Inner join, self join and Outer
join
Sen Zhang
• Joining data together is one of the most
significant strengths of a relational
database.
• A join is a query that combines rows from
two or more relations.
• Joins allow database users to combine
data from one table with data from one or
more other tables or views, or synonyms,
as long as they are relations.
• Tables are “joined” two at a time making a
new relation (a table generated on the fly)
containing all possible combinations of
rows from the original two tables
(sometimes called a “cross join” or
“Cartesian product”).
• See sample script
• A join condition is usually used to limit the
combinations of table data to just those
rows containing columns that match
columns in the other table.
• Most joins are “equi-joins” where the data
from a column in one table exactly
matches data in the column of another
table.
• It is also possible (though usually less
efficient) to join using ranges of values or
other comparisons between the tables
involved.
• A table may be “joined” to another table,
tables, or even itself (reused multiple
times).
• It is important to understand that whenever two
or more tables/views/synonyms (in fact, they are
all relations) are listed in a FROM clause, a join
results.
• Join conditions serve the purpose of limiting the
number of rows returned by the join.
• The absence of a join condition results in all
possible combinations of rows from the involved
tables, i.e. a Cartesian product, which is usually
not useful information.
Inner Joins
• An inner join (sometimes called a simple
join) is a join of two or more tables that
returns only those rows that satisfy the join
condition.
Inner Join
• Traditional inner joins look for rows that match rows in the other
table(s), i.e. to join two tables based on values in one table
being equal to values in another table
• Also known as equality join, equijoin or natural join
• Returns results only if records exist in both tables
Joining Via Linking Table
Self-Join
• A query that joins a table to itself, for example,
employee table can be joined to itself to find out
subordinate - supervisor pairs.
• Used when a table has a foreign key relationship
to itself (usually parent-child relationship)
• Must create a table alias and structure the query
as if you are joining the table to a copy of itself
• FROM table1 alias1, ...
• Use alias, not table name for select and where clauses
Self-Join Example
From inner join to outer join
• A problem with the simple inner join is that
only rows that match between tables are
returned; while it is also possible that a
user might be interested in rows that DO
NOT match rows in the other table(s).
• Finding rows without matches is often
referred as Outer Join.
What is an outer join
• An outer join extends the result of a simple
join (inner join, equ-join, theta join or
natural join).
• An outer join returns all rows that satisfy
the join condition and those rows from one
table for which no rows from the other
satisfy the join condition. Such rows are
not returned by a simple join.
What is an outer join
• To write a query that performs an outer
join of tables A and B and returns all rows
from A, 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 in Oracle 8i
• The syntax for performing an outer join in
SQL is database-dependent.
• For example, in Oracle 8i, we will place an
"(+)" in the WHERE clause on the other
side of the table for which we want to
include all the rows.
• In Oracle 9i, another format is used.
Outer Join
• Returns all rows in one table and matching rows in
joined table
• Inner table: all rows are returned, in other word, if
all rows need to be returned for one table, that
table is called inner table.
• Outer table: matching rows are returned
• Outer table marked with a + in join condition
inner_table.join_field = outer_table.join_field(+)
Null values are inserted for fields in outer table that
are not found
How to interpret “(+)”?
Which table (+) should tail?
• The oracle outer join operator “(+)” simply
tells SQL engine which table to generate
null values for the unmatched row of the
opposite table to match the table.
• Or simply, which side should generate null
rows.
Full join
• How about if we want to include rows in
both tables that cannot find match in the
opposite tables? It is called full join.
• Until now, the only way to accomplish a
full join (values missing on both sides of a
query) was to Union the results of both
left outerjoin and right outerjoin.
Sample scripts in class
• See sample scripts distributed in class for
various outer joins