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<bag name="Users" where="ACTIVE=1">
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Here, the collection Users will contain only users whose ACTIVE value is 1. This approach can be useful, but we recommend considering filters for most scenarios (see section 7.5.2, Collection filters ). You can, of course, use various other comparison operators for restriction.
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Comparison operators A restriction is expressed using ternary logic. The where clause is a logical expression that evaluates to true, false, or null for each tuple of objects. You construct logical expressions by comparing properties of objects to other properties or literal values using HQL s built-in comparison operators.
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Basic queries for objects
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What is ternary logic
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A row is included in a SQL result set if and only if the where clause evaluates to true. In C#, notNullObject==null evaluates to false, and null==null evaluates to true. In SQL, NOT_NULL_COLUMN=null and null=null both evaluate to null, not true. Thus, SQL needs a special operator, IS NULL, to test whether a value is null. This ternary logic is a way of handling expressions that may be applied to null column values. It s a (debatable) SQL extension to the familiar binary logic of the relational model and of typical programming languages such as C#.
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HQL supports the same basic operators as SQL: =, <>, <, >, >=, <=, between, not
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between, in, and not in. For example:
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from Bid bid where bid.Amount between 1 and 10 from Bid bid where bid.Amount > 100 from User u where u.Email in ( 'foo@hibernate.org', 'bar@hibernate.org' )
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In case of criteria queries, all the same operators are available via the Expression class:
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session.CreateCriteria(typeof(Bid)) .Add( Expression.Between("Amount", 1, 10) ) .List(); session.CreateCriteria(typeof(Bid)) .Add( Expression.Gt("Amount", 100) ) .List(); string[] emails = { "foo@NHibernate.org", "bar@NHibernate.org" }; session.CreateCriteria(typeof(User)) .Add( Expression.In("Email", emails) ) .List();
Because the underlying database implements ternary logic, testing for null values requires some care. Remember that null = null doesn t evaluate to true in the database, but to null. All comparisons that use the null operator evaluate to null. Both HQL and the ICriteria API provide an SQL-style is null operator:
from User u where u.Email is null
This query returns all users with no email address. The same semantic is available in the ICriteria API:
session.CreateCriteria(typeof(User)) .Add( Expression.IsNull("Email") ) .List();
You also need to be able to find users who have an email address:
from User u where u.Email is not null session.CreateCriteria(typeof(User)) .Add( Expression.IsNotNull("Email") ) .List();
Finally, the HQL where clause supports arithmetic expressions (but the ICriteria API doesn t):
from Bid bid where ( bid.Amount / 0.71 ) - 100.0 > 0.0
Retrieving objects efficiently
For string-based searches, you need to be able to perform case-insensitive matching and matches on fragments of strings in restriction expressions.
String matching
The like operator allows wildcard searches, where the wildcard symbols are % and _, just as in SQL:
from User u where u.Firstname like "S%"
This expression restricts the result to users with a first name starting with a capital S. You can also negate the like operator, for example by using a substring match expression:
from User u where u.Firstname not like "%Foo S%"
For criteria queries, wildcard searches may either use the same wildcard symbols or specify a MatchMode. NHibernate provides the MatchMode as part of the ICriteria query API; you use it to write string match expressions without string manipulation. These two queries are equivalent:
session.CreateCriteria(typeof(User)) .Add( Expression.Like("Firstname", "S%") ) .List(); session.CreateCriteria(typeof(User)) .Add( Expression.Like("Firstname", "S", MatchMode.Start) ) .List();
The allowed MatchModes are Start, End, Anywhere, and Exact. An extremely powerful feature of HQL is the ability to call arbitrary SQL functions in the where clause. If your database supports user-defined functions (most do), you can put this functionality to all sorts of uses, good or evil. For the moment, let s consider the usefulness of the standard ANSI SQL functions upper() and lower(). They can be used for case-insensitive searching:
from User u where lower(u.Email) = 'foo@hibernate.org'
The ICriteria API doesn t currently support SQL function calls. But it does provide a special facility for case-insensitive searching:
session.CreateCriteria(typeof(User)) .Add( Expression.Eq("Email", "foo@hibernate.org").IgnoreCase() ) .List();
Unfortunately, HQL doesn t provide a standard string-concatenation operator; instead, it supports whatever syntax your database provides. Here is an example for SQL Server:
from User user where ( user.Firstname + ' ' + user.Lastname ) like 'S% K%'
We return to more exotic features of the HQL where clause later in this chapter. We only used single expressions for restrictions in this section; let s combine several with logical operators.
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