JSON_ARRAY Using Subqueries in Oracle Database 23c

From Oracle database 23c onward the JSON_ARRAY function can accept a subquery as input, which brings it in line with the current SQL/JSON standard.

◉ Setup

The examples in this article rely on the following tables.

drop table if exists emp purge;

drop table if exists dept purge;

create table dept (

  deptno number(2) constraint pk_dept primary key,

  dname varchar2(14),

  loc varchar2(13)

) ;

create table emp (

  empno number(4) constraint pk_emp primary key,

  ename varchar2(10),

  job varchar2(9),

  mgr number(4),

  hiredate date,

  sal number(7,2),

  comm number(7,2),

  deptno number(2) constraint fk_deptno references dept

);

insert into dept values (10,'ACCOUNTING','NEW YORK');

insert into dept values (20,'RESEARCH','DALLAS');

insert into dept values (30,'SALES','CHICAGO');

insert into dept values (40,'OPERATIONS','BOSTON');

insert into emp values (7369,'SMITH','CLERK',7902,to_date('17-12-1980','dd-mm-yyyy'),800,null,20);

insert into emp values (7499,'ALLEN','SALESMAN',7698,to_date('20-2-1981','dd-mm-yyyy'),1600,300,30);

insert into emp values (7521,'WARD','SALESMAN',7698,to_date('22-2-1981','dd-mm-yyyy'),1250,500,30);

insert into emp values (7566,'JONES','MANAGER',7839,to_date('2-4-1981','dd-mm-yyyy'),2975,null,20);

insert into emp values (7654,'MARTIN','SALESMAN',7698,to_date('28-9-1981','dd-mm-yyyy'),1250,1400,30);

insert into emp values (7698,'BLAKE','MANAGER',7839,to_date('1-5-1981','dd-mm-yyyy'),2850,null,30);

insert into emp values (7782,'CLARK','MANAGER',7839,to_date('9-6-1981','dd-mm-yyyy'),2450,null,10);

insert into emp values (7788,'SCOTT','ANALYST',7566,to_date('13-JUL-87','dd-mm-rr')-85,3000,null,20);

insert into emp values (7839,'KING','PRESIDENT',null,to_date('17-11-1981','dd-mm-yyyy'),5000,null,10);

insert into emp values (7844,'TURNER','SALESMAN',7698,to_date('8-9-1981','dd-mm-yyyy'),1500,0,30);

insert into emp values (7876,'ADAMS','CLERK',7788,to_date('13-JUL-87', 'dd-mm-rr')-51,1100,null,20);

insert into emp values (7900,'JAMES','CLERK',7698,to_date('3-12-1981','dd-mm-yyyy'),950,null,30);

insert into emp values (7902,'FORD','ANALYST',7566,to_date('3-12-1981','dd-mm-yyyy'),3000,null,20);

insert into emp values (7934,'MILLER','CLERK',7782,to_date('23-1-1982','dd-mm-yyyy'),1300,null,10);

commit;

◉ JSON_ARRAY in Previous Database Versions

The JSON_ARRAY function was introduced in Oracle database 12.1, allowing us to convert a comma-separated list of expressions into a JSON array.

select json_array(empno, ename)

from   emp

where deptno = 10;

JSON_ARRAY(EMPNO,ENAME)

--------------------------------------------------------------------------------

[7782,"CLARK"]

[7839,"KING"]

[7934,"MILLER"]

SQL>

The problem with the initial implementation of JSON_ARRAY was we were not able to supply a subquery as an argument, which limited its flexibility. Suppose we wanted to create a JSON document for a department. We might do the following. We've used JSON_SERIALIZE to pretty the output, but it is not necessary.

select json_serialize(

         json_object('department_number' : d.deptno,

                     'department_name' : d.dname)

         pretty) as output

from   dept d

where  d.deptno = 10;

OUTPUT

--------------------------------------------------------------------------------

{

  "department_number" : 10,

  "department_name" : "ACCOUNTING"

}

SQL>

Now we want to add an array of employees for the department. Logically we would expect to use the JSON_ARRAY function, but instead we are forced to use JSON_ARRAYAGG to achieve this.

select json_serialize(

         json_object('department_number' : d.deptno,

                     'department_name' : d.dname,

                     'employees' : (select json_arrayagg(json_object('employee_number' : e.empno,

                                                                     'employee_name' : e.ename))

                                    from   emp e

                                    where  e.deptno = d.deptno)

         )

       pretty) as output

from   dept d

where  d.deptno = 10;

OUTPUT

--------------------------------------------------------------------------------

{

  "department_number" : 10,

  "department_name" : "ACCOUNTING",

  "employees" :

  [

    {

      "employee_number" : 7782,

      "employee_name" : "CLARK"

    },

    {

      "employee_number" : 7839,

      "employee_name" : "KING"

    },

    {

      "employee_number" : 7934,

      "employee_name" : "MILLER"

    }

  ]

}

SQL>

We have achieved the result we wanted, but we didn't use JSON_ARRAY because of the limitations of its implementation.

◉ JSON_ARRAY in Oracle Database 23c

From Oracle database 23c onward the JSON_ARRAY function can accept a subquery as input, which brings it in line with the current SQL/JSON standard. Now we can rewrite the previous statement as follows.

select json_serialize(

         json_object('department-number' : d.deptno,

                     'department-name' : d.dname,

                     'employees' : json_array(select json_object('employee-number' : e.empno,

                                                                 'employee-name'   : e.ename)

                                              from emp e

                                              where e.deptno = d.deptno)

         )

        pretty) as output

from   dept d

where  d.deptno = 10;

OUTPUT

--------------------------------------------------------------------------------

{

  "department-number" : 10,

  "department-name" : "ACCOUNTING",

  "employees" :

  [

    {

      "employee-number" : 7782,

      "employee-name" : "CLARK"

    },

    {

      "employee-number" : 7839,

      "employee-name" : "KING"

    },

    {

      "employee-number" : 7934,

      "employee-name" : "MILLER"

    }

  ]

}

SQL>

Source: oracle-base.com