To get started export your schema into Querydsl query types like this :

  
java.sql.Connection conn = ...;
MetaDataExporter exporter = new MetaDataExporter();
exporter.setPackageName("com.myproject.mydomain");
exporter.setTargetFolder(new File("src/main/java"));
exporter.export(conn.getMetaData());  

This declares that the database schema is to be mirrored into the com.myproject.domain package in the src/main/java folder.

The generated types have the table name transformed to mixed case as the class name and a similar mixed case transformation applied to the columns which are available as property paths in the query type.

In addition to this primary key and foreign key constraints are provided as fields which can be used for compact join declarations.

This functionality is also available as a Maven plugin. The presented example can be declared like this in the POM :

   
<plugin>
  <groupId>com.mysema.querydsl</groupId>
  <artifactId>querydsl-maven-plugin</artifactId>
  <version>${querydsl.version}</version>
  <executions>
    <execution>
      <goals>
        <goal>export</goal>
      </goals>
    </execution>            
  </executions>
  <configuration>
    <jdbcDriver>org.apache.derby.jdbc.EmbeddedDriver</jdbcDriver>
    <jdbcUrl>jdbc:derby:target/demoDB;create=true</jdbcUrl>
    <packageName>com.myproject.domain</packageName>
    <targetFolder>${project.basedir}/target/generated-sources/java</targetFolder>
	<!--
      optional elements :      
      * jdbcUser = connection user
      * jdbcPassword = connection password
      * namePrefix = name prefix for Q-types (default: Q)
      * schemaPattern = to restrict code generation to certain schemas (default: null)
      * tableNamePattern = to restrict code generation to certain tables (default: null)
      * exportBeans = set to true to generate beans as well (default: false)
      * innerClassesForKeys = set to true to 
    -->     
  </configuration>
  <dependencies>
    <dependency>
      <groupId>org.apache.derby</groupId>
      <artifactId>derby</artifactId>
      <version>${derby.version}</version>
    </dependency>
  </dependencies>
</plugin>  

Use the goal test-export to add the targetFolder as a test compile source root instead of a compile source root.

The ANT task com.mysema.query.sql.ant.AntMetaDataExporter of the querydsl-sql module provides the same functionality as an ANT task. The configuration parameters of the task are jdbcDriverClass, dbUrl, dbUserName, dbPassword, namePrefix, targetPackage, targetSourceFolder, schemaPattern, tableNamePattern, exportBeans and innerClassesForKeys.

Querying with Querydsl SQL is as simple as this :

 
QCustomer customer = new QCustomer("c");

SQLTemplates dialect = new HSQLDBTemplates(); // SQL-dialect
SQLQuery query = new SQLQueryImpl(connection, dialect); 
List<String> lastNames = query.from(customer)
    .where(customer.firstName.eq("Bob"))
    .list(customer.lastName);

which is transformed into the following sql query, assuming that the related table name is customer and the columns first_name and last_name :

     
SELECT c.last_name 
FROM customer c 
WHERE c.first_name = 'Bob'

Querydsl uses SQL dialects to customize the SQL serialization needed for different relational databases. The available dialects are :

Use the the cascading methods of the SQLQuery interface like this

from : Define the query sources here.

innerJoin, join, leftJoin, fullJoin, on : Define join elements using these constructs. For the join methods the first argument is the join source and the second the target (alias).

where : Define the query filters, either in varargs form separated via commas or cascaded via the and-operator.

groupBy : Define the group by arguments in varargs form.

having : Define the having filter of the "group by" grouping as an varags array of Predicate expressions.

orderBy : Define the ordering of the result as an varargs array of order expressions. Use asc() and desc() on numeric, string and other comparable expression to access the OrderSpecifier instances.

limit, offset, restrict : Define the paging of the result. Limit for max results, offset for skipping rows and restrict for defining both in one call.

The syntax for declaring ordering is

 
query.from(customer)
    .orderBy(customer.lastName.asc(), customer.firstName.asc())
    .list(customer.firstName, customer.lastName);

which is equivalent to the following native SQL

  
SELECT c.first_name, c.last_name
FROM customer c 
ORDER BY c.last_name ASC, c.first_name ASC

Grouping can be done in the following form

 
query.from(customer)
    .groupBy(customer.lastName)
    .list(customer.lastName);

which is equivalent to the following native SQL

  
SELECT c.last_name 
FROM customer c 
GROUP BY c.last_name

To create a subquery you create a SQLSubQuery instance, define the query parameters via from, where etc and use unique or list to create a subquery, which is just a type-safe Querydsl expression for the query. unique is used for a unique (single) result and list for a list result.

 
query.from(customer).where(
  customer.status.eq(new SQLSubQuery().from(customer2).unique(customer2.status.max()))
  .list(customer.all())  

Another example

 
query.from(customer).where(  
  customer.status.in(new SQLSubQuery().from(status).where(status.level.lt(3)).list(status.id))  
  .list(customer.all())    

Custom query extensions to support engine specific syntax can be created by subclassing AbstractSQLQuery and adding flagging methods like in the given MySQLQuery example :

          
public class MySQLQuery extends AbstractSQLQuery<MySQLQuery>{
    
    public MySQLQuery(Connection conn) {
        this(conn, new MySQLTemplates(), new DefaultQueryMetadata());
    }
    
    public MySQLQuery(Connection conn, SQLTemplates templates) {
        this(conn, templates, new DefaultQueryMetadata());
    }
    
    protected MySQLQuery(Connection conn, SQLTemplates templates, QueryMetadata metadata) {
        super(conn, new Configuration(templates), metadata);
    }
    
    public MySQLQuery bigResult(){
        return addFlag(Position.AFTER_SELECT, "SQL_BIG_RESULT ");
    }
    
    public MySQLQuery bufferResult(){
        return addFlag(Position.AFTER_SELECT, "SQL_BUFFER_RESULT ");
    }
 
 
    // ...   
}        

The flags are custom SQL snippets that can be inserted at specific points in the serialization. The supported positions are the enums of the com.mysema.query.QueryFlag.Position enum class.

CREATE TABLE commands can be used in fluent form via the CreateTableClause. Here are some examples :

      
new CreateTableClause(conn, templates, "language")
	.column("id", Integer.class).notNull()
	.column("text", String.class).size(256).notNull()
	.primaryKey("PK_LANGUAGE","id")
	.execute();

new CreateTableClause(conn, templates, "symbol")
	.column("id", Long.class).notNull()
	.column("lexical", String.class).size(1024).notNull()
	.column("datatype", Long.class)
	.column("lang", Integer.class)
	.column("intval",Long.class)
	.column("floatval",Double.class)
	.column("datetimeval",Timestamp.class)
	.primaryKey("PK_SYMBOL","id")
	.foreignKey("FK_LANG","lang").references("language","id")
	.execute();

new CreateTableClause(conn, templates, "statement")
	.column("model", Long.class)
	.column("subject", Long.class).notNull()
	.column("predicate", Long.class).notNull()
	.column("object", Long.class).notNull()
	.foreignKey("FK_MODEL","model").references("symbol","id")
	.foreignKey("FK_SUBJECT","subject").references("symbol","id")
	.foreignKey("FK_PREDICATE","predicate").references("symbol","id")
	.foreignKey("FK_OBJECT","object").references("symbol","id")
	.execute();    

The constructor of CreateTableClause takes the connection, the templates and the table name. The rest is declared via column, primaryKey and foreignKey invocations.

Here are the corresponding CREATE TABLE clauses as they are executed.

      
 CREATE TABLE language (
  id INTEGER NOT NULL,
  text VARCHAR(256) NOT NULL,
  CONSTRAINT PK_LANGUAGE PRIMARY KEY(id)
)

 CREATE TABLE symbol (
  id BIGINT NOT NULL,
  lexical VARCHAR(1024) NOT NULL,
  datatype BIGINT,
  lang INTEGER,
  intval BIGINT,
  floatval DOUBLE,
  datetimeval TIMESTAMP,
  CONSTRAINT PK_SYMBOL PRIMARY KEY(id),
  CONSTRAINT FK_LANG FOREIGN KEY(lang) REFERENCES language(id)
)

 CREATE TABLE statement (
  model BIGINT,
  subject BIGINT NOT NULL,
  predicate BIGINT NOT NULL,
  object BIGINT NOT NULL,
  CONSTRAINT FK_MODEL FOREIGN KEY(model) REFERENCES symbol(id),
  CONSTRAINT FK_SUBJECT FOREIGN KEY(subject) REFERENCES symbol(id),
  CONSTRAINT FK_PREDICATE FOREIGN KEY(predicate) REFERENCES symbol(id),
  CONSTRAINT FK_OBJECT FOREIGN KEY(object) REFERENCES symbol(id)
)  

All the DMLClause implementation in the Querydsl SQL module take three parameters, the Connection, the SQLTemplates instance used in the queries and the main entity the DMLClause is bound to.

Insert examples :

    
// with columns
new SQLInsertClause(conn, dialect, survey)
    .columns(survey.id, survey.name)
    .values(3, "Hello").execute();

// without columns
new SQLInsertClause(conn, dialect, survey)
    .values(4, "Hello").execute();

// with subquery
new SQLInsertClause(conn, dialect, survey)
    .columns(survey.id, survey.name)
    .select(new SQLSubQuery().from(survey2).list(survey2.id.add(1), survey2.name))
    .execute();

// with subquery, without columns
new SQLInsertClause(conn, dialect, survey)
    .select(new SQLSubQuery().from(survey2).list(survey2.id.add(10), survey2.name))
    .execute();

Update examples :

  
// update with where
new SQLUpdateClause(conn, dialect, survey)
    .where(survey.name.eq("XXX"))
    .set(survey.name, "S")
    .execute();
  
// update without where
new SQLUpdateClause(conn, dialect, survey)
    .set(survey.name, "S")
    .execute()  

Delete examples :

    
// delete with where
new SQLDelecteClause(conn, dialect, survey)
    .where(survey.name.eq("XXX"))
    .execute();
  
// delete without where
new SQLDelecteClause(conn, dialect, survey)
    .execute()  

Querydsl SQL supports usage of JDBC batch updates through the DML APIs. If you have consecutive DML calls with a similar structure, you can bundle the the calls via addBatch() usage into one DMLClause. See the examples how it works for UPDATE, DELETE and INSERT.

   	

    @Test
    public void updateExample() throws SQLException{
        insert(survey).values(2, "A").execute();
        insert(survey).values(3, "B").execute();

        SQLUpdateClause update = update(survey);
        update.set(survey.name, "AA").where(survey.name.eq("A")).addBatch();
        update.set(survey.name, "BB").where(survey.name.eq("B")).addBatch();
        assertEquals(2, update.execute());
    }

    @Test
    public void deleteExample() throws SQLException{
        insert(survey).values(2, "A").execute();
        insert(survey).values(3, "B").execute();

        SQLDeleteClause delete = delete(survey);
        delete.where(survey.name.eq("A")).addBatch();
        delete.where(survey.name.eq("B")).addBatch();
        assertEquals(2, delete.execute());
    }

    @Test
    public void insertExample(){
        SQLInsertClause insert = insert(survey);
        insert.set(survey.id, 5).set(survey.name, "5").addBatch();
        insert.set(survey.id, 6).set(survey.name, "6").addBatch();
        assertEquals(2, insert.execute());
    }

To create JavaBean DTO types for the tables of your schema use the MetaDataExporter like this :

  
java.sql.Connection conn = ...;
MetaDataExporter exporter = new MetaDataExporter();
exporter.setPackageName("com.myproject.mydomain");
exporter.setTargetFolder(new File("src/main/java"))	;
exporter.setBeanSerializer(new BeanSerializer());   
exporter.export(conn.getMetaData());  

Now you can use the bean types as arguments to the populate method in DML clauses and you can project directly to bean types in queries. Here is a simple example in JUnit form :

    	
    @Test
    public void Insert_Update_Query_and_Delete(){
        QEmployee e = new QEmployee("e");
           
        // Insert
        Employee employee = new Employee();
        employee.setFirstname("John");
        Integer id = insert(e).populate(employee).executeWithKey(e.id);
        employee.setId(id);

        // Update
        employee.setLastname("Smith");
        assertEquals(1l, update(e).populate(employee).where(e.id.eq(employee.getId())).execute());

        // Query
        Employee smith = query().from(e).where(e.lastname.eq("Smith")).uniqueResult(e);
        assertEquals("John", smith.getFirstname());
        
        // Delete 
        assertEquals(1l, delete(e).where(e.id.eq(employee.getId())).execute());
    }

The factory methods used in the previous example are here :

      	
    protected SQLUpdateClause update(RelationalPath<?> e){
        return new SQLUpdateClause(Connections.getConnection(), templates, e);
    }

    protected SQLInsertClause insert(RelationalPath<?> e){
        return new SQLInsertClause(Connections.getConnection(), templates, e);
    }

    protected SQLDeleteClause delete(RelationalPath<?> e){
        return new SQLDeleteClause(Connections.getConnection(), templates, e);
    }

    protected SQLMergeClause merge(RelationalPath<?> e){
        return new SQLMergeClause(Connections.getConnection(), templates, e);
    }

    protected SQLQuery query() {
        return new SQLQueryImpl(Connections.getConnection(), templates);  	
  	}
  	

If you need to specify SQL function calls in Querydsl you can use TemplateExpressions to express them. For general expressions you can use the SimpleTemplate class and for typed expressions BooleanTemplate, ComparableTemplate, DateTemplate, DateTimeTemplate, EnumTemplate, NumberTemplate, StringTemplate and TimeTemplate.

Here is an example for SimpleTemplate usage :

Expression<?> arg1 = ...;
Expression<?> arg2 = ...;
Expression<String> expression = SimpleTemplate.create(String.class, "myfunction({0},{1})", arg1, arg2);		

And here is an example for a Number typed template expression :

Expression<?> arg1 = ...;
Expression<?> arg2 = ...;
NumberExpression<Integer> expression = NumberTemplate.create(Integer.class, "myfunction({0},{1})", arg1, arg2);		

Querydsl SQL provides the possibility to declare custom type mappings for ResultSet/Statement interaction. The custom type mappings can be declared in com.mysema.query.sql.Configuration instances, which are supplied as constructor arguments to the actual queries :

    
    @Test
    public void ForSQLType(){
        Configuration configuration = new Configuration(new H2Templates());
        // overrides the mapping for Types.DATE
        configuration.register(new UtilDateType());
    }
	
    @Test
    public void ForTableColumn(){
        Configuration configuration = new Configuration(new H2Templates());
        // declares a maping for the gender column in the person table
        configuration.register("person", "gender",  new EnumByNameType<Gender>(Gender.class));
        assertEquals(Gender.class, configuration.getJavaType(java.sql.Types.VARCHAR, "person", "gender"));
    }