问题
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
回答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.
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.
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.)
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.
回答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
回答3:
I saw some contradictions in the answers above, I just tried the following on Oracle 12c and the following is correct :
LEFT OUTER JOIN
SELECT *
FROM A, B
WHERE A.column = B.column(+)
RIGHT OUTER JOIN
SELECT *
FROM A, B
WHERE B.column(+) = A.column
回答4:
There is some incorrect information in this thread. I copied and pasted the incorrect information:
LEFT OUTER JOIN
SELECT * FROM A, B WHERE A.column = B.column(+)RIGHT OUTER JOIN
SELECT * FROM A, B WHERE B.column(+) = A.column
The above is WRONG!!!!! It's reversed. How I determined it's incorrect is from the following book:
Oracle OCP Introduction to Oracle 9i: SQL Exam Guide. Page 115 Table 3-1 has a good summary on this. I could not figure why my converted SQL was not working properly until I went old school and looked in a printed book!
Here is the summary from this book, copied line by line:
Oracle outer Join Syntax:
from tab_a a, tab_b b,
where a.col_1 + = b.col_1
ANSI/ISO Equivalent:
from tab_a a left outer join
tab_b b on a.col_1 = b.col_1
Notice here that it's the reverse of what is posted above. I suppose it's possible for this book to have errata, however I trust this book more so than what is in this thread. It's an exam guide for crying out loud...
来源：https://stackoverflow.com/questions/6559261/left-outer-join-using-sign-in-oracle-11g