Left Outer Join using + sign in Oracle 11g

Question

Can any one tell me whether below 2 queries are an example of Left Outer Join or Right Outer Join??

Table Part: Name         Null?       Type PART_ID      NOT NULL    VARCHAR2(4) SUPPLIER_ID              VARCHAR2(4)  PART_ID SUPPLIER_ID P1      S1 P2      S2 P3   P4    Table Supplier: Name            Null?     Type SUPPLIER_ID NOT NULL      VARCHAR2(4) SUPPLIER_NAME   NOT NULL  VARCHAR2(20)  SUPPLIER_ID  SUPPLIER_NAME S1           Supplier#1 S2           Supplier#2 S3           Supplier#3 

Display all the parts irrespective of whether any supplier supplies them or not:

 SELECT P.Part_Id, S.Supplier_Name FROM Part P, Supplier S WHERE P.Supplier_Id = S.Supplier_Id (+)  SELECT P.Part_Id, S.Supplier_Name FROM Part P, Supplier S WHERE S.Supplier_Id (+) = P.Supplier_Id 

Answer 1

TableA LEFT OUTER JOIN TableB is equivalent to TableB RIGHT OUTER JOIN Table A.

In Oracle, (+) denotes the "optional" table in the JOIN. So in your first query, it's a P LEFT OUTER JOIN S. In your second query, it's S RIGHT OUTER JOIN P. They're functionally equivalent.

In the terminology, RIGHT or LEFT specify which side of the join always has a record, and the other side might be null. So in a P LEFT OUTER JOIN S, P will always have a record because it's on the LEFT, but S could be null.

See this example from java2s.com for additional explanation.

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To clarify, I guess I'm saying that terminology doesn't matter, as it's only there to help visualize. What matters is that you understand the concept of how it works.

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RIGHT vs LEFT

I've seen some confusion about what matters in determining RIGHT vs LEFT in implicit join syntax.

LEFT OUTER JOIN

SELECT * FROM A, B WHERE A.column = B.column(+) 

RIGHT OUTER JOIN

SELECT * FROM A, B WHERE B.column(+) = A.column 

All I did is swap sides of the terms in the WHERE clause, but they're still functionally equivalent. (See higher up in my answer for more info about that.) The placement of the (+) determines RIGHT or LEFT. (Specifically, if the (+) is on the right, it's a LEFT JOIN. If (+) is on the left, it's a RIGHT JOIN.)

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Types of JOIN

The two styles of JOIN are implicit JOINs and explicit JOINs. They are different styles of writing JOINs, but they are functionally equivalent.

See this SO question.

Implicit JOINs simply list all tables together. The join conditions are specified in a WHERE clause.

Implicit JOIN

SELECT * FROM A, B WHERE A.column = B.column(+) 

Explicit JOINs associate join conditions with a specific table's inclusion instead of in a WHERE clause.

Explicit JOIN

SELECT * FROM A LEFT OUTER JOIN B ON A.column = B.column 

These Implicit JOINs can be more difficult to read and comprehend, and they also have a few limitations since the join conditions are mixed in other WHERE conditions. As such, implicit JOINs are generally recommended against in favor of explicit syntax.

Answer 2

Those two queries are performing OUTER JOIN. See below

Oracle recommends that you use the FROM clause OUTER JOIN syntax rather than the Oracle join operator. Outer join queries that use the Oracle join operator (+) are subject to the following rules and restrictions, which do not apply to the FROM clause OUTER JOIN syntax:

• You cannot specify the (+) operator in a query block that also contains FROM clause join syntax.

• The (+) operator can appear only in the WHERE clause or, in the context of left- correlation (when specifying the TABLE clause) in the FROM clause, and can be applied only to a column of a table or view.

• If A and B are joined by multiple join conditions, then you must use the (+) operator in all of these conditions. If you do not, then Oracle Database will return only the rows resulting from a simple join, but without a warning or error to advise you that you do not have the results of an outer join.

• The (+) operator does not produce an outer join if you specify one table in the outer query and the other table in an inner query.

• You cannot use the (+) operator to outer-join a table to itself, although self joins are valid. For example, the following statement is not valid:

  -- The following statement is not valid: SELECT employee_id, manager_id    FROM employees    WHERE employees.manager_id(+) = employees.employee_id; 

  However, the following self join is valid:

  SELECT e1.employee_id, e1.manager_id, e2.employee_id    FROM employees e1, employees e2    WHERE e1.manager_id(+) = e2.employee_id    ORDER BY e1.employee_id, e1.manager_id, e2.employee_id; 

• The (+) operator can be applied only to a column, not to an arbitrary expression. However, an arbitrary expression can contain one or more columns marked with the (+) operator.

• A WHERE condition containing the (+) operator cannot be combined with another condition using the OR logical operator.

• A WHERE condition cannot use the IN comparison condition to compare a column marked with the (+) operator with an expression.

If the WHERE clause contains a condition that compares a column from table B with a constant, then the (+) operator must be applied to the column so that Oracle returns the rows from table A for which it has generated nulls for this column. Otherwise Oracle returns only the results of a simple join.

In a query that performs outer joins of more than two pairs of tables, a single table can be the null-generated table for only one other table. For this reason, you cannot apply the (+) operator to columns of B in the join condition for A and B and the join condition for B and C. Refer to SELECT for the syntax for an outer join.

Taken from http://download.oracle.com/docs/cd/B28359_01/server.111/b28286/queries006.htm