A Guide to SQL, Seventh Edition

A Guide to SQL, Seventh Edition

Objectives

Use joins to retrieve data from more than one table

Use the IN and EXISTS operators to query multiple tables

Use a subquery within a subquery

Use an alias


Objectives

Join a table to itself

Perform set operations (union, intersection, and difference)

Use the ALL and ANY operators in a query

Perform special operations (inner join, outer join, and product)


Querying Multiple Tables

When querying more than one table, the tables must be joined

Join tables by finding columns with matching data

Join tables by using a condition in the WHERE clause


Joining Two Tables

In the SELECT clause, list all columns you want to display

In the FROM clause, list all tables involved in the query

In the WHERE clause, restrict to the rows that have common values in matching columns





Comparing JOIN, IN, and EXISTS

Tables can be joined using IN or EXISTS clause

Use IN operator with a subquery

Use the EXISTS operator to retrieve data from more than one table






Correlated Subquery

Subquery involves a table listed in the outer query

In Figure 4.7 the ORDERS table, listed in the FROM clause of the outer query, is used in the subquery

You need to qualify ORDER_NUM column in subquery as ORDERS.ORDER_NUM


Correlated Subquery

For each row in the ORDERS table

Subquery executed using the value of ORDERS.ORDER_NUM that appears in the row

The inner query makes a list of rows in the ORDER_LINE table

Where ORDER_LINE.ORDER_NUM matches this value and

In which PART_NUM is equal to DR93


Using a Subquery within a Subquery

A nested subquery is a subquery within a subquery

SQL evaluates the queries from the innermost query to the outermost

It is possible that there is more than one approach to formulation of the queries

Many DMBS have optimizers that analyze queries for efficiency



Using an Alias

An alias is an alternate name for a table

Used when tables are listed in the FROM clause

Created by typing the name of the table, hitting a space, then typing the name of the alias

Allows for simplicity


Joining a Table to Itself

A self-join is when you are joining a table to itself

A second use for using an alias

Used when comparing records within one table

Alias allows you to treat one table as two separate tables



Using a Self-Join on a Primary Key

It is possible to create a self-join that involves the primary key of the table

Just as in previous examples, you would list the table twice in the FROM clause with aliases



Joining Several Tables Condition shows how the columns are related for each

pair of tables


Joining Several Tables Step-By-Step

In the SELECT clause list all the columns to display

Qualify the column name if needed

In the FROM clause list all tables

Include tables used in the WHERE clause, even if they are not in the SELECT clause


Joining Several Tables Step-By-Step

Take one pair of related tables at a time

Indicate in the WHERE clause the condition that relates the tables

Join conditions with the AND operator

Include any additional conditions in the WHERE clause

Connect them with the AND operator


Set Operations

Set operations are used for taking the union, intersection, and differences of two tables

The union of two tables is a table containing every row that is in either the first table, the second table, or both tables


Set Operations

The intersection (intersect) of two tables is a table containing all rows that are in both tables

The difference (minus) of two tables is the set of all rows that are in the first tables but are not in the second table


Restrictions to Set Operations

Requirements for tables to be union compatible

Have the same number of columns

AND

Their corresponding columns have identical data types and lengths


Special Operations

Inner Join

A join that compares the tables in the FROM clause and lists on those rows that satisfy the condition in the WHERE clause

Outer Join

A join that lists all the rows from one of the tables in a join, regardless of matching


Outer Joins

Left outer join: all rows from the table on the left (listed first in the query) will be included; matching rows only from the table on the right will be included

Right outer join: all rows from the table on the right will be included; matching rows only from the table on the left will be included

Full outer join: all rows from both tables will be included regardless of matches


Product

The product (Cartesian Product) of two tables is the combination of all rows in the first table and all rows in the second table

Omit the WHERE clause to form a product



Summary

Join tables with various methods

A subquery can contain another subquery

An alias can be used to simplify the SQL command as well to create self join

UNION, INTERSECT, MINUS commands are introduced

To form a product of two tables, include both tables in the FROM clause and omit the WHERE clause


Documents

A Guide to SQL, Seventh Edition