I know how to use "DISTINCT" in Doctrine 2, but I really need to use "DISTINCT ON (field)" and I don't know how to do this with the QueryBuilder.
My SQL query looks like:
SELECT DISTINCT ON (currency) currency, amount FROM payments ORDER BY currency
And this query works perfect, but I can't use it with the QueryBuilder. Maybe I could write this query on some other way?
I would suggest that the SELECT DISTINCT ON (..) construct that PostgreSQL supports is outside the Object Relational Model (ORM) that is central to Doctrine. Or, perhaps put another way, because SELECT DISTINCT ON (..) is rare in SQL implementations Doctrine haven't coded for it.
Regardless of the actual logic for it not working, I would suggest you try Doctrine's "Native SQL". You need to map the results of your query to the ORM.
With NativeQuery you can execute native SELECT SQL statements and map
the results to Doctrine entities or any other result format supported
by Doctrine.
In order to make this mapping possible, you need to describe to
Doctrine what columns in the result map to which entity property. This
description is represented by a ResultSetMapping object.
With this feature you can map arbitrary SQL code to objects, such as
highly vendor-optimized SQL or stored-procedures.
SELECT DISTINCT ON (..) falls into vendor-optimized SQL I think, so using NativeQuery should allow you to access it.
Doctrine QueryBuilder has some limitations. Even if I didn't check if it's was possible with query builder, I do not hesitate to use DQL when I do not know how to write the query with query builder.
Check theses examples at
http://doctrine-orm.readthedocs.org/en/latest/reference/dql-doctrine-query-language.html#dql-select-examples
Hope this help.
INDEX BY can be used in DQL, allowing first result rows indexed by the defined string/int field to be overwritten by following ones with the same index:
SELECT
p.currency,
p.amount
FROM Namespace\To\Payments p INDEX BY p.currency
ORDER BY p.currency ASC
DQL - EBNF - INDEX BY
Related
I have a custom query along these lines. I get the list of orderIds from outside. I have the entire order object list with me, so I can change the query in any way, if needed.
#Query("SELECT p FROM Person p INNER JOIN p.orders o WHERE o.orderId in :orderIds)")
public List<Person> findByOrderIds(#Param("orderIds") List<String> orderIds);
This query works fine, but sometimes it may have anywhere between 50-1000 entries in the orderIds list sent from outside function. So it becomes very slow, taking as much as 5-6 seconds which is not fast enough. My question is, is there a better, faster way to do this? When I googled, and on this site, I see we can use ANY, EXISTS: Postgresql: alternative to WHERE IN respective WHERE NOT IN or create a temporary table: https://dba.stackexchange.com/questions/12607/ways-to-speed-up-in-queries-under-postgresql or join this to VALUES clause: Alternative when IN clause is inputed A LOT of values (postgreSQL). All these answers are tailored towards direct SQL calls, nothing based on JPA. ANY keyword is not supported by spring-data. Not sure about creating temporary tables in custom queries. I think I can do it with native queries, but have not tried it. I am using spring-data + OpenJPA + PostgresSQL.
Can you please suggest a solution or give pointers? I apologize if I missed anything.
thanks,
Alice
You can use WHERE EXISTS instead of IN Clause in a native SQL Query as well as in HQL in JPA which results in a lot of performance benefits. Please see sample below
Sample JPA Query:
SELECT emp FROM Employee emp JOIN emp.projects p where NOT EXISTS (SELECT project from Project project where p = project AND project.status <> 'Active')
I want to do something like this in OrmLite
SELECT *, COUNT(title) as titleCount from table1 group by title;
Is there any way to do this via QueryBuilder without the need for queryRaw?
The documentation states that the use of COUNT() and the like necessitates the use of selectRaw(). I hoped for a way around this - not having to write my SQL as strings is the main reason I chose to use ORMLite.
http://ormlite.com/docs/query-builder
selectRaw(String... columns):
Add raw columns or aggregate functions
(COUNT, MAX, ...) to the query. This will turn the query into
something only suitable for using as a raw query. This can be called
multiple times to add more columns to select. See section Issuing Raw
Queries.
Further information on the use of selectRaw() as I was attempting much the same thing:
Documentation states that if you use selectRaw() it will "turn the query into" one that is supposed to be called by queryRaw().
What it does not explain is that normally while multiple calls to selectColumns() or selectRaw() are valid (if you exclusively use one or the other),
use of selectRaw() after selectColumns() has a 'hidden' side-effect of wiping out any selectColumns() you called previously.
I believe that the ORMLite documentation for selectRaw() would be improved by a note that its use is not intended to be mixed with selectColumns().
QueryBuilder<EmailMessage, String> qb = emailDao.queryBuilder();
qb.selectColumns("emailAddress"); // This column is not selected due to later use of selectRaw()!
qb.selectRaw("COUNT (emailAddress)");
ORMLite examples are not as plentiful as I'd like, so here is a complete example of something that works:
QueryBuilder<EmailMessage, String> qb = emailDao.queryBuilder();
qb.selectRaw("emailAddress"); // This can also be done with a single call to selectRaw()
qb.selectRaw("COUNT (emailAddress)");
qb.groupBy("emailAddress");
GenericRawResults<String[]> rawResults = qb.queryRaw(); // Returns results with two columns
Is there any way to do this via QueryBuilder without the need for queryRaw(...)?
The short answer is no because ORMLite wouldn't know what to do with the extra count value. If you had a Table1 entity with a DAO definition, what field would the COUNT(title) go into? Raw queries give you the power to select various fields but then you need to process the results.
With the code right now (v5.1), you can define a custom RawRowMapper and then use the dao.getRawRowMapper() method to process the results for Table1 and tack on the titleCount field by hand.
I've got an idea how to accomplish this in a better way in ORMLite. I'll look into it.
I've written a SQL query that basically selects from a number of tables to determine which ones have rows that were created since a particular date. My SQL looks something like this:
SELECT widget_type FROM(
SELECT 'A' as widget_type
FROM widget_a
WHERE creation_timestamp > :cutoff
UNION
SELECT 'B' as widget_type
FROM widget_b
WHERE creation_timestamp > :cutoff
) types
GROUP BY widget_type
HAVING count(*)>0
That works well in SQL but I recently found that, while JPA may use unions to perform "table per class" polymorphic queries, JPQL does not support unions in queries. So that leaves me wondering whether JPA has an alternative I could use to accomplish the same thing.
In reality, I would be querying against a dozen tables, not just two, so I would like to avoid doing separate queries. I would also like to avoid doing a native SQL query for portability reasons.
In the question I linked to above, it was asked whether the entities that map to widget_a and widget_b are part of the same inheritance tree. Yes, they are. However, if I selected from their base class, I don't believe I would have a way of specifying different string constants for the different child entities, would I? If I could select an entity's class name instead of a string I provide, that might serve my purpose too. But I don't know if that's possible either. Thoughts?
I did a little more searching and found a (seemingly obscure) feature of JPA that serves my purpose perfectly. What I found is that JPA 2 has a type keyword that allows you to limit polymorphic queries to a particular subclass, like so:
SELECT widget
FROM BaseWidget widget
WHERE TYPE(widget) in (WidgetB, WidgetC)
I've found that JPA (or at least Hibernate as a JPA implementation) allows you to use type not only in constraints but also in select lists. This is approximately what my query ended up looking like:
SELECT DISTINCT TYPE(widget)
FROM BaseWidget widget
WHERE widget.creationTimestamp > :cutoff
That query returns a list of Class objects. My original query was selecting string literals because that's closest to what I might have done in SQL. Selecting Class is actually preferable in my case. But if I did prefer to select a constant based on an entity's type, that is the exact scenario that Oracle's documentation uses to illustrate case statements:
SELECT p.name
CASE TYPE(p)
WHEN Student THEN 'kid'
WHEN Guardian THEN 'adult'
WHEN Staff THEN 'adult'
ELSE 'unknown'
END
FROM Person p
Some JPA providers do support UNION,
http://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Querying/JPQL#UNION
but your query seems very complex, and non object-oriented, so using a native SQL query would probably be best.
I'm using Zend Framework for my website and I'd like to retrieve some data from my PostgreSQL database.
I have a request like :
SELECT DISTINCT ON(e.id) e.*, f.*, g.* FROM e, f, g
WHERE e.id = f.id_e AND f.id = g.id_f
This request works well but I don't know how to convert the DISTINCT ON(e.id) with Zend.
It seems that I can get DISTINCT rows but no distinct columns.
$select->distinct()->from("e")->join("f", "e.id = f.id_e")
->join("g", "f.id = g.id_f");
Any idea on how to make a select with distinct column ?
Thanks for help
You probably can't do this with Zend Framework since distinct on is not part of the SQL standard (end of page in Postgres documentation). Although Postgres supports it, I would assume its not part of Zend Framework because you could in theory configure another database connection which does not offer support.
If you know in advance that you're developing for a specific database (Postgres in this case), you could use manually written statements instead. You'll gain more flexibility within the queries and better performance at the cost of no longer being able to switch databases.
You would then instantiate a Zend_Db_Apdapter for Postgres. There a various methods available to get results for SQL queries which are described in the frameworks documentation starting at section Reading Query Results. If you choose to go this route I'd recommend to create an own subclass of the Zend_Db_Adapter_Pgsql class. This is to be able to convert data types and throw exceptions in case of errors instead of returning ambiguous null values and hiding error causes.
Is it possible to sort the data inside of an oracle table? Like ascending/descending via a certain column, alphabetically. Oracle 10g express.
You could try
Select *
from some_table
order by some_column asc
This will sort the results by some_column and place them in ascending order. Use desc instead of asc if you want descending order. Or did you mean to have the ordering in the physical storage itself?
I believe it's possible to specify the ordering/sorting of an indexed column in storage. It's probably closest to what you want. I don't usually use this index sort feature, but for more info see: http://www.stanford.edu/dept/itss/docs/oracle/10g/server.101/b10759/statements_5010.htm#i2062718
Perhaps you could use an index organized table - IOT to ensure that the data is stored ordered by index.
Have a look at the physical properties clause of the CREATE TABLE statement:
http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/statements_7002.htm#i2128663
What is the problem that you are trying to solve though? An IOT may or may not be what you should be using.
As defined by the relational model, the rows and columns in a table are unordered. That's the theory at least.
In practice, if you want the data to come out of a query in a particular order then you should always use the ORDER BY clause. The order of the output is not guarantee unless you provide that.
It would be possible to use an ORDER BY when inserting into a table but that doesn't guarantee the order that data will be output. A query might come out in the same order every time.... but that doesn't mean it will come out in the same order next time.
There were issues when Oracle 10g came out where aggregate queries (with GROUP BY) were not coming out sorted because users had come to rely on the data being sorted as a side-effect of the grouping. With the introduction of the HASH GROUP BY in addition to the SORT GROUP BY people were caught out. This was a useful reminder that ORDER BY should always be used.
What do you really mean ?
Are you just asking for the order by clause ?
http://www.1keydata.com/sql/sqlorderby.html
Oracle 10g Express supports ANSI SQL like most RDBM's so you can sort in the standard manner:
SELECT * FROM Persons ORDER BY LastName
A good tutorial on SQL can be found here: w3schools SQL
Oracle Express does have some limitations compared to the Enterprise Edition but not in the basic SQL dialect it supports.