Hibernate retrieval method (4)

Hibernate retrieval method (4)-[hibernate
]

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Iv. REPORT query

1

Projection Query

Select C from customer C ......

Select C. Name, C. Age from customer C ......

(
1

) Dynamically instantiate the query results.

Select New COM. customerrow (C. ID, C. Name, C. Age)
From customer C ......

Note:
Customerrow

The class does not need to be a persistent class, so it does not need to create its object-link ing file, which is closely used
Select

The link data queried by the statement is packaged
Java

Object.

(
2

) Filter duplicate elements in the query results.

Select distinct C. Name
Customer C ......

 

2

Use Aggregate functions

In
Hql

The following Aggregate functions can be called in a query statement:

L


Count ()

: Number of statistical records

L


Min ()

L


Max ()

L


Sum ()

L


AVG ()

: Average Value

 

3

, Grouping query (
Group... (Having ...)

)

Select C. Name, count (c) from customer C group by C. Name

List ()

Result:
N

Array elements, each object array corresponds to a record in the query results. For example

Iterator it = session. createqurey (


"Select C. Name, count (c) from customer C group by C. Name"
)

. List (). iterator ();


While

(It. hasnext ()){

Object [] pair = (object []) it. Next ();

String name = (string) pair [0];

Integer count
= (Integer) pair [1];

}

 

4

Optimized REPORT query performance

REPORT query features: it usually processes a large amount of data. Generally, it only involves reading data without modifying data.

When
Select

When only some attributes of the persistence class are queried,
Hibernate

The returned query result is relational data (not occupying
Session

Instead of the persistence object (located in
Session

Cache ).

 

If
Select

Statement query improves REPORT query performance. As long as the application does not reference the data, the memory occupied by the data is released. When
From

Type
Hql

A large number of persistent objects remain in
Session

And
Session

Bixun is also responsible for synchronizing these objects to the database.

 

For
Select

Statement query, you can define
JavaBean

To encapsulate the relational data in the query results, so that the application can still access the query results in an object-oriented manner.

 

 

5. Advanced query skills

1

Dynamic query

Hql

And
QBC

Can complete many identical tasks, in contrast,
Hql

Can intuitively express complex query statements, while
QBC

It is difficult to express complex query statements.

If the content of the query statement (also called static query) is specified before the program runs, it should be given priority.
Hql

Query Method. However, if the content of the query statement (also called dynamic query) can be specified only when the program is running ),
QBC

Ratio
Hql

More convenient.

 

QBE

Fuzzy match example:

Customer customer = ....

Example examplecustomer = example. Create (customer );

Examplecustomer. ignorecase (). enablelike (matchmode. Anywhere );

Examplecustomer. excludezeroes ();

Criteria = session. createcriteria (customer.
Class

). Add (examplecustomer );

List list = criteria. List ();

Note:
Ignorecase
Case sensitivity is ignored when comparing strings;
Enablelike
Fuzzy comparison is used for all string type attributes in the sample object;
Excludezeroes
Indicates that the numeric type attribute in the sample object is
0
. If a property is
Null
The query statement does not contain this condition.

You can specify both the customer query conditions and order query conditions in the query window.
QBE

Search.

Customer customer = ....

Example examplecustomer = example. Create (customer );

Examplecustomer. ignorecase (). enablelike (matchmode. Anywhere );

Examplecustomer. excludezeroes ();

Order order =...

Example exampleorder = example. Create (order );

Exampleorder. ignorecase (). enablelike (matchmode. Anywhere );

Exampleorder. excludezeroes ();

Criteria = session. createcriteria (customer.
Class

)

. Add (examplecustomer)

. Createcriteria (
"Orders"
)

. Add (exampleorder );

List list = criteria. List ();

 

To be continued...