Difference between Oracle's plus (+) notation and ansi JOIN notation?

AFAIK, the (+) notation is only present for backwards compatibility because Oracle debuted it before the ANSI standard for joins was put in place. It's specific to Oracle and you should avoid using it in new code when there's an equivalent standards-compliant version available.

It seems there are differences between the two, and the (+) notation has restrictions that the ANSI join syntax does not have. Oracle themselves recommend that you not use the (+) notation. Full description here in the Oracle® Database SQL Language Reference11g Release 1 (11.1):

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:

SELECT employee_id, manager_idFROM employeesWHERE employees.manager_id(+) = employees.employee_id;

However, the following self join is valid:

SELECT e1.employee_id, e1.manager_id, e2.employee_idFROM employees e1, employees e2WHERE 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.

The most comprehensive answer obviously is the one by nagul.

An addition for those who are looking for quick translation/mapping to the ANSI syntax:

---- INNER JOIN--SELECT *FROM EMP eINNER JOIN DEPT d ON d.DEPTNO = e.DEPTNO; -- Synonym in deprecated oracle (+) syntaxSELECT *FROM EMP e,     DEPT dWHERE d.DEPTNO = e.DEPTNO;---- LEFT OUTER JOIN--SELECT *FROM EMP eLEFT JOIN DEPT d ON d.DEPTNO = e.DEPTNO; -- Synonym in deprecated oracle (+) syntaxSELECT *FROM EMP e,     DEPT dWHERE d.DEPTNO (+) = e.DEPTNO;---- RIGHT OUTER JOIN--SELECT *FROM EMP eRIGHT JOIN DEPT d ON d.DEPTNO = e.DEPTNO;-- Synonym in deprecated oracle (+) syntaxSELECT *FROM EMP e,     DEPT dWHERE d.DEPTNO = e.DEPTNO(+);---- CROSS JOIN--SELECT *FROM EMP eCROSS JOIN DEPT d; -- Synonym in deprecated oracle (+) syntaxSELECT *FROM EMP e,     DEPT d;---- FULL JOIN--SELECT *FROM EMP eFULL JOIN DEPT d ON d.DEPTNO = e.DEPTNO;-- Synonym in deprecated oracle (+) syntax !NOT WORKING!SELECT *FROM EMP e,     DEPT dWHERE d.DEPTNO (+) = e.DEPTNO(+);

The notation is still supported as of Oracle 10 (and I believe 11). It's use is considered "old fashioned", and also is not as database portable as the ANSI JOIN syntax. It's also considered much less readable, although if you come from the + background getting used to ANSI JOIN can take a little time. The important thing to know before hurling brickbats at Oracle is that they developed their + syntax before the ANSI committee had completed the definitions for the joins.

There is no performance difference; they are expressing the same thing.

Edit: By "not as portable" I should have said "only supported in Oracle SQL"