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From Thomas Fischer <Fisc...@seitenbau.net>
Subject submitted patch which allows left joins, right joins and explicit inner joins
Date Fri, 18 Jun 2004 14:27:36 GMT




Hi,

I have added functionality to Torque that allows one to do left joins,
right joins (and explicit inner joins, in case anybody needs it). I posted
the diffs on scarab, the ticket id is TRQS219.
(http://nagoya.apache.org/scarab/issues/id/TRQS219)

To create a left join, the following code has to be used:
criteria.addJoin(AuthorPeer.AUTHOR_ID, BookPeer.AUTHOR_ID,
Criteria.LEFT_JOIN);
It is, of course, still possible to use
criteria.addJoin(AuthorPeer.AUTHOR_ID, BookPeer.AUTHOR_ID);
This is equivalent to
criteria.addJoin(AuthorPeer.AUTHOR_ID, BookPeer.AUTHOR_ID, null);
and produces a join in the as it was done up to now, i.e. by adding the
join condition to the "where" clause of the query.
I tested my changes on an Oracle9i database. I do not know whether the code
runs on any other database (it produces SQL like "SELECT author.AUTHOR_ID,
author.NAME FROM book RIGHT JOIN author ON book.AUTHOR_ID=author.AUTHOR_ID
LEFT JOIN book b ON author.AUTHOR_ID=b.AUTHOR_ID").

Note that I did NOT change the code in generator which generates code for
selets with joins, e.g. BookPeer.doSelectJoinAuthor(). In my opinion, it
would be nice to use a left join there to get also the books without an
author, but I did not want to make too many changes at one time.

I hope that I have made the changes such that fully qualified table names
(database.schema.columnname) are possible, though I did not test it.

Below, I summarize the changes which I made, then I give an example how the
joins are handled internally, and then I discuss some special cases.

Any comments or questions are welcome.

    Cheers

            Thomas


Changes:

added the necessary Constants (Join types and "ON") to SqlEnum and Criteria

added the methods equals(Object) and hashCode() to SqlEnum. Thinking it
through, this would not have been strictly necessary, because SqlEnum has
no public constructor and only the predefined constants are used, so
Object.equals() should give the same result as the new equals()-method.
However, in my opinion adding the new method makes the code more robust and
better readable, but this modification can be left out if the developers
think it's not necessary.)

Added a new method addJoin(String left, String right, SqlEnum joinType) to
the Criteria class

Added an inner class called Join to the Criteria class. A Join contains the
left column, the right column and the join type. Replaced the Lists joinL
and joinR by a new list joins which contains elements of type Join.

Added an inner Class called FromElement to Query. A fromElement contains
the table name in the FROM clause. It might also contain the join type and
the join condition.

Added a private helper function Criteria.getTablenameForFromClause(String
tableOrAliasName, Criteria criteria) which returns either the tablename
itself if tableOrAliasName is not an alias, or a String of the form
"tableName tableOrAliasName" if tableOrAliasName is an alias for a table
name.

Added a public helper function getRawColumnName(String
fullyQualifiedColumnName) which strips the database (and schema) names from
columns. This is useful for working with aliases, as the constants for
column names in the peer classes are fully qualified.

Found that most code in BasePeer.createQuery(Criteria) was duplicated in
BasePeer.createPreparedStatement(Criteria, StringBuffer, List). As I had to
make changes in both methods anyway, I decided to split the code and put
the parts which are (almost) equal in both methods into private the methods
processSelectColumns(..), processAsColumns(..), processJoins(..),
processModifiers(..), processCriterions(..) and processOrderBy(..). I am
not sure why BasePeer.createQuery(..) processes the groupBy- and
Having-Lists of Criteria while BasePeer.createPreparedStatement(..)
doesn't, but I left it as it was, though I think it's a bit fishy. Also,
the criteria.getLimit() and criteria.getOffset() are processed differently
by BasePeer.createQuery(..) and BasePeer.createPreparedStatement(..). I
could see no reason for this, but I left it also as it was.

Changed the treatment of select clauses in BasePeer.createQuery(..) and
basePeer.createPreparedStatement(..). Both treatments are quite similar,
except that the checks on column names differ: In normal statements, a
column does not have to contain a "." to be valid, if it contains a "*".
However, criteria.addSelectColumn("*") fails in Torque 3.1 with an
StringIndexOutOfBoundsException. Does anybody know why this is so ? I did
not really know what to do, so I changed the behaviour in
basePeer.createPreparedStatement(..) so that it is like the behaviour in
BasePeer.createQuery(..). Should not "*" be a valid "column name" to select
from ???


Example illustrating the used algorithm:

A Criteria should contain a single left join. The SQL snippet which is
expected is
table_a left join table_b on table_a.id = table_b.a_id

First, the method criteria.addJoin("table_a.id", "table_b.a_id",
Criteria.LEFT_JOIN) is called.
In this function call, criteria.joins would be populated with a single
element of type Join where the left column is table_a.id, the right column
is table_b.a_id and the join type is SqlEnum.LEFT_JOIN.

calling BasePeer.createQuery(criteria) would create a Query element where
fromTables contains two elements of type FromElement. In the first
FromElement, the table name is table_a and the join type and the join
condition are both null. In the second FromElement, the table name is
table_b, the join type is SqlEnum.LEFT_JOIN and the join condition is
table_a.id = table_b.a_id

In BasePeer.toString(), the first element of the fromTables would be
appended at the appropriate place by calling its toString() method. Every
other element is checked whether it contains a join type. If not, a comma
is appended before the element itself is appended; if yes, a space is
appended before the element itself is appended. This leads to the desired
result.


Special cases which are considered in the code:

1) criteria.addJoin("table_a.id", "table_b.a_id") is called (corresponds to
criteria.addJoin("table_a.id", "table_b.a_id", null) ).
In this case, criteria.joins would still be populated with a single element
of type Join where the left column is table_a.id and the right column is
table_b.a_id. However, in contrast to the previous example, the join type
is null. On calling BasePeer.createQuery(criteria), a query element would
be created where the to fromElements contain only the two tablenames, and
the join condition would be added to the where clause (and not to the
second fromElement, as was done in the previous example).

2) "Simple" multiple joins are considered.
criteria.addJoin("table_a.id", "table_b.a_id", Criteria.LEFT_JOIN);
criteria.addJoin("table_b.id", "table_c.b_id", Criteria.LEFT_JOIN);
The sql should be
table_a left join table_b on table_a.id = table_b.a_id left join table_c on
table_b.id = table_c.second_a_id
i.e. it must be taken into account that table_b is already "in use", it
must not be added a second time to query.fromTables.
This is implemented as follows: In the past, this behaviour was ensured by
using a uniqueList in the fromTables. To retain that behaviour, one could
define that two FromElements are equal if their tablenames are equal.
However, this is not the "intuitive" behaviour of equals() which is
expected from a FromElement, therefore this approach was not implemented.
Instead, the method BasePeer.createQuery(criteria) checks "by hand" (i.e.
by using the method BasePeer.fromClauseContainsTableName(..)) to see if a
tablename already exists

3) "More difficult" multiple joins : consider the following code snippet:
criteria.addAlias("b", "table_b");
criteria.addJoin("table_a.id", "table_b.a_id", Criteria.LEFT_JOIN);
criteria.addJoin("b.second_a_id", "table_a.id", Criteria.RIGHT_JOIN);
The expected SQL is NOT
table_a left join table_b on table_a.id = table_b.a_id, table_b b right
join table_a on b.second_a_id = table_a.id
but is is instead
table_a left join table_b on table_a.id = table_b.a_id left join table_b b
on table_a.id = b.second_a_id
i.e. if the second table name is already "in use", the join order and the
join type should be "reversed". This is done in
BasePeer.createQuery(criteria). (Note that this works only because joins
are now processed before other code which may also add tables to the from
clause). If both columns are already "in use", an exception is thrown.

4) Tablenames can also be added to a criteria by adding columns from
foreign tables explicity to criteria : Consider
criteria.addSelectColumn("table_b.id");
criteria.addJoin("table_a.id", "table_b.a_id", Criteria.LEFT_JOIN);
as opposed to
criteria.addSelectColumn("table_c.id");
criteria.addJoin("table_a.id", "table_b.a_id", Criteria.LEFT_JOIN);
It is ensured that in the first case, table_b is not added (again) to the
from clause, while in the second case, table_c must be added to the from
clause. (This was not an issue in Torque 3.1 because the table names were
stored in an UniqueList, but now this has to be considered.)


Special cases where I don't know whether they are treated correctly :

1) I am not sure whether I have treated added Criterion's correctly
(criteria.add(Criterion)). The tablenames for added Criterions are added
like the colums from the select Columns (see 4), which is the treatment
which they got in BasePeer.java in version 1.78 from the CVS. Therefore
this should work with the old join type (join type null), but I have no
idea if this treatment is ok for the join types LEFT_JOIN, RIGHT_JOIN and
INNER_JOIN. This is mainly because I have no clue what
criteria.add(Criterion) does.

If anybody can think of another special case which I did not consider,
please tell me.


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