MySQL 5.7

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CREATE TABLE employees(id INT PRIMARY KEY, name VARCHAR(45), age INT);
INSERT INTO employees(id, name, age) VALUES (1,'John', 34);
INSERT INTO employees(id, name, age) VALUES (2,'Mary', 40);
INSERT INTO employees(id, name, age) VALUES (3,'Mike', 44);
SET @jsonempl=(SELECT JSON_ARRAYAGG(
 JSON_OBJECT("id", id, "name", name, "age", age)) 
 FROM employees);
SELECT JSON_PRETTY(@jsonempl);


JSON_PRETTY(@jsonempl)


[
  {
    "id": 1,
    "age": 34,
    "name": "John"
  },
  {
    "id": 2,
    "age": 40,
    "name": "Mary"
  },
  {
    "id": 3,
    "age": 44,
    "name": "Mike"
  }
]


SELECT JSON_PRETTY(JSON_ARRAYAGG(
 JSON_OBJECT("id", id, "name", name, "age", age)
 )) json
 FROM employees


json


[
  {
    "id": 1,
    "age": 34,
    "name": "John"
  },
  {
    "id": 2,
    "age": 40,
    "name": "Mary"
  },
  {
    "id": 3,
    "age": 44,
    "name": "Mike"
  }
]