Why do I need to configure the SQL dialect of a data source

Question

When we configure a data source using Hibernate  we should add the hibernate dialect property  or eclipselink target-database if you are using EclipseLink    I want to know what is the meaning of dialect  I configure this property according to the documentation of Hibernate but I don t know what s the meaning of it

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Databases implement subtle differences in the SQL they use  Things such as data types for example vary across databases  e g  in Oracle You might put an integer value in a number field and in SQL Server use an int field   Or database specific functionality - selecting the top n rows is different depending on the database  The dialect abstracts this so you don t have to worry about it

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Dialect is the SQL dialect that your database uses   List of SQL dialects for Hibernate   Either provide it in hibernate cfg xml as     lt hibernate-configuration gt      lt session-factory name  session-factory  gt         lt property name  hibernate dialect  gt org hibernate dialect SQLServerDialect lt  property gt                 lt  session-factory gt   lt  hibernate-configuration gt    or in the properties file as    hibernate dialect org hibernate dialect SQLServerDialect

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The SQL dialect converts the HQL query which we write in our java or any other object oriented program to the specific database SQL   For example in the java suppose I write List employees   session createQuery  FROM Employee   list     but when my dialect is  lt property name  hibernate dialect  gt        org hibernate dialect MySQLDialect   The HQL   FROM Employee   gets converted to  SELECT   FROM EMPLOYEE  before hitting the MySQL database

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Short answer   quot The irony of JDBC is that  although the programming interfaces are portable  the SQL language is not  Despite the many attempts to standardize it  it is still rare to write SQL of any complexity that will run unchanged on two major database platforms  Even where the SQL dialects are similar  each database performs differently depending on the structure of the query  necessitating vendor-specific tuning in most cases  quot     stolen from Pro JPA 2 Mastering the Java Persistence API  chapter 1  page 9   So  we might think of JDBC as the ultimate specification that abstracts away everything related to databases  but it isn t  A quote from the JDBC specification  chapter 4 4  page 20   The driver layer may mask differences between standard SQL 2003 syntax and the native dialect  supported by the data source   May is no guarantee that the driver will  and therefore we should provide the dialect in order to have a working application  In a best-case scenario  the application will work but might not run as effectively as it could if the persistence provider knew which dialect to use  In the case of Hibernate he will refuse to deploy your application unless you feed him the dialect  What about JPQL then  The JDBC specification does not mention the word JPQL  JDBC is a standardized way of database access  Go read this JavaDoc and you will find that once the application can access the database  what must be fed into the JDBC compliant driver is vanilla   undecorated SQL  It is worth noting that JPQL is a query language  not a data definition language  DDL   So even if we could feed the JDBC driver with JPQL  that would be of no use for the persistence provider during the phase of parsing the persistence xml file and setting up tables  Closer look at the property For your reference  here is an example for Hibernate and EclipseLink on how to specify a Java DB dialect in the persistence xml file   lt property name  quot hibernate dialect quot  value  quot org hibernate dialect DerbyTenSevenDialect quot   gt   lt property name  quot eclipselink target-database quot  value  quot JavaDB quot   gt   Is the property mandatory  In theory  the property has not been standardized and the JPA 2 1 specification says not a word about SQL dialects  So we re out of luck and must turn to vendor specific empirical studies and documentation thereof  Hibernate refuse to accept a deployment archive that hasn t specified the property rendering the archive undeployable  Hibernate documentation says   Always set the hibernate dialect property to the correct org hibernate dialect Dialect subclass for your database   So that is pretty clear  Do note that the dialects listed in the documentation are specifically targeting one or the other vendor  There is no  quot generic quot  dialect or anything like that  Given then that the property is an absolute requirement for a successful deployment  you would expect that the documentation of the WildFly application server which bundles Hibernate should say something  but it doesn t  EclipseLink on the other hand is a bit more forgiving  If you don t provide the property  the deployment deploys  without warning too   EclipseLink documentation says   Use the eclipselink target-database property to specify the database to use  controlling custom operations and SQL generation for the specified database   The talk is about  quot custom operations and SQL generation quot   meaning it is bit vague if you ask me  But one thing is clear  They don t say that the property is mandatory  Also note that one of the available values is  quot Database quot  which represent  quot a generic database quot  target  Hmm  what  quot dialect quot  would that be  SQL 2 0   But then again  the property is called  quot target-database quot  and not  quot dialect quot  so maybe  quot Database quot  translates to no SQL at all lol  Moving on to the GlassFish server which bundles EclipseLink  Documentation  page  quot 6-3 quot   says   You can specify the optional eclipselink target-database property to guarantee that the database type is correct   So GlassFish argues that the property is  quot optional quot  and the value added is a  quot guarantee quot  that I am actually using Java DB - in case I didn t know  Conclusion Copy-paste whatever you can find on google and pray to God

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The dialect in the Hibernate context  will take care of database data type  like in orace it is integer however in SQL it is int  so this will by known in hibernate by this property  how to map the fields internally

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Dialect means  the variant of a language   Hibernate  as we know  is database agnostic  It can work with different databases  However  databases have proprietary extensions native SQL variations  and set sub-set of SQL standard implementations  Therefore at some point hibernate has to use database specific SQL  Hibernate uses  dialect  configuration to know which database you are using so that it can switch to the database specific SQL generator code wherever whenever necessary

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Short answer  hibernate dialect property makes Hibernate to generate the appropriate SQL statements for the chosen database

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A dialect is a form of the language that is spoken by a particular group of people    Here  in context of hibernate framework  When hibernate wants to talk using queries  with the database it uses dialects     The SQL dialect s are derived from the Structured Query Language which uses human-readable expressions to define query statements  A hibernate dialect gives information to the framework of how to convert hibernate queries HQL  into native SQL queries      The dialect of hibernate can be configured using below property   hibernate dialect   Here  is a complete list of hibernate dialects   Note  The dialect property of hibernate is not mandatory

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Dialect property is used by hibernate in following ways   To generate Optimized SQL queries  If you have more than one DB then to talk with particular DB you want  To set default values for hibernate configuration file properties based on the DB software we use even though they are not specifed in configuration file

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Hibernate dialect property tells Hibernate to generate the appropriate SQL statements for the chosen database   A list of available dialects can be found here  http   javamanikandan blogspot in 2014 05 sql-dialects-in-hibernate html  RDBMS                   Dialect DB2                     org hibernate dialect DB2Dialect DB2 AS 400              org hibernate dialect DB2400Dialect DB2 OS390               org hibernate dialect DB2390Dialect PostgreSQL              org hibernate dialect PostgreSQLDialect MySQL                   org hibernate dialect MySQLDialect MySQL with InnoDB       org hibernate dialect MySQLInnoDBDialect MySQL with MyISAM       org hibernate dialect MySQLMyISAMDialect Oracle  any version     org hibernate dialect OracleDialect Oracle 9i 10g           org hibernate dialect Oracle9Dialect Sybase                  org hibernate dialect SybaseDialect Sybase Anywhere         org hibernate dialect SybaseAnywhereDialect Microsoft SQL Server    org hibernate dialect SQLServerDialect SAP DB                  org hibernate dialect SAPDBDialect Informix                org hibernate dialect InformixDialect HypersonicSQL           org hibernate dialect HSQLDialect Ingres                  org hibernate dialect IngresDialect Progress                org hibernate dialect ProgressDialect Mckoi SQL               org hibernate dialect MckoiDialect Interbase               org hibernate dialect InterbaseDialect Pointbase               org hibernate dialect PointbaseDialect FrontBase               org hibernate dialect FrontbaseDialect Firebird                org hibernate dialect FirebirdDialect

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Hibernate uses  dialect  configuration to know which database you are using so that it can convert hibernate query to database specific query

[hibernate] Why do I need to configure the SQL dialect of a data source?

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