Displaying Data
from Multiple Tables
(SQL99 Syntax with examples)
SQL99 Syntax
Cross Join
• The CROSS keyword indicates that a
cross join is being performed. A cross join
produces the cross-product of two
relations and is essentially the same as
the comma-delimited Oracle notation.
Cross Join Example
Select all employees and departments
• SQL99
SELECT ename, dname
FROM emp CROSS JOIN dept;
• Oracle
SELECT ename, dname
FROM emp, dept;
Natural Join
• The NATURAL keyword indicates that a natural
join is being performed. A natural join is based
on all columns in the two tables that have the
same name. It selects rows from the two tables
that have equal values in the relevant columns.
When specifying columns that are involved in
the natural join, do not qualify the column name
with a table name or table alias.
• Restriction: You cannot specify a LOB column or
a collection column as part of a natural join.
Natural Join Example (Equijoin)
Select employees, their job and their department, and
departments location
• SQL99
SELECT empno, ename, job, dname, loc
FROM emp NATURAL JOIN dept;
• Oracle
SELECT a.empno, a.ename, a.job,
b.dname, b.loc
FROM emp a, dept b
WHERE a.deptno = b.deptno;
USING column
• When you are specifying an equijoin of columns
that have the same name in both tables, the
USING column clause indicates the columns to
be used. You can use this clause only if the join
columns in both tables have the same name. Do
not qualify the column name with a table name
or table alias.
• Restriction: You cannot specify a LOB column or
a collection column in the USING column clause.
USING column Example
Select employees and their department, and departments
location
• SQL99
SELECT a.empno, a.ename, b.dname,
b.loc
FROM emp a JOIN dept b
USING (deptno);
ON condition
• Use the ON clause to specify a join
condition. Doing so lets you specify join
conditions separate from any search or
filter conditions in the WHERE clause.
ON condition Example
Select employees and their department, and departments
location
• SQL99
SELECT a.empno, a.ename, b.dname,
b.loc
FROM emp a JOIN dept b
ON (a.deptno = b.deptno);
Self Join
• A self join is a join of a table to itself. This
table appears twice in the FROM clause
and is followed by table aliases that qualify
column names in the join condition. To
perform a self join, Oracle combines and
returns rows of the table that satisfy the
join condition.
Self Join Example
Select employees and their managers
• SQL99
SELECT a.ename employee, b.ename
manager
FROM emp a JOIN emp b
ON (a.empno = b.mgr);
Three-Way Join Example (1)
Select employees and their departments and customers
that they work with
• SQL99
SELECT a.ename employee, b.dname
departmant,
c.name customer
FROM emp a JOIN dept b
ON (a.deptno = b.deptno)
JOIN customer c
ON (a.empno = c.repid);
Three-Way Join Example (2)
Select employees and their departments and customers
that they work with
• Oracle
SELECT a.ename employee, b.dname
departmant,
c.name customer
FROM emp a, dept b, customer c
WHERE a.deptno = b.deptno
AND a.empno = c.repid;
Outer Join (1)
An outer join extends the result of a simple join. An
outer join returns all rows that satisfy the join
condition and also returns some or all of those
rows from one table for which no rows from the
other satisfy the join condition.
– To write a query that performs an outer join of tables A
and B and returns all rows from A (a left outer join),
use the ANSI LEFT [OUTER] JOIN syntax, or apply
the outer join operator (+) to all columns of B in the
join condition. For all rows in A that have no matching
rows in B, Oracle returns null for any select list
expressions containing columns of B.
Outer Join (2)
– To write a query that performs an outer join of tables A
and B and returns all rows from B (a right outer join),
use the ANSI RIGHT [OUTER] syntax, or apply the
outer join operator (+) to all columns of A in the join
condition. For all rows in B that have no matching
rows in A, Oracle returns null for any select list
expressions containing columns of A.
– To write a query that performs an outer join and
returns all rows from A and B, extended with nulls if
they do not satisfy the join condition (a full outer join),
use the ANSI FULL [OUTER] JOIN syntax.
Left Outer Join Example
Select all employees, their job and customers that they
work with and customers addresses
• SQL99
SELECT a.ename, a.job, b.name,
b.address
FROM emp a LEFT OUTER JOIN
customer b
ON (a.empno = b.repid);
Right Outer Join Example
Select all departments, their location and employees that
work in them
• SQL99
SELECT a.ename, b.dname, b.loc
FROM emp a RIGHT OUTER JOIN dept b
ON (a.deptno = b.deptno);