Working With JOINS in Spring Data JPA

Working With JOINS in Spring Data JPA | Before seeing JOINS in Spring Data JPA, let us see in short how JOIN works in SQL.

SQL Joins

SQL Joins:- Joins are used to fetch data [SELECT] from multiple tables [2 or more tables]
using a single query [one SQL query].

Equal Join
Cross Join
Self Join
JOIN | INNER JOIN:- Rows which are connected in both tables.
OUTER JOINS
- LEFT OUTER JOIN | LEFT JOIN:- All Left side table rows and connected right side table rows.
- RIGHT OUTER JOIN | RIGHT JOIN:- All Right side table rows and connected left side table rows.
- FULL OUTER JOIN | RIGHT JOIN:- All rows from both tables.

Among these Spring Data JPA only supports INNER & OUTER Joins. It does not support Equal, Cross and Self joins.
Word OUTER IS optional. ie LEFT OUTER JOIN and LEFT JOIN are the same.
Condition for using Joins:- One Table primary key must be used as another table foreign key i.e. two tables must have a relation(1…1 / 1…* / *…1/ *…*)

Consider the given depttab and emptab tables and write SQL queries for given JOINS:-

-- Create the Parent Table (depttab)
CREATE TABLE depttab (
    did INT PRIMARY KEY,
    dname VARCHAR(50)
);

-- Create the Child Table (emptab)
CREATE TABLE emptab (
    eid INT PRIMARY KEY,
    ename VARCHAR(50),
    esal DECIMAL(10, 2),
    didFk INT,
    FOREIGN KEY (didFk) REFERENCES depttab(did)
);

-- Insert Records into depttab
INSERT INTO depttab (did, dname) VALUES
(101, 'DEV'),
(102, 'QA'),
(103, 'BA'),
(104, 'MG');

-- Insert Records into emptab
INSERT INTO emptab (eid, ename, esal, didFk) VALUES
(55, 'AA', 30.00, NULL),
(56, 'BB', 40.00, 101),
(57, 'CC', 50.00, 103),
(58, 'DD', 90.00, NULL);

mysql> select * from depttab;
+-----+-------+
| did | dname |
+-----+-------+
| 101 | DEV   |
| 102 | QA    |
| 103 | BA    |
| 104 | MG    |
+-----+-------+

mysql> select * from emptab;
+-----+-------+-------+-------+
| eid | ename | esal  | didFk |
+-----+-------+-------+-------+
|  55 | AA    | 30.00 |  NULL |
|  56 | BB    | 40.00 |   101 |
|  57 | CC    | 50.00 |   103 |
|  58 | DD    | 90.00 |  NULL |
+-----+-------+-------+-------+

Inner JOIN Example:-

mysql > SELECT * 
	FROM depttab d
	INNER JOIN emptab e
	ON d.did = e.didFk;

+-----+-------+-----+-------+-------+-------+
| did | dname | eid | ename | esal  | didFk |
+-----+-------+-----+-------+-------+-------+
| 101 | DEV   |  56 | BB    | 40.00 |   101 |
| 103 | BA    |  57 | CC    | 50.00 |   103 |
+-----+-------+-----+-------+-------+-------+

Fetch the Dept name and emp name using RIGHT JOIN

mysql > SELECT d.dname, e.ename 
	FROM depttab d
	RIGHT JOIN emptab e
	ON d.did = e.didFk;

+-------+-------+
| dname | ename |
+-------+-------+
| NULL  | AA    |
| DEV   | BB    |
| BA    | CC    |
| NULL  | DD    |
+-------+-------+

Print ename, esal and dname using left join.

mysql > SELECT e.ename , e.esal, d.dname
	FROM depttab d
	LEFT JOIN emptab e
	ON d.did = e.didFk;

+-------+-------+-------+
| ename | esal  | dname |
+-------+-------+-------+
| BB    | 40.00 | DEV   |
| NULL  |  NULL | QA    |
| CC    | 50.00 | BA    |
| NULL  |  NULL | MG    |
+-------+-------+-------+

Print all Dept that do not have employees.

mysql > SELECT e.ename , e.esal, d.dname
	FROM depttab d
	LEFT JOIN emptab e
	ON d.did = e.didFk 
	WHERE e.didFk is null;

+-------+------+-------+
| ename | esal | dname |
+-------+------+-------+
| NULL  | NULL | QA    |
| NULL  | NULL | MG    |
+-------+------+-------+

Another Example:-

Product ————–<> Vendor
Product & Vendor are connected to Many-To-One i.e. 1 vendor can have many products.

Product:- pid, pcode, pcost, vidFk
Vendor:- vid, vname, addr, vcode

Try to find out which joins will be used.

Display all products which are having vendor links. => INNER Join.
Display All products and their vendor names. => LEFT JOIN.
Display all Vendors and their product names. => RIGHT JOIN.

Spring Data JPA Join Example – 1

SQL JOINS:-

SELECT <P>.<COLUMN> , <C>.<COLUMN>
FROM <PARENT-TABLE> <P>
    [JOIN - TYPE ]
    <CHILD-TABLE> <C>
ON <P>.<PK> = <C>.<FK> | <P>.<FK> = <C>.<PK>
WHERE <CONDITION>;

HQL/JPQL Joins:-

SELECT <P>.<VARIABLES> , <C>.<VARIABLES>
FROM <PARENT-CLASS> <P>
    [JOIN - TYPE ]
    <P>.<HAS-A VARIABLE> AS <C>
WHERE <CONDITION>;

Here <P> and <C> are alias names given for mode classes.

// child class
class Project {
    Integer pid;
    String pcode;
}

// parent class
class Employee {
    Integer eid;
    String ename;

    @ManyToOne
    @JoinColumn(name = "_")
    Project pob;
}

HQL/JPQL syntax for Inner JOIN:-

SELECT e.ename, p.pcode
FROM Employee e
    INNER JOIN
    e.Product AS p

To fetch employees who are not connected to the project.

SELECT e.ename, p.pcode
FROM Employee e
    LEFT OUTER JOIN
    e.Product AS p
WHERE p IS NULL;

HQL/JPQL syntax is the same for collection/non-collection type. But the output is different.
HQL/JPQL will be finally converted to SQL by dialect.
SQL is database-dependent whereas HQL/JPQL is database-independent.
The word ‘AS’ indicates an alias name (Also Know As) which is optional.
Words related to SQL are case-insensitive (SELECT, FROM, WHERE).
Words related to Java are case-sensitive (Employee, empId..)
INNER JOIN and JOIN get the same result. Word INNER is Optional.
A few databases and Data JPA do not support FULL JOIN. This means that there is less join that gets all rows from DB tables. In that case, better use findAll();

Create a spring starter project using dependencies:- Spring Web, Spring Data JPA, and MySQL Driver.

In the properties file:-

spring.datasource.url=jdbc:mysql://localhost:3306/test
spring.datasource.username=root
spring.datasource.password=root
spring.jpa.show-sql=true
spring.jpa.hibernate.ddl-auto=update
spring.jpa.properties.hibernate.format_sql=true
spring.jpa.database-platform: org.hibernate.dialect.MySQL8Dialect

Model classes & repositories:-

@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
@Table(name = "ven_tab")
public class Vendor {

    @Id
    @Column(name = "vid_col")
    private Integer vid;

    @Column(name = "vcode_col")
    private String vcode;

    @Column(name = "vloc_col")
    private String vloc;
}

@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
@Table(name = "prod_tab")
public class Product {

    @Id
    @Column(name = "pid_col")
    private Integer pid;

    @Column(name = "pcode_col")
    private String pcode;

    @Column(name = "pcost_col")
    private Double pcost;

    @Column(name = "pmodel_col")
    private String pmodel;

    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "vid_fk")
    private Vendor vob;
}

public interface ProductRepository extends JpaRepository<Product, Integer> { }

public interface VendorRepository extends JpaRepository<Vendor, Integer> { }

@Component
public class DataInsertRunner implements CommandLineRunner {

    @Autowired
    private ProductRepository productRepository;

    @Autowired
    private VendorRepository vendorRepository;

    @Override
    public void run(String... args) throws Exception {
        // first insert vendor
        Vendor vendor1 = new Vendor(101, "ABC", "HYD");
        Vendor vendor2 = new Vendor(102, "DEF", "DHL");
        Vendor vendor3 = new Vendor(103, "PQR", "CHN");
        Vendor vendor4 = new Vendor(104, "IJK", "MUM");
        vendorRepository.saveAll(List.of(vendor1, vendor2, vendor3, vendor4));

        // then insert product
        Product product1 = new Product(10, "PEN", 20.0, "A", vendor1);
        Product product2 = new Product(11, "BOOK", 40.0, "B", null);
        Product product3 = new Product(12, "BTL", 80.0, "A", null);
        Product product4 = new Product(13, "INK", 50.0, "A", vendor2);
        productRepository.saveAll(List.of(product1, product2, product3, product4));
    }
}

Let us see some code examples on Joins:-

public interface ProductRepository extends JpaRepository<Product, Integer> {
    
    @Query("SELECT p.pcode, v.vcode FROM Product p INNER JOIN p.vob v")
    public List<Object[]> getProductAndVendorCodes();
}

@Component
public class TestRunnner implements CommandLineRunner {

    @Autowired
    private ProductRepository productRepository;

    @Override
    public void run(String... args) throws Exception {
        List<Object[]> list = productRepository.getProductAndVendorCodes();
        for (Object[] obj : list) {
            System.out.println(obj[0] + " " + obj[1]);
        }
    }
}

Generated Query:-

Hibernate: 
    select
        p1_0.pcode_col,
        v1_0.vcode_col 
    from
        prod_tab p1_0 
    join
        ven_tab v1_0 
            on v1_0.vid_col=p1_0.vid_fk

Output:-

PEN ABC
INK DEF

More Examples:-

@Query("SELECT p.pcode, v.vcode FROM Product p LEFT OUTER JOIN p.vob v")
public List<Object[]> getProductAndVendorCodes();

Generated Query:-

Hibernate: 
    select
        p1_0.pcode_col,
        v1_0.vcode_col 
    from
        prod_tab p1_0 
    left join
        ven_tab v1_0 
            on v1_0.vid_col=p1_0.vid_fk

Output:-

PEN ABC
BOOK null
BTL null
INK DEF

If you are using the Oracle database then you may get an exception:- QuerySyntaxException: expecting “join”, found ‘OUTER’ near line 1, column 71 [SELECT p.pcode, v.vcode FROM package.Product p LEFT OUTER JOIN p.vob v ]. In that case, you can use Dialect: Oracle9iDialect, Oracle10gDialect.

@Query("SELECT p.pcode, v.vcode FROM Product p RIGHT OUTER JOIN p.vob v")
public List<Object[]> getProductAndVendorCodes();

Generated Query:-

Hibernate: 
    select
        p1_0.pcode_col,
        v1_0.vcode_col 
    from
        prod_tab p1_0 
    right join
        ven_tab v1_0 
            on v1_0.vid_col=p1_0.vid_fk

Output:-

PEN ABC
INK DEF
null PQR
null IJK

@Query("SELECT p.pcode, v.vcode FROM Product p FULL OUTER JOIN p.vob v")
public List<Object[]> getProductAndVendorCodes();

It gives error:- Caused by: java.sql.SQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'full join ven_tab v1_0 on v1_0.vid_col=p1_0.vid_fk' at line 1

FULL JOIN is not supported by a few databases and Data JPA. And it is meaningless join that gets all rows from database tables. In that case, better use findAll();

Spring Data JPA Join Example – 2

@Data
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Table(name = "emptab")
public class Employee {
    @Id
    @Column(name = "eid_col")
    private Integer eid;
    
    @Column(name = "ename_col")
    private String ename;
    
    @Column(name = "esal_col")
    private Double esal;
}

@Data
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Table(name = "depttab")
public class Department {
    @Id
    @Column(name = "did_col")
    private Integer did;

    @Column(name = "dcode_col")
    private String deptCode;

    @Column(name = "aname_col")
    private String adminName;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinColumn(name = "did_fk")
    private List<Employee> emps;
}

public interface DepartmentRepository extends JpaRepository<Department, Integer> {

    // @Query("SELECT d.deptCode, e.ename FROM Department d INNER JOIN d.emps e")
    // @Query("SELECT d.deptCode, e.ename FROM Department d LEFT JOIN d.emps e")
    @Query("SELECT d.deptCode, e.ename FROM Department d RIGHT JOIN d.emps e")
    public List<Object[]> getDeptEmpData();
}

public interface EmployeeRepository extends JpaRepository<Employee, Integer> { }

@Component
@Order(1)
public class DataInsertRunner implements CommandLineRunner {

    @Autowired
    private DepartmentRepository departmentRepository;

    @Autowired
    private EmployeeRepository employeeRepository;

    @Override
    public void run(String... args) throws Exception {
        Employee e1 = new Employee(10, "A", 3.3);
        Employee e2 = new Employee(11, "B", 4.3);
        Employee e3 = new Employee(12, "C", 5.3);
        Employee e4 = new Employee(13, "D", 6.6);
        Employee e5 = new Employee(14, "E", 8.6);
        employeeRepository.saveAll(List.of(e1, e2, e3, e4, e5));

        Department d1 = new Department(521, "DEV", "SAM", Arrays.asList(e1, e2));
        Department d2 = new Department(522, "QA", "SYED", Arrays.asList(e3, e4));
        Department d3 = new Department(523, "BA", "AJAY", null);
        departmentRepository.saveAll(List.of(d1, d2, d3));
    }
}

@Component
@Order(2)
public class DataFetchRunner implements CommandLineRunner {

    @Autowired
    private DepartmentRepository departmentRepository;

    @Override
    public void run(String... args) throws Exception {
        List<Object[]> list = departmentRepository.getDeptEmpData();
        for (Object[] obj : list) {
            System.out.println(obj[0] + " " + obj[1]);
        }
    }
}

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SQL Joins

Spring Data JPA Join Example – 1

Spring Data JPA Join Example – 2

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