I’m currently reviewing a draft of Roger Troller’s updated PL/SQL and SQL Coding Guidelines version 3.0. One guideline recommends to use ANSI join syntax. The mentioned reasons are
ANSI join syntax does not have as many restrictions as the ORACLE join syntax has. Furthermore ANSI join syntax supports the full outer join. A third advantage of the ANSI join syntax is the separation of the join condition from the query filters.
While I read this I wondered which restrictions still exist for ORACLE join syntax nowadays and searched for “(+)” in the current Error Messages documentation (E49325-06) and found the following error messages:
- ORA-01417: a table may be outer joined to at most one other table
- ORA-01719: outer join operator (+) not allowed in operand of OR or IN
- ORA-01799: a column may not be outer-joined to a subquery
- ORA-25156: old style outer join (+) cannot be used with ANSI joins
- ORA-30563: outer join operator (+) is not allowed here
In the 9.2 documentation (A96525-01) I found the following additional messages:
- ORA-01416: two tables cannot be outer-joined to each other
- ORA-01468: a predicate may reference only one outer-joined table
I’ve written SQL statements to produce the error message listed above on a 9.2.0.8 Oracle database and ran them on a 12.1.0.2 database as well to see which restrictions still exist for the outer join operator (+) as basis for my feedback to Roger. While writing the queries I thought this might be an interesting topic to blog about.
Examples
SELECT s.*, p.*
FROM sh.sales s, sh.products p
WHERE p.prod_id = s.prod_id(+)
AND p.supplier_id(+) = s.channel_id;An ORA-01416 is thrown in 9.2.0.8 and in 12.1.0.2. You cannot formulate such a query using ANSI join. Doing something like that does not make sense. It is not a relevant restriction. But it is interesting to see that an ORA-01416 is thrown in Oracle 12.1.0.2, even if this error message is not documented anymore.
SELECT s.*, c.*, p.*
FROM sh.sales s, sh.customers c, sh.products p
WHERE p.prod_id = s.prod_id(+)
AND c.cust_id = s.cust_id(+);An ORA-01417 is thrown in 9.2.0.8 but not in 12.1.0.2.
SELECT s.*, p.* FROM sh.sales s, sh.products p WHERE p.prod_id(+) = s.prod_id(+);
An ORA-01468 is thrown in 9.2.0.8 and in 12.1.0.2. You cannot formulate such a query using ANSI join. It could have been a way to formulate a full outer join, but something like that is not supported with Oracle join syntax. ORA-01468 is not documented in Oracle 12.1.0.2, but nonetheless this error is thrown. I do not consider this a relevant restriction for Oracle join-syntax.
SELECT s.*, p.*
FROM sh.sales s, sh.products p
WHERE p.prod_id(+) = s.prod_id
AND p.prod_category(+) IN ('Boys', 'Girls');An ORA-01719 is thrown in 9.2.0.8 but not in 12.1.0.2.
SELECT s.*
FROM sh.sales s
WHERE s.time_id(+) = (SELECT MAX(t.time_id)
FROM sh.times t);An ORA-01799 is thrown in 9.2.0.8 and in 12.1.0.2. You cannot formulate such a query using ANSI join. Of course you may rewrite this to a valid Oracle join or ANSI join query. Here’s an example:
SELECT s.*, t.max_time_id
FROM sh.sales s,
(SELECT MAX(t.time_id) AS max_time_id
FROM sh.times t) t
WHERE s.time_id(+) = t.max_time_id;Because the restriction applies to ANSI join as well, I do not consider this a relevant restriction for Oracle join syntax.
SELECT s.*, c.*, p.*
FROM sh.sales s, sh.customers c
JOIN sh.products p
ON (p.prod_id = s.prod_id)
WHERE c.cust_id = s.cust_id(+);An ORA-25156 is thrown in 9.2.0.8 and in 12.1.0.2. This is not really a restriction for Oracle join syntax. The grammar simply does not support to mix join syntax variants.
SELECT lpad(' ', (LEVEL - 1) * 3) || to_char(e.empno) || ' ' ||
e.ename(+) ||
' ' || d.dname AS emp_name
FROM scott.emp e, scott.dept d
WHERE e.deptno(+) = d.deptno
CONNECT BY PRIOR e.empno(+) = e.mgr
START WITH e.ename(+) = 'KING'
ORDER BY rownum, e.empno(+);An ORA-30563 is thrown in 9.2.0.8 and 12.1.0.2. Interesting is that if you remove the (+) on the highlighted line 2 the query works on 9.2.0.8 but not on 12.1.0.2. Using the (+) in a CONNECT BY clause, START WITH clause, or ORDER BY clause does not make sense. It is not possible using ANSI-join as well. The important part is the join itself on line 5 and this is working in conjunction with a CONNECT BY. Therefore I do consider this an irrelevant restriction for the Oracle join syntax.
Summary
The results of the example relevant statements are summarized in the following table.
| Error message by test SQL | Relevant outer join restriction? | Result in 9.2.0.8 | Result in 12.1.0.2 |
|---|---|---|---|
| ORA-01416 two tables cannot be outer-joined to each other | No | Error | Error |
| ORA-01417: a table may be outer joined to at most one other table | Yes | Error | OK |
| ORA-01468 a predicate may reference only one outer-joined table | No | Error | Error |
| ORA-01719: outer join operator (+) not allowed in operand of OR or IN | Yes | Error | OK |
| ORA-01799: a column may not be outer-joined to a subquery | No | Error | Error |
| ORA-25156: old style outer join (+) cannot be used with ANSI joins | No | Error | Error |
| ORA-30563: outer join operator (+) is not allowed here | No | Error | Error |
Table 1: Outer join operator (+) restrictions in 9.2.0.8 and 12.1.0.2
In the most current Oracle version no relevant limitations exist regarding the Oracle join syntax. Hence choosing ANSI join syntax just because in the past some limitations existed is doing the right for the wrong reasons… I favor the ANSI join syntax because filter and join conditions are clearly separated. For full outer joins, there is simply no better performance option than to use ANSI join syntax. See also also Chris Antognini’s post about native full outer join.