1 JPQL
1.1 What is jpql
jpql: jpa is a query language provided, similar to sql.
1.2 jpql and differentiated sql
(1) jpql inside the operation target and the object attributes sql operating tables and table columns inside the case-sensitive
(2) can not appear in jpql inside *, table name can not appear
(3) jpql sql and they are the same keywords, keywords are case insensitive
1.3 jpql syntax
sql syntax:
select *
from table
where the filter conditions
group by grouping condition
avg after having filtered packet (score)> 98
order by ordering
jpql similar
select o.name, o (object attribute)
from the class O the Join O. inside class attribute name p
where o. attribute name
group by o. attribute name
order by
…
1.4 jpql which join
sql: query link
External connection: the connection is divided into left outer (left table and the intersection of all the data must contain the right part of the table.) And a right outer connection (and vice versa) left join right join
En: data on the intersection portion join
JPQL:
select o from Employee o join o.department d
1.5 aggregate functions - and the same sql
sql: find the following values for a set of data
count the number of records seeking seeking maximum sum sum max min avg averaging for the Minimum
1 5 6 8 2 -->count 5 max 8 min 1 sum 22 avg 22/5
With the use of group by group
1.6 sub-queries - and as sql
Subquery:
As the result of a query or a query further condition table used
Subquery: a nested query
select p from Employee p where p.salary > (select avg(o.salary) from Employee o)
1.7 jpql use collection
There may be used a value determined size
select o from Project o where o.employees.size = 0
1.8 Paging
sql syntax:
select * from table where limit beginIndex,pageSize
currentPage: Current Page
pageSize: the number of page
beginIndex: starting position beginIndex = (currentPage-1) * pageSize
pageSize: the number of page
select * from table where limit 0,5
select * from table where limit 5,5
2 SQL support
To sql
Using this method
createNativeQuery
3 transaction concurrency
3.1 What is a transaction
transaction
Services: a set of logical operations either all succeed or fail (with the total death)
3.2 What are the characteristics of the transaction (ACID)
Atomicity: A
This operation is a transaction which has a minimum unit, can not be split again
Consistency: C
The same amount of data consistency, before the operation, the total amount of 10,000, and after completing the operation, the total amount of 10 000
Isolation today :() I
Reflect more than two affairs operations, should have isolation between transactions, your transaction can not operate my business, if you operate my business transactions occur, it will bring the issue of concurrent transactions
Persistence: D
When we commit a transaction submitted data on the permanent presence down
3.3 Transaction Concurrency
Transaction concurrency:
A plurality of transaction (or two or more of the transaction),SimultaneouslyIn operationThe same dataTime, this phenomenon is called concurrent transactions
Transaction concurrency will bring some problems: - solved through database isolation mechanism
The first update appears lost - handles
The second category is missing Update - handles
Dirty read - database isolation mechanism
Waste (Magic degrees) - Database isolation mechanism
Non-repeatable read - database isolation mechanism
3.4 database isolation mechanism
Four kinds
READ UNCOMMITTED (uncommitted read) Read Fantasy (dummy read), not repeat the reading and allow dirty reads.
READ COMMITTED allow fantasy to read, non-repeatable read, do not allow dirty read
REPEATABLE READ allowed to read fantasy, do not allow dirty reads and non-repeatable read
SERIALIZABLE fantasy reads, non-repeatable reads and dirty reads are not allowed - performance certainly become low
3.5 The first and second categories lost updates
Lock mechanism
Pessimistic locking: very pessimistic locks need to wait for a lock release, in order to manipulate data, performance is not high
Optimistic locking: no need to wait, after the version number of the underlying need to control the first modification, the version number change, the second person can not operate on this product
4JPA usage rules or experience
. \ 1 bidirectional many association, not a one-way-to-many - a little high efficiency
. \ 2 flexible one-way one association - foreign key parties to maintain high efficiency point
\ 3. Not one to one, with many-to-replace (do not use a shared one primary key, foreign key with a unique one to one)
\ 4. The secondary cache configuration object (configuration Cacheable), query cache (query JPQL),
No query only use the query cache (a hit if the conditions are very low)
Query cache:
EntityManagerFactory different entityManager same same same transmission condition values jpql
\ 5 combining relationship set the list (sequence repeat) - The second project document modules, many-to-use collection set
\ 6. Design table, the table field less, do not be afraid table associated with secondary cache support, design table as far as possible to reach the third paradigm (foreign key)
Database paradigm: database rules
1NF: design time, which try to ensure that the table can not store a plurality of column values, a minimum unit column, can not be divided - body atomic Series
2NF: inside each row of data table should have a unique value can be distinguished, it is recommended you create a primary key
3NF: Table should not be stored inside another non-primary key information in a table - I suggest you a foreign key
