
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Querying Multiple Tables
 To retrieve data from two or more tables


Join the tables
Formulate a query using the same commands as for
single tables
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Create a Join
 Indicate in the SELECT clause all columns to
display
 List in the FROM clause all tables involved in the
query
 Give condition(s) in the WHERE clause to restrict
the data to be retrieved to only those rows that have
common values in matching columns
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Qualify Columns
 SLSREP_NUMBER is in both the SALES-REP
table and the CUSTOMER table, which causes
ambiguity among column names
 Qualify columns by separating the table name and
the column name with a period.
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Examples
 List the customer number, last name, and first
name for every customer, together with the sales
rep number, last name, and fist name for the sales
rep who represents each customer
 List the customer number, last name, and first
name of every customer whose credit limit is
$1,000 together with the sales rep number, last
name, and first name of the sales rep who
represents each customer
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JOIN, IN, and EXISTS
 Include the where condition to ensure that matching
columns contain equal values

Similar results are obtained using IN or EXISTS operator
within a subquery
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Nested Subquery
 When a subquery is within another subquery, it is
called a nested subquery
 Find the order number and order date for every
order that includes a part located in warehouse
number 3
 Rewrite above query using 2 table joins
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Nested Subquery Evaluation
 Innermost subquery is evaluated first, producing
a temporary table of part numbers located in
warehouse 3
 Intermediate subquery is evaluated, producing a
second temporary table with a list of order
numbers
 Outer query is evaluated last, producing the
desired list of order numbers and order dates
using only those orders whose numbers are in
the temporary table produced in step 2
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Using an Alias
 Tables listed in the FROM clause can be given an
alternative name
 List the sales rep number, last name, and first name
for every sales rep together with the customer
number, last name, and first name for each
customer the sales rep represents
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Complex Joins
 Joining a table to itself
 Find every pair of customers who have the same first and
last names
 For each pair of tables joined, include a condition indicating
how the columns are related
 For every part on order, list the part number, number
ordered, order number, order date, customer number, last
name, and first name of the customer who placed the order,
and the last name and first name of the sales rep who
represents each customer
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Set Operations
 Union: a table containing every row that is in either
the first table or the second table, or both (must be
union-compatible: same number of columns and
corresponding columns have identical data types
and lengths)
 Intersection (intersect): a table containing every
row that is in both tables
 Difference (minus): set of every row that is in the
first table but not in the second table
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Set Operations
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Set Operations
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UNION/INTERSECT/MINUS
 List the customer number, last name, and first name for
every customer who is either represented by sales rep
number 12 or who currently has orders on file, or both
 List the customer number, last name, and first name for
every customer who is represented by sales rep number 12
and who currently has orders on file
 List the customer number, last name, and first name for
every customer who is either represented by sales rep
number 12 or who does not have orders currently on file
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ALL and ANY
 ALL and ANY operators are used with subqueries
to produce a single column of numbers


ALL: condition is true only if it satisfies all values
produced by the subquery
ANY: condition is true if it satisfies any value (one or
more) produced by the subquery
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ALL and ANY
 Find the customer number, last name, first name,
current balance, and sales rep number for every
customer whose balance is greater than the individual
balances of every customer of sales rep 12
 Find the customer number, last name, first name,
current balance, and sales rep number of every
customer whose balance is larger than the balances of
at least one customer of sales rep number 12
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Summary
 To join tables together, indicate in the SELECT clause all columns to
display, list in the FROM clause all tables to join, and then include in the
WHERE clause any conditions requiring values in matching columns to
be equal
 When referring to matching columns in different tables, you must
qualify the column names to avoid confusion

You qualify column names using the following format: table name.column
name
 Use the IN operator or the EXISTS command with an appropriate
subquery as an alternate way of performing a join
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Summary
 A subquery can contain another subquery. The innermost
subquery is executed first
 The name of a table in a FROM clause can be followed by
an alias, which is an alternate name for the table

The alias can be used in place of the table name throughout the SQL
command
 By using two different aliases for the same table in a single
SQL command, you can join a table to itself
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Summary
 The UNION command creates a union of two tables; that is,
the collection of rows that are in either or both tables



The INTERSECT command creates the intersection of two tables;
that is, the collection of rows that are in both tables
The MINUS command creates the difference of two tables; that is,
the collection of rows that are in the first table but not in the second
table
To perform any of these operation, the tables must be unioncompatible
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Summary
 Two tables are union-compatible if they have the
same number of columns, and if their corresponding
columns have identical data types and lengths
 If a subquery is preceded by the ALL command, the
condition is true only if it is satisfied by all values
produced by the subquery
 If a subquery is preceded by the ANY command, the
condition is true if it is satisfied by any value (one or
more) produced by the subquery
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