mysql中变量赋值

Chapter 14. HQL: The Hibernate Query Language

14.1. Case Sensitivity

14.2. The from clause

14.3. Associations and joins

14.4. Forms of join syntax

14.5. Referring to identifier property

14.6. The select clause

14.7. Aggregate functions

14.8. Polymorphic queries

14.9. The where clause

14.10. Expressions

14.11. The order by clause

14.12. The group by clause

14.13. Subqueries

14.14. HQL examples

14.15. Bulk update and delete

14.16. Tips & Tricks

14.17. Components

14.18. Row value constructor syntax

Hibernate uses a powerful query language (HQL) that is similar in appearance to SQL. Compared with SQL, however, HQL is fully object-oriented and understands notions like inheritance, polymorphism and association.

 

14.1. Case Sensitivity

With the exception of names of Java classes and properties, queries are case-insensitive. So SeLeCT is the same as sELEct is the same as SELECT, but org.hibernate.eg.FOO is not org.hibernate.eg.Foo, and foo.barSetis not foo.BARSET.

This manual uses lowercase HQL keywords. Some users find queries with uppercase keywords more readable, but this convention is unsuitable for queries embedded in Java code.

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14.2. The from clause

The simplest possible Hibernate query is of the form:

from eg.Cat

This returns all instances of the class eg.Cat. You do not usually need to qualify the class name, since auto-import is the default. For example:

from Cat

In order to refer to the Cat in other parts of the query, you will need to assign an alias. For example:

from Cat as cat

This query assigns the alias cat to Cat instances, so you can use that alias later in the query. The as keyword is optional. You could also write:

from Cat cat

Multiple classes can appear, resulting in a cartesian product or "cross" join.

from Formula, Parameter

from Formula as form, Parameter as param

It is good practice to name query aliases using an initial lowercase as this is consistent with Java naming standards for local variables (e.g. domesticCat).

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14.3. Associations and joins

You can also assign aliases to associated entities or to elements of a collection of values using a join. For example:

from Cat as cat
    inner join cat.mate as mate
    left outer join cat.kittens as kitten

from Cat as cat left join cat.mate.kittens as kittens

from Formula form full join form.parameter param

The supported join types are borrowed from ANSI SQL:

  • inner join

  • left outer join

  • right outer join

  • full join (not usually useful)

The inner joinleft outer join and right outer join constructs may be abbreviated.

from Cat as cat
    join cat.mate as mate
    left join cat.kittens as kitten

You may supply extra join conditions using the HQL with keyword.

from Cat as cat
    left join cat.kittens as kitten
        with kitten.bodyWeight > 10.0

A "fetch" join allows associations or collections of values to be initialized along with their parent objects using a single select. This is particularly useful in the case of a collection. It effectively overrides the outer join and lazy declarations of the mapping file for associations and collections. See Section 19.1, “Fetching strategies” for more information.

from Cat as cat
    inner join fetch cat.mate
    left join fetch cat.kittens

A fetch join does not usually need to assign an alias, because the associated objects should not be used in the where clause (or any other clause). The associated objects are also not returned directly in the query results. Instead, they may be accessed via the parent object. The only reason you might need an alias is if you are recursively join fetching a further collection:

from Cat as cat
    inner join fetch cat.mate
    left join fetch cat.kittens child
    left join fetch child.kittens

The fetch construct cannot be used in queries called using iterate() (though scroll() can be used). Fetch should be used together with setMaxResults() or setFirstResult(), as these operations are based on the result rows which usually contain duplicates for eager collection fetching, hence, the number of rows is not what you would expect. Fetch should also not be used together with impromptu with condition. It is possible to create a cartesian product by join fetching more than one collection in a query, so take care in this case. Join fetching multiple collection roles can produce unexpected results for bag mappings, so user discretion is advised when formulating queries in this case. Finally, note that full join fetch and right join fetch are not meaningful.

If you are using property-level lazy fetching (with bytecode instrumentation), it is possible to force Hibernate to fetch the lazy properties in the first query immediately using fetch all properties.

from Document fetch all properties order by name

from Document doc fetch all properties where lower(doc.name) like '
			

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