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select item.description from Item item
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It s difficult to see how it could possibly be meaningful to have two identical rows in a query result, so if you think duplicates are likely, you should use the distinct keyword:
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select distinct item.description from Item item
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This eliminates duplicates from the returned list of Item descriptions.
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Calling SQL functions It s also possible (for some Hibernate SQL dialects) to call database-specific SQL functions from the select clause (remember, you can freely do it in the where clause). For example, the following query retrieves the current date and time from the database server (Oracle syntax), together with a property of Item:
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select item.startDate, sysdate from Item item
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The technique of database functions in the select clause is of course not limited to database-dependent functions, but to other, more generic (or standardized) SQL functions as well:
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select item.startDate, item.endDate, upper(item.name) from Item item
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This query returns an Object[] with the starting and ending date of an item auction, and the name of the item all in uppercase. In particular, it s possible to call SQL aggregate functions.
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7.4.2 Using aggregation
Hibernate recognizes the following aggregate functions: count(), min(), max(), sum(), and avg(). This query counts all the Items:
select count(*) from Item
The result is returned as an Integer:
Integer count = (Integer) session.createQuery("select count(*) from Item") .uniqueResult();
Notice how we use *, which has the same semantics as in SQL. The next variation of the query counts all Items that have a successfulBid:
select count(item.successfulBid) from Item item
This query calculates the total of all the successful Bids:
select sum(item.successfulBid.amount) from Item item
The query returns a BigDecimal. Notice the use of an implicit join in the select clause: We navigate the association (successfulBid) from Item to Bid by referencing it with a dot. The next query returns the minimum and maximum bid amounts for a particular Item:
Writing report queries
select min(bid.amount), max(bid.amount) from Bid bid where bid.item.id = 1
The result is an ordered pair of BigDecimals (two instances of BigDecimal in an Object[] array). The special count(distinct) function ignores duplicates:
select count(distinct item.description) from Item item
When you call an aggregate function in the select clause without specifying any grouping in a group by clause, you collapse the result down to a single row containing your aggregated value(s). This means (in the absence of a group by clause) any select clause that contains an aggregate function must contain only aggregate functions. So, for more advanced statistics and reporting, you ll need to be able to perform grouping.
7.4.3 Grouping
Just like in SQL, any property or alias that appears in HQL outside of an aggregate function in the select clause must also appear in the group by clause. Consider the next query, which counts the number of users with each particular last name:
select u.lastname, count(u) from User u group by u.lastname
Now look at the generated SQL:
select U.LAST_NAME, count(U.USER_ID) from USER U group by U.LAST_NAME
In this example, the u.lastname isn t inside an aggregate function; we use it to group the result. We also don t need to specify the property we d like to count in HQL. The generated SQL will automatically use the primary key if we use an alias that has been set in the from clause. The next query finds the average bid amount for each item:
select bid.item.id, avg(bid.amount) from Bid bid group by bid.item.id
This query returns ordered pairs of Item identifiers and average bid amount. Notice how we use the id special property to refer to the identifier of a persistent class no matter what the identifier s real property name is.
Retrieving objects efficiently
The next query counts the number of bids and calculates the average bid per unsold item:
select bid.item.id, count(bid), avg(bid.amount) from Bid bid where bid.item.successfulBid is null group by bid.item.id
This query uses an implicit association join. For an explicit ordinary join in the from clause (not a fetch join), we can re-express it as follows:
select bidItem.id, count(bid), avg(bid.amount) from Bid bid join bid.item bidItem where bidItem.successfulBid is null group by bidItem.id
To initialize the bids collection of the Items, we can use a fetch join and refer to the associations starting on the other side:
select item.id, count(bid), avg(bid.amount) from Item item fetch join item.bids bid where item.successfulBid is null group by item.id
Sometimes, you ll want to further restrict the result by selecting only particular values of a group.
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